vt1200/harness/udpstream/vidstream.c

/*
 * vidstream.c -- terminal side of the VT1200 video stream.
 *
 * Task 3 claimed the working set with ONE malloc at startup and published
 * where it landed.  Task 4 added the receive representation: two 24 KB frame
 * slots at the head of that pool and a poll loop that hands a completed frame
 * to fp_parse() (frame_parse.c, shared with the host unit test) and counts what
 * it accepted.  Task 5 blits an accepted strip frame to the screen: one
 * PIXBLT B,XY per source row, repeated down the row `stretch` times by
 * SPTCH=0 (vram_b.asm).  Task 6 adds the back-channel: the LANCE is taken over
 * for transmit and a KIND_STATUS frame is built and sent every 64 accepted
 * frames (send_status() below).  Task 7 replaces Task 4's debugger-injected
 * buffer with the payload's own LANCE RX ring: sixteen descriptors whose
 * buffers tile a slot, armed per slot, consumed in ring order, so the chip
 * DMAs the wire bytes into the memory fp_parse() and the blit read.
 * Plan 2's Task 3 (this one) adds the tile dictionary: KIND_DICT_CHUNK frames
 * are assembled into a 32 KB block after the two slots, and the dispatcher
 * learns the TILES_FULL/TILES_DELTA kinds so a frame that arrives before the
 * dictionary is complete is refused and counted instead of being mistaken for
 * a strip.  Plan 2's Task 4 paints TILES_FULL: one PIXBLT B,XY per 32x32 tile
 * from the dictionary (vram_tile.asm), 1,280 of them per frame inside one
 * dint/eint pair.  Plan 2's Task 5 decodes TILES_DELTA: a validated changed-tile
 * list painted by the same per-tile primitive, only the tiles it names.
 *
 * Pool layout inside the single allocation (POOL_BYTES = 249856 = 244 KB):
 *
 *      +0x00000   frame slot A   49152 bytes   (32 x 1526 B RX buffers)
 *      +x0C000   frame slot B   49152 bytes   (32 x 1526 B RX buffers)
 *      +0x18000   dictionary     32768 bytes   (256 x 128 B, Task 3)
 *      +0x20000   slack         114688 bytes   (unused; the LANCE block's
 *                                               fixed 0x3C000 tail follows)
 *      +0x3C000   LANCE block     4096 bytes   (init block, TX ring, status
 *                                               frame, RX ring)
 *
 * The dictionary sits at POOL_PTR + 2 * SLOT_BYTES, which is the placement the
 * plan fixes and plan 2's Task 4 consumes.  The 160 KB framebuffer reserve
 * follows it: this decoder paints straight from the RX fragments (there is no
 * per-frame CPU copy, spec 8.1) and never touches that region, so it is slack
 * today, but the layout adds up to exactly POOL_BYTES -- 49152 + 49152 + 32768
 * + 114688 + 4096 = 249856.  The pool size did NOT have to grow for the deeper
 * ring: the 160 KB framebuffer reserve was unused slack, and 114,688 B of it
 * is still free between the dictionary and the LANCE block.  dict_slack_check below pins the part Task 3
 * depends on (the slot pair and the 32 KB dictionary); the LANCE block's
 * offsets at 0x3C000 were fixed before it and are not re-checked here.  Had
 * the dictionary gone after the framebuffer, the plan's base would not be
 * POOL_PTR + 2 * SLOT_BYTES and Task 4 would read 160 KB of zeroes.
 *
 * The LANCE block is the 4 KB tail.  Its ring bases are aligned at run time
 * (lance_start() explains why the offset alone is not enough), and the status
 * frame sits right after the 8 TX descriptors, the RX ring after that.
 *
 * A slot is RX_DESC packet buffers of RXBUFLEN = 1526 bytes (32 x 1526 =
 * 48832, so it fits 49152 with 320 bytes to spare).  The chip writes ONE Ethernet frame
 * per buffer -- 14-byte header, the VS frame's chunk, and the FCS it appends --
 * so a VS frame arrives as a series of datagrams in ring order, and the frame's
 * bytes are NOT contiguous in the slot: they are 14 bytes into each buffer,
 * 1526 bytes apart.  This payload therefore keeps a fragment table (offset,
 * buffer, length per datagram) and `paint()` reads each source row across the
 * fragments that hold it; there is no per-frame CPU copy of the picture (the
 * design's "DMA, no CPU copy" -- spec 8.1).  fp_parse() runs over the first
 * fragment's body, which holds the VS header, with the accumulated length as
 * its bound.
 *
 * Descriptors are re-armed per slot: when a frame completes, all 16 are pointed
 * at the OTHER slot BEFORE the picture is painted, so the chip can fill N+1
 * while we draw N and can never write into the slot being painted.  A frame
 * that needs more than 16 datagrams cannot be received at all: the ring is
 * exhausted, the partial frame is dropped and counted (RX_RING_DROPS).  128
 * rows of 160 bytes is 20,496 bytes = 15 datagrams, so the design's strip frame
 * fits with one descriptor to spare.
 *
 * Screen geometry.  VRAM is at bit 0x04000000 and a screen row is 0x800 bits
 * (256 bytes), the real unit's pitch (harness/c-hello/README.md).  The harness
 * now models the real unit's VR319 monitor by default, so MAME's renderer uses
 * the same 256-byte pitch; the draw test still compares VRAM as memory, not the
 * rendered picture, because that is what the payload actually writes.
 *
 * OS repaint canary (spec 8.9).  The blit covers screen rows 0..1023 and
 * columns 0..1279 only.  One 16-bit sentinel word sits in the unused right
 * edge of the last screen row (row 1023, column 1280), outside every pixel the
 * blit writes.  If something else -- the resident OS -- paints over us it also
 * disturbs that corner; OS_TOUCH counts how often that was seen.  This
 * decoder has no partial update: an accepted frame repaints the whole screen
 * anyway, so detecting the touch and repainting are the same act here, and
 * OS_TOUCH is the record.
 *
 * Marker layout: see the table in frame_parse.h.  Markers are 32-bit cells on
 * the byte-aligned page at bit 0x01810000, 0x20 bits (4 bytes) apart.  The
 * design plan names them 0x01810020, 0x01810024, ... but the TMS34010 is
 * bit-addressed, so 0x01810024 is four BITS after 0x01810020 -- inside the
 * same cell.  The plan's numbers are byte offsets into the page; the rule is
 *
 *      marker named 0x018100NN  ->  bit 0x01810000 + NN * 8
 *
 * (POOL_PTR = 0x01810100, ...).  Task 5's OS_TOUCH is 0x01810220 (ruling R12),
 * i.e. cell 0x44 -- free between the RX_* counters and Task 4's retired
 * injection cells.  Task 7's ring record takes cells 0x10, 0x14, 0x18, 0x1c,
 * 0x40 and 0x48 (the free page prefix R12 left for the RX work, at the same
 * four-byte spacing every other cell uses); vidstream.lua uses the same rule.
 * Plan 2's Task 3 takes cells 0x50 (DICT_READY), 0x54 (DICT_BYTES), 0x58
 * (DICT_BAD) and 0x5c (TILES_DROP) -- the gap between RX_FCS_DROP and
 * RX_SLOT_A, still at four-byte spacing.
 *
 * K&R C only: no prototypes, no '//', declarations before statements.
 */
#include "romapi.h"
#include "poolcfg.h"		/* POOL_BYTES, written by build-stream.sh */
#include "frmpsr.h"		/* frame_parse.h under an 8.3 name; see that file */

#define HEAP_FREE   0x1E8BA0	/* firmware free-byte counter, IN BITS     */
#define MARK        0x01A00000	/* .text must end before this bit address  */

/*
 * The decode-time instrument (plan 2, the 12 risk: B,XY at 0.32 us/word is
 * MAME-only).  REFCNT is the TMS34010's own refresh counter, an I/O register
 * at bit 0xC00001F0 -- the same one the firmware's POST reads twice and
 * requires to have moved (harness/mame/BRINGUP_LOG.md, the REFCNT fix).  It is
 * a real register, not a harness invention, and reading it costs a MOVE.
 *
 * What it is NOT: microseconds.  MAME returns (total_cycles()/16) & 0xfffc,
 * i.e. a tick every 64 CPU cycles with the low two bits masked, and the real
 * part's DRAM-refresh rate has not been measured on the unit.  The status
 * field therefore carries REFCNT ticks, and the unit conversion is still the
 * M2 measurement the plan asks for.  The counter is 16 bits and wraps; every
 * sample below extends the difference modulo 65,536.  Stock MAME advances it
 * by 4 every 64 cycles (a 21 ms wrap at 50 MHz); the documented local REFCNT
 * patch advances it every cycle (a 1.3 ms wrap).  A tile row is well under one
 * period in either build, so sampling once per row -- and every 64 entries in
 * the delta validation pass -- keeps the extension unambiguous.  If a future
 * payload cannot read REFCNT, it must report that instead of substituting a
 * made-up clock.
 */
#define REFCNT_REG  0xC00001F0	/* TMS34010 refresh counter, 16 bits       */

static unsigned int decode_mark;	/* REFCNT at the last sample        */
static unsigned int decode_ticks;	/* this frame so far, wrap-extended  */
static unsigned int decode_worst;	/* worst since the last status sent */

#define STATIC_BYTES 249856	/* the designed working set, 244 KB        */

/* Screen memory and the strip geometry (see the header comment). */
#define VRAM_BASE   0x04000000	/* video RAM, bit address                  */
#define VRAM_ROW    0x800	/* 2048 bits = 256 bytes a screen row      */
#define SCREEN_ROWS 1024	/* rows in the framebuffer                 */
#define ROW_PIXELS  1280	/* a strip row is 1280 pixels              */
#define ROW_BYTES   160		/* 1280 pixels, 1 bpp                      */

/*
 * The sentinel word: last screen row, first column past the 1280 the blit
 * writes.  Outside the blitted area by construction, inside mapped VRAM (the
 * next 256-byte row would be bit 0x06000000, which the driver maps as the NVR,
 * not as video memory).
 */
#define SENT_ADDR   (VRAM_BASE + (SCREEN_ROWS - 1) * VRAM_ROW + ROW_PIXELS)
#define SENT_VALUE  0xA5C3

/*
 * The two frame slots.  SLOT_BYTES holds a whole RX ring: 32 descriptors x
 * RXBUFLEN (1526) = 48,832 B, rounded up.  It also still holds a 128-row strip
 * frame (128 x 160 B + 16 B header = 20,496 B = 15 packets).
 *
 * 32, not 16, because 16 could not absorb the tile dictionary: it arrives as
 * 23 back-to-back datagrams (one TILES_DICT_CHUNK each), and with a 16-deep
 * ring the chip dropped the tail while the consumer was still copying -- the
 * measured zero-gap upload stalled at 24,616 of 32,768 B.  The heap has
 * megabytes free; the ring, not the memory, was the limit.  Slot addresses are bit addresses, because that is what a
 * pointer holds in this toolchain (see harness/c-hello/README.md: "pointers
 * are bit addresses").
 */
#define SLOT_BYTES  49152

/*
 * Plan 2's Task 3: the tile dictionary.
 *
 * 256 entries of 32x32 1-bpp tiles, 128 bytes each = 32 KB, held in the
 * startup pool right after the slot pair -- POOL_PTR + 2 * SLOT_BYTES, the
 * placement the plan fixes and the tile blit (Task 4) consumes.  The chunks
 * arrive as KIND_DICT_CHUNK frames whose payload is a u16 little-endian
 * offset plus up to 1448 dictionary bytes (tiledict.py); dict_chunk() is the
 * assembler and states the rules it enforces.
 *
 * The pool layout in the header comment is exactly POOL_BYTES, so a build with
 * a smaller request cannot hold the dictionary: dict_slack_check below refuses
 * it at build time, and the run-time guard in main() refuses to point the
 * assembler at memory outside the allocation.  (POOL_BYTES is generated into
 * poolcfg.h by build-stream.sh; the fallback pool is STATIC_BYTES, the same
 * size.)
 */
#define DICT_OFF    (2 * SLOT_BYTES)	/* 0x0C000: after the two slots     */
#define DICT_ENTRY  128			/* one 32x32 1-bpp tile, in bytes   */
#define DICT_ENTRIES 256		/* entries: an index is one byte    */
#define DICT_SIZE   (DICT_ENTRY * DICT_ENTRIES)	/* 256 entries = 32 KB    */

/*
 * Task 4: a TILES_FULL frame is 1,280 dictionary indices, row-major, one byte
 * per tile: 40 tiles across the 1280-pixel screen, 32 tile bands down the 1024
 * rows.  Tile (tx, ty) is dictionary entry `index[ty * 40 + tx]`, painted by
 * one PIXBLT B,XY from dict_base + index*1024 bits (128 bytes = 1024 bits;
 * ruling R4 corrected the spec's old index << 7) with SPTCH = 32 bits,
 * DYDX = (32, 32) and destination XY (ty*32, tx*32) -- vram_tile.asm owns the
 * operand setup, tile_blit() below is its only caller.
 */
#define TILE_PIXEL  32			/* a tile is 32x32 pixels           */
#define TILE_COLS   40			/* 1280 px / 32                     */
#define TILE_ROWS   32			/* 1024 px / 32                     */
#define TILES_SCREEN (TILE_COLS * TILE_ROWS)	/* 1,280 indices a frame    */
#define TILE_SHIFT  10			/* 128 bytes = 1024 bits an entry   */

/*
 * The arithmetic check the brief asks for, as a compile-time assertion.
 *
 * A #if here does NOT work: this preprocessor compiled the file with
 * POOL_BYTES = 50000 and no complaint, so it does not fold the macro
 * arithmetic in #if.  An `extern' array with a negative length is accepted
 * too (measured).  A DEFINED array whose length is 1 or -1 does fail the
 * build (measured: POOL_BYTES=50000 -> "1 Error, No Warnings" and no
 * VIDSTRM.OUT), at the cost of one byte of .bss on a good build.  The
 * dictionary must fit after the slot pair, which is the arithmetic below; the
 * full layout is exactly POOL_BYTES (header comment).
 */
char dict_slack_check[(POOL_BYTES >= (DICT_OFF + DICT_SIZE)) ? 1 : -1];

/*
 * Task 5: the wire's tile count and the decoder's geometry must be the same
 * grid.  frame_parse.h pins it (FP_TILES_SCREEN, the layout block) and this is
 * the build-time half: a disagreement is a compile error rather than a paint
 * that lands one tile off.  Same trick as dict_slack_check above.
 */
char tile_grid_check[(TILES_SCREEN == FP_TILES_SCREEN) ? 1 : -1];

/*
 * Task 6: the terminal's status frame.
 *
 * The payload takes the LANCE over -- STOP, its own init block, RX and TX
 * rings, INIT, IDON, STRT -- and every 64 accepted frames builds a 60-byte
 * Ethernet frame in its own buffer and hands it to the chip with a TX
 * descriptor.  The descriptor setup is lance.c's (harness/udpstream/lance.c):
 * the ring words are TMD0/TMD1/TMD2/TMD3 at byte offsets 0/2/4/6, the buffer
 * address's high byte lives in TMD1, and the length is (0x10000 - bytes) in
 * TMD2.  This is NOT a second driver: it is the same init/descriptor sequence,
 * with one buffer instead of a loopback probe's.
 *
 * Where the pieces live: the pool's last 4 KB was slack, so the LANCE block
 * goes there -- 24 bytes of init block, 64 bytes of TX ring (8 descriptors), the
 * 60-byte status frame and the RX ring (16 descriptors), all aligned at run
 * time in lance_start().  All of it is memory this payload owns, which is the
 * point: the firmware's own rings are in low memory with 32 descriptors (see
 * lance.c's note on the OS init block), and arming a slot in a ring whose
 * position the chip owns is not something this payload can do reliably.
 *
 * Task 7 arms the receiver (MODE = 0, so CSR0's STRT sets RXON) against those
 * 16 descriptors.  `drops` in the status body is three counts, all meaning "a
 * frame that arrived and did not reach the screen": the chip's CSR0 MISS bit,
 * counted once per report window in which it was latched (the bit cannot
 * distinguish two misses in one window, and send_status() says so), plus the
 * payload's own count of frames the ring was too small to finish
 * (RX_RING_DROPS), plus the datagrams the chip flagged bad (RX_FCS_DROP) --
 * those deliver no bytes at all, so without the third count a body-corrupting
 * packet would be invisible.  The chip's field is not replaced, and the
 * payload's own counts are added to it rather than invented.
 */
#define LANCE_RAP    0x05900000	/* bit address of the register-select port  */
#define LANCE_RDP    0x05800000	/* bit address of the data port             */

#define CSR0_INIT    0x0001
#define CSR0_STRT    0x0002
#define CSR0_STOP    0x0004
#define CSR0_TDMD    0x0008
#define CSR0_IDON    0x0100
#define CSR0_MISS    0x1000	/* no RX descriptor: a missed-packet event  */

#define TMD1_OWN     0x8000
#define TMD1_ENP     0x0100
#define TMD1_STP     0x0200

/*
 * The RX ring (Task 7).  RX_DESC descriptors of 8 bytes each, each pointing at
 * its own RXBUFLEN-byte buffer inside the armed slot.  RXBUFLEN is one full
 * 1514-byte Ethernet frame plus margin, the figure lance_cmd.c uses on the
 * unit: a datagram from vidsrc.py is 14 + 1450 = 1464 bytes, the chip appends
 * the 4-byte FCS, writes it after the frame and reports the total in RMD3.
 * RX_DESC * RXBUFLEN = 48832 bytes, which fits a 49152-byte slot.
 *
 * The depth is the burst size the protocol actually uses: a whole 32 KB
 * dictionary upload is 23 datagrams (MAX_BODY 1448), so 32 descriptors let the
 * chip take the entire upload before the consumer has copied the first chunk.
 * It is not a substitute for the sender pacing -- any ring can be outrun by a
 * longer burst -- but it removes the accidental failure of the one burst the
 * protocol defines.
 */
#define RX_DESC      32
#define RXBUFLEN     1526

#define RMD1_OWN     0x8000	/* the chip owns the descriptor             */
#define RMD1_ENP     0x0100	/* end of packet                            */
#define RMD1_STP     0x0200	/* start of packet                          */
#define RMD1_BUFF    0x0400	/* buffer (descriptor) error                */
#define RMD1_CRC     0x0800	/* CRC/FCS error                            */
#define RMD1_OFLO    0x1000	/* overflow error                           */
#define RMD1_FRAM    0x2000	/* framing error                            */
#define RMD1_ERR     0x4000	/* any of the above: the datagram is bad    */

#define ETH_FCS      4		/* the chip stores this after the frame     */

#define L_INIT_OFF   0x3C000	/* 12-word init block (24 bytes)            */
#define L_RING_OFF   0x3C040	/* 8 TX descriptors, 8 bytes each           */
#define L_RING_LEN   8		/* TLEN code 3 = 2^3 descriptors            */

#define STATUS_EVERY 64		/* frames per status report                 */
#define STATUS_LEN   60		/* Ethernet minimum; the chip adds the FCS  */

/* Offset of the VS header inside the Ethernet frame, and of the body. */
#define ETH_HDR_LEN  14
#define STAT_BODY    (ETH_HDR_LEN + FP_HEADER_SIZE)

/*
 * Static fallback.  pool_static is global so the linker map names it and the
 * MAME check can assert the exact address a failed malloc falls back to.
 * stack_cushion and the five VRAM words below sit between it and ENTRY.ASM's
 * _cstack: TMS34010 pushes grow downward, so the 4 KB stack spills into those
 * 4 KB, never into the pool.
 */
unsigned int pool_static[STATIC_BYTES / 4];
unsigned int stack_cushion[1024];

/*
 * VRAM.ASM's externals.  vram.asm is linked into every payload in this
 * directory; the C side owns the values.  vram_b.asm (PIXBLT B,XY) shares
 * blit_src/blit_daddr/blit_dydx and ignores blit_sptch, because its SPTCH is
 * always 0 -- that is what makes one source row fill `stretch` screen rows.
 */
unsigned int vram_pattern;
unsigned int blit_src, blit_sptch, blit_daddr, blit_dydx;

/* Task 4's tile primitive (vram_tile.asm): the source bit address and the
 * destination XY word.  DYDX and SPTCH are the constant 32x32 tile's, and are
 * set in the assembly; only these two vary per call. */
unsigned int tile_src, tile_daddr;

extern void vram_blit_b_();	/* vram_b.asm: PIXBLT B,XY, SPTCH = 0       */
extern void tile_blit_();	/* vram_tile.asm: PIXBLT B,XY, 32x32 tile   */
extern void vid_dint_();	/* vram_b.asm: DINT                          */
extern void vid_eint_();	/* vram_b.asm: EINT                          */

/*
 * The firmware's free-space counter is a 32-bit count IN BITS at
 * 0x1E8BA0 (dwt/notes/HEAP.md).  POOL_FREE reports bytes, because the
 * marker is for humans and the check compares against POOL_BYTES.
 */
static unsigned int heap_bytes()
{
	return *(unsigned int *)HEAP_FREE / 8;
}

/*
 * Task 6 state.  The toolchain never runs .cinit, so main() clears these
 * before they are used (same rule as the markers).
 */
static unsigned int lance_ok;		/* 1 once the LANCE is ours         */
static unsigned int lance_txring;	/* bit address of the TX ring       */
static unsigned int lance_rxring;	/* bit address of the RX ring       */
static unsigned int status_buf;		/* bit address of the status frame  */
static unsigned int lance_txslot;	/* next TX descriptor to arm        */
static unsigned int status_seq;		/* reports sent, the status frame_no*/
static unsigned int status_seen;	/* frames_rx at the last report     */
static unsigned int drops_total;	/* CSR0 MISS windows, lower bound on misses */

/*
 * Task 7 receive state.
 *
 *   rx_slot_of[]  which slot each descriptor points into (0 = A, 1 = B).
 *              Per descriptor, so descriptors can be recycled one at a time
 *              and nothing the chip has already delivered is re-pointed
 *              unread (see rx_release()).
 *   rx_pos     the next descriptor the consumer expects.  The chip fills
 *              descriptors strictly in ring order (am79c90.cpp's
 *              recv_complete_cb advances m_rx_ring_pos by one per packet and
 *              INIT resets it to 0), and this payload consumes them in that
 *              same order, so rx_pos *is* the chip's position -- there is no
 *              second, shadow copy of it that could drift.  It is published as
 *              RX_RING_POS so the MAME check can inject at the right descriptor
 *              without guessing.
 *   rx_acc     bytes of the current VS frame accumulated so far, i.e. the sum
 *              of the delivered datagram bodies.  It doubles as the "still
 *              receiving" flag: 0 means no frame is in progress.
 *   rx_nfrag   fragments recorded for the frame being received.
 *   rx_frag_*  one entry per datagram: where its body sits in the slot, which
 *              frame byte it starts at, and how many body bytes it delivered.
 *              paint() walks this instead of copying the picture into a
 *              contiguous buffer.
 * RX_RING_DROPS counts frames abandoned because the ring ran out of descriptors
 * before the frame completed: a frame larger than 16 datagrams cannot be
 * received at all, so it is dropped and reported rather than half-painted.
 * RX_FCS_DROP counts datagrams the chip flagged bad (RMD1_ERR) whose ethertype
 * says they are ours: they deliver no bytes at all -- the chip writes RMD3 = 0
 * for them -- so without this cell a body-corrupting packet would be invisible
 * to both crc_err and drops.  Each one abandons the frame it belongs to.
 */
static unsigned int rx_pos;
static unsigned int rx_acc;
static unsigned int rx_nfrag;
/* Which slot each descriptor currently points into.  Per descriptor, not one
 * global "armed" slot: a burst is handled by releasing descriptors one at a
 * time as their data is consumed, so a descriptor the chip has already filled
 * is never re-pointed behind the consumer's back. */
static unsigned int rx_slot_of[RX_DESC];
/* Descriptors consumed for the frame being assembled -- their data must stay
 * put until the frame is parsed or abandoned, so they are released together
 * at the end. */
static unsigned int rx_hold[RX_DESC];
static unsigned int rx_nhold;
/* The slot the first fragment of the current frame landed in (RX_SLOT). */
static unsigned int rx_frame_slot;
static unsigned int rx_frag_off[RX_DESC];
static unsigned int rx_frag_ptr[RX_DESC];
static unsigned int rx_frag_len[RX_DESC];

/*
 * Plan 2's Task 3 state.  Like every other static here it is cleared by main()
 * before use: the toolchain never runs .cinit.
 *
 *   dict_base   bit address of the dictionary in the pool (POOL_PTR + DICT_OFF)
 *   dict_bytes  bytes accepted, always a contiguous prefix from entry 0
 *   dict_bad    chunks refused (gap/overlap/duplicate, overrun, after ready)
 *   dict_ready  1 once dict_bytes is exactly DICT_SIZE
 *   tiles_drop  tile frames that could not become pixels
 *
 * dict_bytes is the assembler's only cursor.  It is not a copy of anything the
 * sender says: the offset in a chunk is checked against it, never trusted, so a
 * missing or replayed chunk is visible as an offset mismatch rather than a hole
 * in the dictionary.
 */
static unsigned int dict_base;
static unsigned int dict_bytes;
static unsigned int dict_bad;
static unsigned int dict_ready;
static unsigned int tiles_drop;

/* Set by dict_refuse(); main() turns it into an out-of-cadence status frame.
 * That early report is the difference between the host seeing "dictionary
 * incomplete" and seeing nothing at all: after one lost chunk every later
 * chunk is refused, no frame is ever accepted, and the 64-frame cadence never
 * fires.  See the note on the report loop in main(). */
static unsigned int status_now;

/*
 * blit_row(src, daddr_xy, dydx)
 *
 * One PIXBLT B,XY.  The three values are published where vram_b.asm reads
 * them (see the note there on the calling form; the assembly entry point takes
 * no arguments).  `src` is the packed 1-bpp source row as a bit address,
 * `daddr_xy` is the screen position as (y << 16) | x, and `dydx` is the
 * rectangle as (screen rows << 16) | pixels.  The primitive always sets
 * SPTCH = 0, so the "screen rows" dimension reads the same source row that
 * many times: that is the vertical stretch.
 */
static void blit_row(src, daddr_xy, dydx)
unsigned int src;
unsigned int daddr_xy;
unsigned int dydx;
{
	blit_src = src;
	blit_daddr = daddr_xy;
	blit_dydx = dydx;
	vram_blit_b_();
}

/*
 * The decode-time instrument around a paint (REFCNT_REG above).
 *
 * decode_begin() marks the start of one frame's decode, decode_sample()
 * extends the elapsed count by however many REFCNT ticks have passed since the
 * last sample (mod 65,536 -- the counter is 16 bits), and decode_end() folds
 * the frame's total into decode_worst, which send_status() reports and main()
 * clears on a successful send.  The per-row sample is what makes the wrapped
 * counter unambiguous over a full repaint.
 */
static void decode_begin()
{
	decode_mark = *(unsigned short *)REFCNT_REG;
	decode_ticks = 0;
}

static void decode_sample()
{
	unsigned int now;

	now = *(unsigned short *)REFCNT_REG;
	decode_ticks = decode_ticks + ((now - decode_mark) & 0xFFFF);
	decode_mark = now;
}

static void decode_end()
{
	decode_sample();
	if (decode_ticks > decode_worst) {
		decode_worst = decode_ticks;
	}
}

/*
 * tile_blit(src, tx, ty)
 *
 * One PIXBLT B,XY of a 32x32 dictionary tile.  `src` is the entry's bit
 * address (dict_base + index * TILE_BITS), (tx, ty) the tile's position on the
 * 40x32 grid.  The assembly entry takes no arguments (ruling R13); the two
 * values are published where vram_tile.asm reads them, and the destination XY
 * is formed here because that is the only per-call arithmetic: x = tx*32,
 * y = ty*32, packed as (y << 16) | x.
 */
static void tile_blit(src, tx, ty)
unsigned int src;
unsigned int tx;
unsigned int ty;
{
	tile_src = src;
	tile_daddr = ((ty * TILE_PIXEL) << 16) | (tx * TILE_PIXEL);
	tile_blit_();
}

/*
 * paint(h) -- draw one accepted strip frame from the fragment table.
 *
 * Source row r goes down screen rows r*stretch .. r*stretch+stretch-1.  The
 * frame's bytes do not sit contiguously in the slot (each datagram's body is 14
 * bytes into its own buffer), so a row is cut into the pieces the fragments
 * hold it in: the row starts at frame byte payload_off + r*ROW_BYTES, and each
 * piece is one PIXBLT from the fragment that holds it, with the destination x
 * advanced by the bytes already drawn.  A 1450-byte datagram holds 9 rows and a
 * fraction, so only about one row in nine is split at all.
 *
 * A frame the screen cannot hold is not drawn: zero rows or zero stretch, more
 * than SCREEN_ROWS screen rows, or a payload shorter than the rows it claims
 * (fp_parse() and the accumulator have already proved that payload_len body
 * bytes arrived, so that bound is real).  A refused frame never touches VRAM,
 * so a malformed header cannot paint garbage over a good one.
 *
 * The whole burst runs with interrupts masked (spec 8.9) so the resident OS
 * cannot repaint between two of our source rows and tear the picture.
 */
static void paint(h)
fp_header *h;
{
	unsigned int r, rows, stretch, sent, pos, done, k, f, room, n;

	rows = h->rows;
	stretch = h->stretch;
	if (h->kind != FP_KIND_STRIP || rows == 0 || stretch == 0) {
		return;
	}
	if (rows * stretch > SCREEN_ROWS) {
		return;
	}
	if (rows * ROW_BYTES > h->payload_len) {
		return;
	}
	if (rx_nfrag == 0) {
		return;
	}

	/* The canary word lives outside every pixel this frame writes.  If it
	 * is not ours any more, something else painted VRAM: count it and put
	 * it back.  There is no partial update here -- this frame repaints the
	 * whole screen anyway -- so OS_TOUCH is the record, not a trigger. */
	sent = (unsigned int)*(unsigned short *)SENT_ADDR;
	if (sent != SENT_VALUE) {
		*(unsigned int *)OS_TOUCH = *(unsigned int *)OS_TOUCH + 1;
		*(unsigned short *)SENT_ADDR = SENT_VALUE;
	}

	vid_dint_();
	pos = h->payload_off;		/* the frame's first body byte */
	k = 0;
	for (r = 0; r < rows; r++) {
		done = 0;
		while (done < ROW_BYTES) {
			while (k < rx_nfrag
			       && pos >= rx_frag_off[k] + rx_frag_len[k]) {
				k = k + 1;
			}
			if (k >= rx_nfrag) {
				vid_eint_();
				return;		/* truncated: stop drawing */
			}
			if (pos < rx_frag_off[k]) {
				vid_eint_();
				return;		/* a gap: never paint across it */
			}
			f = pos - rx_frag_off[k];
			room = rx_frag_len[k] - f;
			n = ROW_BYTES - done;
			if (room < n) {
				n = room;
			}
			blit_row(rx_frag_ptr[k] + f * 8,
				 ((r * stretch) << 16) | (done * 8),
				 (stretch << 16) | (n * 8));
			done = done + n;
			pos = pos + n;
		}
	}
	vid_eint_();
}

/*
 * paint_tiles(h) -- draw one accepted TILES_FULL frame, 1,280 tiles.  1 when
 * the picture was painted, 0 when the frame could not be decoded (the caller
 * counts the refusal in TILES_DROP).
 *
 * The payload is 1,280 dictionary indices in a single byte, row-major: index
 * i is tile (tx, ty) with ty = i / 40, tx = i % 40.  The nested loop avoids
 * the division, which the TMS34010 has no instruction for.  Each tile is one
 * PIXBLT B,XY of its 128-byte dictionary entry from dict_base + index*1024
 * (the shift by TILE_SHIFT, because a shift by a named constant is emitted
 * without a multiply).
 *
 * A frame is refused, and nothing is painted, when:
 *   - the dictionary is not complete (DICT_READY 0 or no base): there is
 *     nothing to decode with -- this is the case Task 3's dict mode pins;
 *   - the payload is not exactly 1,280 bytes: the TILES_FULL format is one
 *     byte per tile and nothing else, so a short frame does not name every
 *     tile and a longer one is not this kind's wire form.  Painting a partial
 *     screen over a good one would be worse than leaving the old picture, and
 *     the arbitrary-length tolerance would be a rule nobody wrote down.
 * A gap in the fragment table in the middle of the burst is impossible for a
 * completed frame (fp_parse() accepted it and rx_acc is the accumulated length,
 * so every payload byte is covered); the check is there so such a frame stops
 * drawing instead of painting from a stale pointer, and it is deliberately not
 * counted as a drop because the frame was not refused up front.  That branch
 * leaves a partial picture, exactly as paint()'s truncated case does.
 *
 * The whole burst runs with interrupts masked (spec 8.9): 1,280 blits take
 * long enough that a resident OS repaint between two tiles would tear the
 * picture.  The loop is kept small on purpose -- it is the code the
 * instruction cache holds while the blits run: no call per tile except the
 * primitive, no division, no per-tile function.
 *
 * The OS-repaint canary is the same one paint() uses; a frame that sees it
 * disturbed counts OS_TOUCH and puts it back.
 */
static int paint_tiles(h)
fp_header *h;
{
	unsigned int tx, ty, k, pos, idx, sent;

	if (!dict_ready || dict_base == 0) {
		return 0;
	}
	if (h->payload_len != TILES_SCREEN) {
		return 0;
	}
	if (rx_nfrag == 0) {
		return 0;
	}

	sent = (unsigned int)*(unsigned short *)SENT_ADDR;
	if (sent != SENT_VALUE) {
		*(unsigned int *)OS_TOUCH = *(unsigned int *)OS_TOUCH + 1;
		*(unsigned short *)SENT_ADDR = SENT_VALUE;
	}

	vid_dint_();
	decode_begin();
	k = 0;
	pos = h->payload_off;		/* the frame's first index byte */
	for (ty = 0; ty < TILE_ROWS; ty++) {
		for (tx = 0; tx < TILE_COLS; tx++) {
			while (k < rx_nfrag
			       && pos >= rx_frag_off[k] + rx_frag_len[k]) {
				k = k + 1;
			}
			if (k >= rx_nfrag) {
				vid_eint_();
				decode_end();
				return 1;	/* truncated: stop drawing */
			}
			if (pos < rx_frag_off[k]) {
				vid_eint_();
				decode_end();
				return 1;	/* a gap: never paint across it */
			}
			idx = (unsigned int)*(unsigned char *)
				(rx_frag_ptr[k] + (pos - rx_frag_off[k]) * 8);
			tile_blit(dict_base + (idx << TILE_SHIFT), tx, ty);
			pos = pos + 1;
		}
		decode_sample();
	}
	vid_eint_();
	decode_end();
	return 1;
}

/*
 * frag_get(pos, vp) -- one frame byte at frame offset `pos`.  1 on success, 0
 * when the fragment table does not hold it (a gap, or a frame shorter than the
 * position asked for).
 *
 * paint_delta() reads single bytes at offsets its own arithmetic derives -- the
 * count, each position and each value -- where frag_copy() moves one contiguous
 * run.  Same table, same rules: no copy across a gap and nothing past the last
 * delivered byte.
 */
static int frag_get(pos, vp)
unsigned int pos;
unsigned int *vp;
{
	unsigned int k;

	k = 0;
	while (k < rx_nfrag && pos >= rx_frag_off[k] + rx_frag_len[k]) {
		k = k + 1;
	}
	if (k >= rx_nfrag) {
		return 0;
	}
	if (pos < rx_frag_off[k]) {
		return 0;
	}
	*vp = (unsigned int)*(unsigned char *)
		(rx_frag_ptr[k] + (pos - rx_frag_off[k]) * 8);
	return 1;
}

/*
 * paint_delta(h) -- draw one accepted TILES_DELTA frame.  1 when the list was
 * decoded and painted, 0 when the frame was refused (the caller counts that in
 * TILES_DROP and VRAM is untouched).
 *
 * The wire form (frame_parse.h, tiledict.py's delta_frame) is
 *
 *     count u16 | count x position u16 | count x entry u8
 *
 * with payload_len exactly 2 + 3 * count.  A position is the row-major tile
 * index ty * TILE_COLS + tx and must be strictly ascending; an entry is a
 * dictionary index, one byte.
 *
 * Two passes, and the first one is the safety property: EVERY position and
 * entry is read and bounds-checked before the first blit.  A frame with one bad
 * entry therefore paints nothing at all, where a decoder that painted as it
 * read would leave half a delta on the screen -- and half a delta is worse than
 * none, because the next delta is computed against a frame this side never had.
 *
 * Refused (nothing painted), in the order the checks run:
 *   - no complete dictionary (DICT_READY 0 or no base), as for TILES_FULL;
 *   - payload_len < 2: no count field;
 *   - count 0 or above TILES_SCREEN: an empty list is not sent (the encoder
 *     returns None) and a screen cannot have more changed tiles than tiles;
 *   - payload_len != 2 + 3 * count: the list would run past the frame, or the
 *     frame carries bytes that are not part of the list;
 *   - a position >= TILES_SCREEN: off the 40x32 grid (a blit aimed there would
 *     write past the last tile band);
 *   - a position not greater than the previous one: the list is not canonical,
 *     and a repeated tile would be painted twice for no reason.  This is also
 *     what lets the paint pass run with one cursor: strictly ascending means
 *     the nested grid loop visits every named tile exactly once;
 *   - an entry >= DICT_ENTRIES: one byte cannot express it, so this check is
 *     dead today, but it is the bound that keeps a future wider encoding from
 *     reading past the 32 KB dictionary.  It is written out rather than left to
 *     the byte width.
 *
 * The paint pass walks the 40x32 grid once, comparing the running tile position
 * with the next named one -- no division, no search, and the same
 * one-PIXBLT-per-tile primitive (and dint/eint pair and canary) as
 * paint_tiles().  A tile the list does not name is not touched at all, which is
 * the whole point of a delta.
 *
 * Because the list was validated a moment ago from the same slot, the reads in
 * the paint pass cannot fail; their return values are ignored deliberately, and
 * the slot cannot be written while this frame is being decoded (rx_switch()
 * armed the OTHER slot before the dispatch).
 */
static int paint_delta(h)
fp_header *h;
{
	unsigned int count, k, pos, val, prev, lo, hi, sent;
	unsigned int tile, tx, ty;

	decode_begin();
	if (!dict_ready || dict_base == 0) {
		return 0;
	}
	if (h->payload_len < FP_DELTA_HDR) {
		return 0;
	}
	if (!frag_get(h->payload_off, &lo)
	    || !frag_get(h->payload_off + 1, &hi)) {
		return 0;
	}
	count = lo | (hi << 8);
	if (count == 0 || count > TILES_SCREEN) {
		return 0;
	}
	if (h->payload_len != FP_DELTA_HDR + count * 3) {
		return 0;
	}

	/* Pass 1: the whole list, before anything is painted. */
	prev = 0;
	for (k = 0; k < count; k++) {
		if ((k & 0x3F) == 0) {
			decode_sample();
		}
		if (!frag_get(h->payload_off + FP_DELTA_HDR + k * 2, &lo)
		    || !frag_get(h->payload_off + FP_DELTA_HDR + k * 2 + 1,
				 &hi)) {
			return 0;
		}
		pos = lo | (hi << 8);
		if (pos >= TILES_SCREEN) {
			return 0;
		}
		if (k > 0 && pos <= prev) {
			return 0;
		}
		prev = pos;
		if (!frag_get(h->payload_off + FP_DELTA_HDR + count * 2 + k,
			      &val)) {
			return 0;
		}
		if (val >= DICT_ENTRIES) {
			return 0;
		}
	}

	/* The same canary rule as paint_tiles(): outside every tile, so an
	 * unchanged value means nobody painted over this frame's screen. */
	sent = (unsigned int)*(unsigned short *)SENT_ADDR;
	if (sent != SENT_VALUE) {
		*(unsigned int *)OS_TOUCH = *(unsigned int *)OS_TOUCH + 1;
		*(unsigned short *)SENT_ADDR = SENT_VALUE;
	}

	/* Pass 2: walk the grid, paint the named tiles.  `pos` holds the next
	 * named position; `tile` is the grid position the loop is on.  The
	 * checkpoint here ends the validation stretch (which is O(count)) and
	 * the per-row samples keep the 16-bit REFCNT counter unambiguous. */
	decode_sample();
	vid_dint_();
	frag_get(h->payload_off + FP_DELTA_HDR, &lo);
	frag_get(h->payload_off + FP_DELTA_HDR + 1, &hi);
	pos = lo | (hi << 8);
	k = 0;
	tile = 0;
	for (ty = 0; ty < TILE_ROWS; ty++) {
		for (tx = 0; tx < TILE_COLS; tx++) {
			if (k < count && pos == tile) {
				frag_get(h->payload_off + FP_DELTA_HDR
					 + count * 2 + k, &val);
				tile_blit(dict_base + (val << TILE_SHIFT),
					  tx, ty);
				k = k + 1;
				if (k < count) {
					frag_get(h->payload_off + FP_DELTA_HDR
						 + k * 2, &lo);
					frag_get(h->payload_off + FP_DELTA_HDR
						 + k * 2 + 1, &hi);
					pos = lo | (hi << 8);
				}
			}
			tile = tile + 1;
		}
		decode_sample();
	}
	vid_eint_();
	decode_end();
	return 1;
}

/*
 * frag_copy(dst, pos, n) -- copy n frame bytes starting at frame offset `pos`
 * into memory at bit address `dst`.  1 on success, 0 when the fragment table
 * does not hold the whole range (a truncated frame or a gap).
 *
 * The frame's bytes are not contiguous in the slot: each datagram's body sits
 * 14 bytes into its own 1526-byte buffer, so a dictionary chunk that spans two
 * datagrams -- every 1450-byte chunk does, since 16 + 1450 = 1466 -- has to be
 * stored piece by piece.  paint() walks the same table for the blit, so this is
 * one reading of the fragment map, not a second one.
 */
static int frag_copy(dst, pos, n)
unsigned int dst;
unsigned int pos;
unsigned int n;
{
	unsigned int k, f, room, i;

	k = 0;
	while (n > 0) {
		while (k < rx_nfrag
		       && pos >= rx_frag_off[k] + rx_frag_len[k]) {
			k = k + 1;
		}
		if (k >= rx_nfrag) {
			return 0;		/* truncated */
		}
		if (pos < rx_frag_off[k]) {
			return 0;		/* a gap: never copy across it */
		}
		f = pos - rx_frag_off[k];
		room = rx_frag_len[k] - f;
		if (room > n) {
			room = n;
		}
		for (i = 0; i < room; i++) {
			*(unsigned char *)(dst + i * 8) =
				*(unsigned char *)(rx_frag_ptr[k] + (f + i) * 8);
		}
		dst = dst + room * 8;
		pos = pos + room;
		n = n - room;
	}
	return 1;
}

/* One refused dictionary chunk: count it, ask main() for an immediate status
 * report, and leave the accepted prefix alone so the sender's next good chunk
 * still lands.  The early report is diagnostic: without it a lost chunk makes
 * every later chunk fail and no frame is ever accepted, so the 64-frame report
 * never fires and the host only sees silence. */
static void dict_refuse()
{
	dict_bad = dict_bad + 1;
	*(unsigned int *)DICT_BAD = dict_bad;
	status_now = 1;
}

/*
 * dict_chunk(h) -- assemble one slice of the 32 KB tile dictionary.
 *
 * The wire form (tiledict.py, restated in frame_parse.h) is the VS header,
 * then a u16 little-endian dictionary offset, then up to 1448 dictionary bytes.
 * The rules, and what each refuses:
 *
 *   payload_len >= 3        the offset and at least one dictionary byte.
 *   off == dict_bytes       ascending and contiguous.  A gap, an overlap, a
 *                           duplicate and a reordered chunk all fail this one
 *                           test.  It is also the 128-byte rule Task 2's
 *                           review carried: an offset inside an entry is legal
 *                           only as a continuation of the accepted prefix --
 *                           which is exactly "off equals the prefix end" -- and
 *                           a first chunk that does not begin at entry 0 fails
 *                           it too.
 *   off + body <= DICT_SIZE past the last entry; the tail chunk that would
 *                           overrun 32,768 is the "partial final entry" case.
 *   frag_copy() succeeds    the bytes are really in the fragment table.
 *
 * DICT_READY is set only by the chunk that makes dict_bytes exactly DICT_SIZE:
 * 256 whole entries.  A short upload is therefore never ready, so a tile frame
 * that follows it is still refused -- the wire has no end-of-upload marker to
 * do that earlier.
 *
 * After DICT_READY a further chunk is refused rather than restarting the
 * upload: plan ruling R1 makes the dictionary fixed for the clip, and a
 * re-upload would need a coordinated paint stall the protocol does not have.
 *
 * The offset is read out of the first fragment, whose body always holds the VS
 * header and the first bytes after it; frag_copy() then handles the body
 * proper, which may cross into the next datagram.
 */
static void dict_chunk(h)
fp_header *h;
{
	unsigned int off, body, p;

	if (dict_ready || dict_base == 0 || h->payload_len < 3) {
		dict_refuse();
		return;
	}
	if (rx_frag_len[0] < h->payload_off + 2) {
		dict_refuse();
		return;
	}
	p = rx_frag_ptr[0] + h->payload_off * 8;
	off = (unsigned int)*(unsigned char *)p
	    | ((unsigned int)*(unsigned char *)(p + 8) << 8);
	body = h->payload_len - 2;
	if (off != dict_bytes || off + body > DICT_SIZE) {
		dict_refuse();
		return;
	}
	if (!frag_copy(dict_base + off * 8, h->payload_off + 2, body)) {
		dict_refuse();
		return;
	}
	dict_bytes = off + body;
	*(unsigned int *)DICT_BYTES = dict_bytes;
	*(unsigned int *)DICT_BAD = dict_bad;
	if (dict_bytes == DICT_SIZE) {
		dict_ready = 1;
		/* Published last, after DICT_BYTES: a reader that sees readiness
		 * sees the complete record. */
		*(unsigned int *)DICT_READY = 1;
	}
}

/*
 * The LANCE, transmit only.  The sequence is lance.c's, and the comment on
 * each step names what makes it necessary (harness/mame/README.md rule 4: a
 * step taken from another driver carries its source).
 */
static void w16(bit, v)
unsigned int bit, v;
{
	*(unsigned short *)bit = v;
}

static unsigned int r16(bit)
unsigned int bit;
{
	return *(unsigned short *)bit;
}

static void wb(bit, v)
unsigned int bit, v;
{
	*(unsigned char *)bit = v;
}

/*
 * The chip's register file takes a select write (RAP) then a data access
 * (RDP), and needs a settle between them: am79c90.cpp's regs_w consumes the
 * address register on the next access, and lance.c measured that a store
 * immediately after the select lands in the wrong register.  Same 30-pass
 * settle as lance.c.
 */
static void lance_settle()
{
	unsigned int i;

	for (i = 0; i < 30; i++) {
	}
}

static void lance_csr_w(reg, v)
unsigned int reg, v;
{
	*(unsigned short *)LANCE_RAP = reg;
	lance_settle();
	*(unsigned short *)LANCE_RDP = v;
	lance_settle();
}

static unsigned int lance_csr_r(reg)
unsigned int reg;
{
	unsigned int v;

	*(unsigned short *)LANCE_RAP = reg;
	lance_settle();
	v = *(unsigned short *)LANCE_RDP;
	lance_settle();
	return v;
}

/* The two slots' bit addresses, published by main() and never changed. */
static unsigned int slot_base[2];

/*
 * rx_arm(slot) -- point all 16 descriptors at `slot`, ownership set last.
 *
 * The chip masks the ring ADDRESS it is given (RING_ADDR_MASK 0xFFFFF8,
 * am79c90.h) but uses a descriptor's buffer address as written, in RMD1 bits
 * 0-7 (high byte) and RMD0 (low 16).  NetBSD's am7990_rint() re-arms a used
 * descriptor with "length first, ownership last" and leaves the address alone;
 * here the address changes on every frame (the slot alternates), so the whole
 * descriptor is rewritten, and RMD1 is written FIRST with OWN CLEAR.
 *
 * The order is not cosmetic: the chip fetches RMD1 BEFORE the address.  In
 * am79c90.cpp's receive path it reads `m_rx_md[1] = m_dma_in_cb(ring_address |
 * 2)` and only then `m_rx_md[0] = m_dma_in_cb(ring_address | 0)` and
 * `m_rx_md[2] = ... | 4`.  With OWN set in the same write that carries the new
 * high byte, a fetch inside the window pairs the NEW slot's high byte with the
 * PREVIOUS slot's RMD0 -- a mixed buffer address.  That is harmless only while
 * both slots share a 64 KB page; on the unit a 48 KB slot pair can cross one,
 * and the chip would then DMA into memory outside the pool.
 *
 * Publishing the address and length under OWN clear closes it: a fetch inside
 * the window reads OWN clear and the packet is a missed packet
 * (recv_complete_cb() case -2 latches CSR0 MISS), so it is counted and dropped
 * instead of being written somewhere wrong.  OWN is then set in the last write.
 *
 * RMD2 is the buffer size as a negative two's complement count (0x10000 -
 * RXBUFLEN), the form am79c90's get_buf_length() decodes and lance_cmd.c arms
 * with.  RMD3 is the message count the chip fills in; zeroing it first means a
 * descriptor completed by anything but the chip reads as a zero-length packet
 * rather than as the previous frame's length.
 */
static void rx_arm_one(i, slot)
unsigned int i, slot;
{
	unsigned int cb;

	cb = (slot_base[slot] >> 3) + i * RXBUFLEN;
	/* OWN clear, with the new high byte: a fetch here is a MISS. */
	w16(lance_rxring + i * 64 + 16, (cb >> 16) & 0xFF);
	w16(lance_rxring + i * 64 + 0, cb & 0xFFFF);
	w16(lance_rxring + i * 64 + 32, (0x10000 - RXBUFLEN) & 0xFFFF);
	w16(lance_rxring + i * 64 + 48, 0);
	/* Ownership last, over a complete descriptor. */
	w16(lance_rxring + i * 64 + 16, RMD1_OWN | ((cb >> 16) & 0xFF));
	rx_slot_of[i] = slot;
}

static void rx_arm(slot)
unsigned int slot;
{
	unsigned int i;

	for (i = 0; i < RX_DESC; i++) {
		rx_arm_one(i, slot);
	}
}

/*
 * rx_release() -- give the frame's descriptors back, pointing each at the OTHER
 * slot (spec 8.1's overlap: the buffer the chip next writes must not be one the
 * consumer is still reading).
 *
 * This replaces a per-frame "switch slots and re-arm all 16".  That flip was
 * the burst bug: an all-16 re-arm re-points descriptors the chip had ALREADY
 * completed but the consumer had not yet read, so their frames were silently
 * lost -- measured on a 23-datagram dictionary upload: the chip delivered all
 * 23 (LANCE DMA log, 1.13 ms apart) and only 4 reached these counters.  Holding
 * and releasing each descriptor individually means nothing the chip delivered
 * is ever re-pointed unread, and the ring recycles as fast as the consumer.
 */
static void rx_release()
{
	unsigned int k, i;

	for (k = 0; k < rx_nhold; k++) {
		i = rx_hold[k];
		rx_arm_one(i, 1 - rx_slot_of[i]);
	}
	rx_nhold = 0;
}

/*
 * rx_dgram(i) -- consume descriptor i, which the chip has completed.
 *
 * One Ethernet frame per buffer, so STP and ENP must both be set.  A packet
 * longer than one buffer would be chained across descriptors, and every
 * reference driver (4.4BSD pmax if_le.c, NetBSD am7990.c, Linux 7990.c) drops a
 * chained frame rather than reassembling it -- lance_cmd.c cites the same rule
 * -- so a chained descriptor contributes nothing here.  RMD3 is the delivered
 * length INCLUDING the 4-byte FCS the chip appends, so the body is RMD3 minus
 * the Ethernet header and the FCS.
 *
 * The datagram is appended to the fragment table and the frame's own
 * payload_len decides when the frame is complete (spec 8.1: payload_len plus
 * the bytes actually delivered are authoritative, not the descriptor count).
 * RMD3 is per datagram here, because each datagram is its own Ethernet packet.
 */
static void rx_dgram(i)
unsigned int i;
{
	unsigned int rmd1, mlen, len, src, plen, total, lim, slot;
	unsigned int accepted, refused;
	fp_header h;

	accepted = 0;
	refused = 0;
	rmd1 = r16(lance_rxring + i * 64 + 16);
	mlen = r16(lance_rxring + i * 64 + 48) & 0x0FFF;
	src = slot_base[rx_slot_of[i]] + i * RXBUFLEN * 8 + ETH_HDR_LEN * 8;
	len = 0;
	if (rmd1 & RMD1_ERR) {
		/*
		 * A datagram the chip flagged -- CRC/FCS, buffer, overflow or
		 * framing -- delivers NO bytes: am79c90.cpp's
		 * recv_complete_cb() writes RMD3 as
		 *   (m_rx_md[1] & RMD1_ERR) ? 0 : (result & RMD3_MCNT)
		 * so the length checks below would drop it silently and a
		 * body-corrupting packet would never appear in any counter.
		 *
		 * The bytes ARE in the buffer (dma_out() ran before the FCS
		 * was checked), so a datagram of ours is still identifiable by
		 * its ethertype -- the Ethernet header is written before the
		 * body, so a body-corrupting error leaves it intact.  Count one
		 * drop for the frame it breaks, abandon that frame and re-arm
		 * the other slot: with a datagram missing the accumulation can
		 * never reach the frame's payload_len, and letting later
		 * datagrams keep appending would mix two frames into one
		 * picture.  Traffic that is not ours is ignored, exactly as at
		 * a frame start.
		 *
		 * If the corruption landed on the ethertype the datagram cannot
		 * be attributed and is not counted.  That is the limit of what
		 * the chip leaves readable; it is stated in the report rather
		 * than papered over.
		 */
		if (*(unsigned char *)(src - ETH_HDR_LEN * 8 + 12 * 8) == 0x88
		    && *(unsigned char *)(src - ETH_HDR_LEN * 8 + 13 * 8) == 0xB7) {
			*(unsigned int *)RX_FCS_DROP =
				*(unsigned int *)RX_FCS_DROP + 1;
			rx_acc = 0;
			rx_nfrag = 0;
			rx_release();
		}
		return;
	}
	if ((rmd1 & (RMD1_STP | RMD1_ENP)) == (RMD1_STP | RMD1_ENP)
	    && mlen > ETH_HDR_LEN + ETH_FCS
	    && mlen - ETH_HDR_LEN - ETH_FCS <= RXBUFLEN - ETH_HDR_LEN) {
		len = mlen - ETH_HDR_LEN - ETH_FCS;
	}
	if (len == 0) {
		return;			/* runt or chained: nothing appended */
	}
	if (rx_acc == 0) {
		/* A frame starts here.  Only a VS frame is ours: the chip
		 * accepts broadcast and its own address, so other traffic can
		 * land in this ring, and fp_parse() would count every such
		 * datagram as a bad frame.  Resync on the magic instead. */
		if (len < 2
		    || *(unsigned char *)src != FP_MAGIC0
		    || *(unsigned char *)(src + 8) != FP_MAGIC1) {
			return;
		}
	}

	if (rx_nfrag == 0) {
		rx_frame_slot = rx_slot_of[i];	/* what RX_SLOT reports */
	}
	rx_frag_off[rx_nfrag] = rx_acc;
	rx_frag_ptr[rx_nfrag] = src;
	rx_frag_len[rx_nfrag] = len;
	rx_nfrag = rx_nfrag + 1;
	rx_acc = rx_acc + len;

	if (rx_acc < FP_HEADER_SIZE) {
		return;
	}
	plen = (unsigned int)*(unsigned char *)(rx_frag_ptr[0] + 6 * 8)
	     | ((unsigned int)*(unsigned char *)(rx_frag_ptr[0] + 7 * 8) << 8);
	total = FP_HEADER_SIZE + plen;
	if (total > SLOT_BYTES) {
		/* The header claims a frame larger than the slot: it can never be
		 * completed, and no part of it is drawn.  Reject it now, then hand
		 * the OTHER slot to the chip: all 16 descriptors are re-armed,
		 * including the one this datagram arrived in, so the sender's next
		 * frame starts on a clean ring.  What is left of the rejected frame
		 * is ignored, exactly as any frame that does not open with the
		 * magic is. */
		*(unsigned int *)RX_BAD = *(unsigned int *)RX_BAD + 1;
		rx_acc = 0;
		rx_nfrag = 0;
		rx_release();
		return;
	}
	if (rx_acc < total) {
		return;			/* still arriving */
	}

	/* The frame is complete.  Its descriptors stay held -- so the chip
	 * cannot overwrite the picture while it is parsed and painted -- and are
	 * released just below, each into the other slot. */
	slot = rx_frame_slot;

	/*
	 * Task 4 finding (b): the length handed to fp_parse() is clamped to the
	 * slot.  rx_acc is bounded by RX_DESC * RXBUFLEN and the claim above,
	 * but fp_parse() walks a caller-supplied length, and a slot-sized bound
	 * is the contract both of them can be read against.
	 */
	lim = rx_acc;
	if (lim > SLOT_BYTES) {
		lim = SLOT_BYTES;
	}
	if (fp_parse((unsigned char *)rx_frag_ptr[0], lim, &h)) {
		if (h.kind == FP_KIND_STRIP) {
			*(unsigned int *)RX_LAST_LEN = h.payload_len;
			*(unsigned int *)RX_SLOT = slot;
			paint(&h);
			accepted = 1;
		} else if (h.kind == FP_KIND_DICT_CHUNK) {
			/* A chunk is not a picture: it is counted by DICT_BYTES
			 * and DICT_BAD, never by RX_OK. */
			dict_chunk(&h);
		} else if (h.kind == FP_KIND_TILES_FULL) {
			/*
			 * Task 4: a full frame of indices is painted from the
			 * dictionary, one PIXBLT per tile.  RX_OK and the record
			 * cells describe only frames that reached the screen, so
			 * they move after the paint, not before it.  A frame the
			 * dictionary cannot decode (no DICT_READY, short payload)
			 * is counted in TILES_DROP and touches nothing -- the
			 * same rule Task 3's pre-dictionary branch had.
			 */
			if (paint_tiles(&h)) {
				*(unsigned int *)RX_LAST_LEN = h.payload_len;
				*(unsigned int *)RX_SLOT = slot;
				accepted = 1;
			} else {
				refused = 1;
			}
		} else if (h.kind == FP_KIND_TILES_DELTA) {
			/*
			 * Task 5: a changed-tile list.  paint_delta() validates
			 * the whole list before it paints, so a refused frame
			 * touches no pixel; a frame it accepts moves the same
			 * record cells a TILES_FULL does, and only then.
			 */
			if (paint_delta(&h)) {
				*(unsigned int *)RX_LAST_LEN = h.payload_len;
				*(unsigned int *)RX_SLOT = slot;
				accepted = 1;
			} else {
				refused = 1;
			}
		} else {
			/* A well-formed header whose kind nothing here dispatches
			 * (KIND_STATUS is the terminal's own, travelling the other
			 * way): received and refused, so it is a bad frame. */
			*(unsigned int *)RX_BAD = *(unsigned int *)RX_BAD + 1;
		}
	} else {
		*(unsigned int *)RX_BAD = *(unsigned int *)RX_BAD + 1;
	}
	rx_acc = 0;
	rx_nfrag = 0;
	/* Released only now: every byte this frame needed has been read. */
	rx_release();
	/*
	 * Published after the release, not inside the dispatch.  RX_OK is the
	 * "frame done" signal a reader polls, and it must not become visible
	 * while rx_release() is still rewriting descriptors: a sample taken then
	 * catches a descriptor mid-update (measured: slot B's high byte with
	 * slot A's low word).  A reader that sees RX_OK move now sees a painted
	 * picture AND a consistent ring.
	 */
	if (refused) {
		tiles_drop = tiles_drop + 1;
		*(unsigned int *)TILES_DROP = tiles_drop;
	}
	if (accepted) {
		*(unsigned int *)RX_OK = *(unsigned int *)RX_OK + 1;
	}
}

/*
 * rx_poll() -- drain every descriptor the chip has completed, in ring order.
 *
 * Task 4 finding (a): there is no level-triggered "frame ready" cell any more.
 * The handshake is the OWN bit, per descriptor: the chip clears it when the
 * packet is in the buffer and this side sets it again (in rx_arm) only after
 * reading the descriptor's data, so there is no shared flag for two writers to
 * race on.  `rx_pos` is the only cursor and it advances only after a descriptor
 * has been read; the chip advances its own position by one per packet and INIT
 * resets both to 0, so consuming in ring order keeps the two in step.
 *
 * A frame that uses the whole ring without completing is dropped: the ring
 * cannot hold it, so it can never complete, and leaving the partial frame in
 * place would make every later frame look like its tail.
 */
static void rx_poll()
{
	unsigned int i, n;

	for (n = 0; n < RX_DESC; n++) {
		i = rx_pos;
		if (r16(lance_rxring + i * 64 + 16) & RMD1_OWN) {
			break;		/* the chip has not completed it */
		}
		rx_pos = (i + 1) & (RX_DESC - 1);
		*(unsigned int *)RX_DGRAM = *(unsigned int *)RX_DGRAM + 1;
		rx_hold[rx_nhold] = i;
		rx_nhold = rx_nhold + 1;
		rx_dgram(i);
		if (rx_nfrag == RX_DESC && rx_acc != 0) {
			*(unsigned int *)RX_RING_DROPS =
				*(unsigned int *)RX_RING_DROPS + 1;
			rx_acc = 0;
			rx_nfrag = 0;
			rx_release();
		}
	}
	*(unsigned int *)RX_RING_POS = rx_pos;
	*(unsigned int *)RX_RING_ACC = rx_acc;
	*(unsigned int *)RX_RING_ARMED = rx_slot_of[rx_pos];
}

/*
 * lance_start(ptr) -- take the LANCE over for receive and transmit.  1 on
 * success.
 *
 * STOP comes first because the init block address (CSR1/CSR2/CSR3) is only
 * writable while the chip is stopped -- am79c90.cpp regs_w guards each of them
 * with "if (m_csr[0] & CSR0_STOP)" -- and because STOP resets the chip, which
 * clears INEA: the network interrupt is off before this payload touches
 * anything, so the firmware's ISR cannot act on a half-programmed ring.  The
 * caller also masks interrupts for the whole sequence.
 *
 * The init block's words are the C-LANCE's own order: MODE, PADR (three
 * words), LADRF (four), RX ring address/length, TX ring address/length.  A
 * ring word is the address's bits 16-23 in its low byte and the length code
 * (2^code descriptors) in bits 13-15; the pool sits above 64 KB, so unlike
 * lance.c's low-memory rings the high byte is not zero and has to be written.
 *
 * Task 7 removed MODE_DRX and gave the receiver a ring, because that is what a
 * receive path is: MODE = 0 (no loopback, no transmitter disable, FCS kept and
 * appended) and STRT afterwards sets RXON as well as TXON (am79c90.cpp's STRT
 * handler: "if (m_mode & MODE_DRX) clear RXON else set RXON").  The descriptors
 * are armed for slot A before INIT, so the chip's first packet -- which can
 * arrive the instant STRT lands -- finds a buffer.
 */
static int lance_start(ptr)
unsigned int ptr;
{
	unsigned int init, ring, buf, rxring, cb, i, idon;

	/*
	 * Alignment is the CHIP's requirement, not a convenience.  The Am7990
	 * masks what it is given: INIT_ADDR_MASK 0xFFFFFE for the init block and
	 * RING_ADDR_MASK 0xFFFFF8 for a ring (am79c90.h), because a descriptor
	 * is eight bytes.  The pool base is only word-aligned (0x0090C8D0 is
	 * byte 0x12191A), so a fixed offset does NOT give a ring the chip can
	 * find: the first run of this code put the ring at byte 0x15D93A, the
	 * chip masked that to 0x15D938 and read our TMD0 word as TMD1 -- it never
	 * fetched the status buffer at all (proved from VT1200_LANCE_DMA_LOG:
	 * no read of the buffer address, and the chip writing 0x599A into our
	 * TMD0 words).  Round up to the alignment the chip will keep.
	 */
	init = (ptr + L_INIT_OFF * 8 + 0x0F) & 0xFFFFFFF0;	/* 16-bit words */
	ring = (ptr + L_RING_OFF * 8 + 0x3F) & 0xFFFFFFC0;	/* 8-byte      */
	buf = ring + L_RING_LEN * 64;				/* after the ring */
	rxring = (buf + STATUS_LEN * 8 + 0x3F) & 0xFFFFFFC0;	/* after status */
	lance_txring = ring;
	status_buf = buf;
	lance_rxring = rxring;

	lance_csr_w(0, CSR0_STOP);

	w16(init + 0 * 16, 0);			/* MODE: no loopback, FCS on */
	w16(init + 1 * 16, 0x0008);		/* PADR 08:00:2b:16:4b:5a    */
	w16(init + 2 * 16, 0x162b);
	w16(init + 3 * 16, 0x5a4b);
	w16(init + 4 * 16, 0);			/* LADRF: physical + broadcast */
	w16(init + 5 * 16, 0);
	w16(init + 6 * 16, 0);
	w16(init + 7 * 16, 0);
	cb = rxring >> 3;			/* BYTE address of the RX ring */
	w16(init + 8 * 16, cb & 0xFFFF);
	w16(init + 9 * 16, ((cb >> 16) & 0xFF) | (5 << 13));	/* 32 desc */
	cb = ring >> 3;				/* BYTE address of the ring  */
	w16(init + 10 * 16, cb & 0xFFFF);
	w16(init + 11 * 16, ((cb >> 16) & 0xFF) | (3 << 13));

	/* Every TX descriptor points at the one status buffer; TMD1's OWN stays
	 * clear until a frame is ready, and TMD2 holds the length as a two's
	 * complement (0x10000 - 60), lance.c's descriptor layout. */
	cb = buf >> 3;
	for (i = 0; i < L_RING_LEN; i++) {
		w16(ring + i * 64 + 0, cb & 0xFFFF);
		w16(ring + i * 64 + 16, (cb >> 16) & 0xFF);
		w16(ring + i * 64 + 32, (0x10000 - STATUS_LEN) & 0xFFFF);
		w16(ring + i * 64 + 48, 0);
	}
	for (i = 0; i < STATUS_LEN; i++) {
		wb(buf + i * 8, 0);
	}

	/* The RX ring, armed for slot A: this is the state the first packet
	 * finds, and it is what the MAME ring check asserts. */
	rx_nhold = 0;
	rx_arm(0);
	*(unsigned int *)RX_RING_BASE = rxring;

	lance_csr_w(1, (init >> 3) & 0xFFFF);
	lance_csr_w(2, ((init >> 3) >> 16) & 0xFF);
	lance_csr_w(3, 0);
	lance_csr_w(0, CSR0_INIT);

	idon = 0;
	for (i = 0; i < 20000; i++) {
		idon = lance_csr_r(0);
		if (idon & CSR0_IDON) {
			break;
		}
	}
	*(unsigned int *)ST_INIT = idon;
	if (!(idon & CSR0_IDON)) {
		return 0;
	}

	/* STRT turns the receiver and the transmitter on; no INEA, so no
	 * interrupt reaches the firmware's network ISR while this payload owns
	 * the chip.  RXON is what makes the descriptors armed above live. */
	lance_csr_w(0, CSR0_STRT);
	for (i = 0; i < 30; i++) {
	}
	return 1;
}

/*
 * send_status(frames_rx) -- build and transmit one status frame.
 *
 * The frame is an ordinary Ethernet frame: broadcast destination (the LANCE
 * accepts broadcast with an empty LADRF, and the host's capture sees it), the
 * terminal's own address from its Ethernet ROM, the stream's ethertype 0x88B7
 * (frame.py ETHERTYPE), then the same 16-byte VS header every other frame
 * carries, with rows and stretch zero because a status has no geometry.  The
 * body is 16 bytes, LITTLE-endian as frame.py's pack_status() writes it:
 * frames_rx u32, crc_err u16, drops u16, csr0 u16, heap_free_kb u16,
 * dict_state u8, reserved u8, decode_worst u16.  (The command protocol in
 * lance_cmd.c is big-endian; this one is not, and getting it backwards reads
 * 64 frames as 0x40000000 -- which is how the Task 6 MAME run first found it.)
 *
 * dict_state is two bits: 0x01 DICT_READY (the 32 KB dictionary is complete)
 * and 0x02 DICT_BAD (at least one chunk was refused since the payload started).
 * Both can be set -- a chunk refused after the dictionary was ready leaves the
 * dictionary usable -- which is why it is a bit set rather than an enum.
 *
 * decode_worst is the worst decode time, in REFCNT ticks, since the last
 * status frame that was actually sent (decode_begin() above says why ticks and
 * not microseconds).  It is the instrument for the plan's 12 risk: whether the
 * B,XY tile path is fast enough on the unit.  A report sent before any frame
 * was painted carries 0.
 *
 * crc_err is RX_BAD: the frames fp_parse() rejected (header CRC, magic,
 * version or length).  Since ruling R14 a corrupted *body* cannot appear
 * here -- the LANCE validated each datagram's FCS before the bytes were
 * stored -- so this is the only CRC error the terminal can count.  A datagram
 * whose FCS the CHIP rejected is not a frame at all: it delivers no bytes
 * (RMD3 = 0) and is counted in drops, via RX_FCS_DROP.
 *
 * drops is three counts of frames that arrived and did not reach the screen:
 *
 *   drops_total   CSR0 MISS, counted per REPORT WINDOW in which the latched bit
 *                 was set -- the chip gives no per-packet count, so this is one
 *                 per window at most (a lower bound, see send_status()).
 *   RX_RING_DROPS frames too large for the 16 descriptors (Task 7).
 *   RX_FCS_DROP   datagrams the chip flagged bad; each abandons its frame.
 *
 * Neither number is inferred from another.
 *
 * Returns 1 when the frame was handed to the chip, 0 when the ring slot was
 * still owned and nothing was armed.  Overwriting a descriptor the chip still
 * owns is what every reference driver refuses to do (Linux stops the queue,
 * NetBSD prints "missing buffer"), and the caller must not count a report it
 * never sent: it advances status_seen only on 1.
 */
static int send_status(frames_rx)
unsigned int frames_rx;
{
	unsigned int i, crc, csr0, tmd1, off, hb, miss, bad, rdrops, fdrops,
		     free_kb, dstate;

	hb = status_buf;

	/* Ethernet header. */
	for (i = 0; i < 6; i++) {
		wb(hb + i * 8, 0xFF);
	}
	wb(hb + 6 * 8, 0x08);  wb(hb + 7 * 8, 0x00);
	wb(hb + 8 * 8, 0x2B);  wb(hb + 9 * 8, 0x16);
	wb(hb + 10 * 8, 0x4B); wb(hb + 11 * 8, 0x5A);
	wb(hb + 12 * 8, 0x88); wb(hb + 13 * 8, 0xB7);

	/* The VS header, laid out exactly as frame.py's pack_header() does. */
	off = hb + ETH_HDR_LEN * 8;
	wb(off + 0 * 8, FP_MAGIC0);
	wb(off + 1 * 8, FP_MAGIC1);
	wb(off + 2 * 8, FP_VERSION);
	wb(off + 3 * 8, FP_KIND_STATUS);
	wb(off + 4 * 8, status_seq & 0xFF);
	wb(off + 5 * 8, (status_seq >> 8) & 0xFF);
	wb(off + 6 * 8, FP_STATUS_SIZE);
	wb(off + 7 * 8, 0);
	wb(off + 8 * 8, 0);			/* rows: none */
	wb(off + 9 * 8, 0);			/* stretch: none */
	wb(off + 10 * 8, 0);			/* reserved */
	wb(off + 11 * 8, 0);
	crc = fp_crc((unsigned char *)off, FP_PREFIX_SIZE);
	wb(off + 12 * 8, crc & 0xFF);
	wb(off + 13 * 8, (crc >> 8) & 0xFF);
	wb(off + 14 * 8, 0);
	wb(off + 15 * 8, 0);

	/* The chip's status word, and the missed-packet bit it latches.  CSR0
	 * MISS is ONE BIT: am79c90.cpp's recv_complete_cb() only ORs it in
	 * (case -2), so two missed packets inside one report window look exactly
	 * like one.  drops_total therefore counts report windows in which at
	 * least one packet was missed -- a lower bound on missed packets, not an
	 * event count -- and the bit is cleared here after the report is built
	 * (write-1-to-clear) so the next window starts clean.  The status field
	 * is a "something was missed" indicator, and this comment says so
	 * instead of claiming a count the hardware cannot give. */
	csr0 = lance_csr_r(0);
	miss = 0;
	if (csr0 & CSR0_MISS) {
		drops_total = drops_total + 1;
		miss = 1;
	}

	bad = *(unsigned int *)RX_BAD;
	/* The three drop counts described above: the chip's MISS windows, the
	 * payload's count of frames too large for the ring, and the datagrams
	 * the chip flagged bad (they carry no bytes, so nothing else sees
	 * them). */
	rdrops = *(unsigned int *)RX_RING_DROPS;
	fdrops = *(unsigned int *)RX_FCS_DROP;
	free_kb = heap_bytes() >> 10;
	off = hb + STAT_BODY * 8;
	wb(off + 0 * 8, frames_rx & 0xFF);
	wb(off + 1 * 8, (frames_rx >> 8) & 0xFF);
	wb(off + 2 * 8, (frames_rx >> 16) & 0xFF);
	wb(off + 3 * 8, (frames_rx >> 24) & 0xFF);
	wb(off + 4 * 8, bad & 0xFF);
	wb(off + 5 * 8, (bad >> 8) & 0xFF);
	wb(off + 6 * 8, (drops_total + rdrops + fdrops) & 0xFF);
	wb(off + 7 * 8, ((drops_total + rdrops + fdrops) >> 8) & 0xFF);
	wb(off + 8 * 8, csr0 & 0xFF);
	wb(off + 9 * 8, (csr0 >> 8) & 0xFF);
	wb(off + 10 * 8, free_kb & 0xFF);
	wb(off + 11 * 8, (free_kb >> 8) & 0xFF);
	/* The two Task 6 review fields: dictionary state and the worst decode
	 * since the last report (frame.py's pack_status() writes the same
	 * offsets; both bits and the tick unit are in the docstring above). */
	dstate = (dict_ready ? 0x01 : 0) | (dict_bad ? 0x02 : 0);
	wb(off + 12 * 8, dstate & 0xFF);
	wb(off + 13 * 8, 0);			/* reserved: keep the u16 aligned */
	wb(off + 14 * 8, decode_worst & 0xFF);
	wb(off + 15 * 8, (decode_worst >> 8) & 0xFF);

	/* Arm the next descriptor: TMD3, then the length, then ownership last,
	 * so the chip cannot take a descriptor whose length is still stale
	 * (lance_cmd.c does the same, and NetBSD's am7990_rint() says why). */
	off = lance_txring + lance_txslot * 64;
	for (i = 0; i < 10000; i++) {
		if (!(r16(off + 16) & TMD1_OWN)) {
			break;
		}
	}
	if (r16(off + 16) & TMD1_OWN) {
		return 0;			/* still the chip's: arm nothing */
	}
	w16(off + 48, 0);
	w16(off + 32, (0x10000 - STATUS_LEN) & 0xFFFF);
	w16(off + 16, TMD1_OWN | TMD1_STP | TMD1_ENP
	     | ((status_buf >> 3 >> 16) & 0xFF));
	/* Where the harness must look to watch the chip finish this frame. */
	*(unsigned int *)ST_TMLAST = off + 16;

	/* One write: a bare TDMD would clear STRT (CSR0 is a command shadow;
	 * lance.c's comment and every reference driver agree). */
	lance_csr_w(0, CSR0_STRT | CSR0_TDMD);

	tmd1 = 0;
	for (i = 0; i < 20000; i++) {
		tmd1 = r16(off + 16);
		if (!(tmd1 & TMD1_OWN)) {
			break;
		}
	}

	lance_txslot = (lance_txslot + 1) & (L_RING_LEN - 1);
	status_seq = status_seq + 1;
	*(unsigned int *)ST_FRAMES = frames_rx;
	*(unsigned int *)ST_TMD1 = tmd1;
	/* ST_SENT last: a reader that sees it move reads a frame that was
	 * built, transmitted and recorded, never one in progress. */
	*(unsigned int *)ST_SENT = *(unsigned int *)ST_SENT + 1;

	if (miss) {
		lance_csr_w(0, CSR0_MISS);
	}
	return 1;
}

int main()
{
	unsigned int before, after, ptr, frames, sent;

	*(unsigned int *)VID_ALIVE = 0;
	*(unsigned int *)POOL_PTR = 0;
	*(unsigned int *)POOL_FREEB = 0;
	*(unsigned int *)POOL_FREEA = 0;

	/* Receive record: counters and the two slots are cleared before anything
	 * is published, so a reader never sees a counter from a previous run or a
	 * slot address that has no allocation behind it.  The ring record is
	 * published by lance_start() once the descriptors are armed. */
	*(unsigned int *)RX_OK = 0;
	*(unsigned int *)RX_BAD = 0;
	*(unsigned int *)RX_LAST_LEN = 0;
	*(unsigned int *)RX_SLOT = 0;
	*(unsigned int *)RX_SLOT_A = 0;
	*(unsigned int *)RX_SLOT_B = 0;
	*(unsigned int *)RX_RING_BASE = 0;
	*(unsigned int *)RX_RING_POS = 0;
	*(unsigned int *)RX_RING_ACC = 0;
	*(unsigned int *)RX_RING_ARMED = 0;
	*(unsigned int *)RX_RING_DROPS = 0;
	*(unsigned int *)RX_FCS_DROP = 0;
	*(unsigned int *)RX_DGRAM = 0;
	*(unsigned int *)OS_TOUCH = 0;

	/* Plan 2's Task 3: the dictionary record starts empty and is published
	 * as zero before the pool is known, so a reader never sees an earlier
	 * run's readiness on a pool that has no dictionary behind it. */
	*(unsigned int *)DICT_READY = 0;
	*(unsigned int *)DICT_BYTES = 0;
	*(unsigned int *)DICT_BAD = 0;
	*(unsigned int *)TILES_DROP = 0;
	dict_ready = 0;
	dict_bytes = 0;
	dict_bad = 0;
	tiles_drop = 0;
	status_now = 0;
	decode_mark = 0;
	decode_ticks = 0;
	decode_worst = 0;

	/* Arm the VRAM canary before any frame is accepted, so a repaint
	 * between now and the first blit is counted rather than mistaken for
	 * our own last frame. */
	*(unsigned short *)SENT_ADDR = SENT_VALUE;

	before = heap_bytes();
	ptr = malloc_(POOL_BYTES);	/* malloc_ takes BYTES, returns BITS */
	after = heap_bytes();

	if (ptr == 0) {
		/* The fallback is the 244 KB static pool by definition; a larger
		 * request still reports the failure through the low-bit flag. */
		ptr = (unsigned int)pool_static;
		*(unsigned int *)POOL_FREEB = before | 1;
	} else {
		*(unsigned int *)POOL_FREEB = before;
	}
	*(unsigned int *)POOL_FREEA = after;

	/* Slot B follows slot A in the same pool.  Both are published before
	 * POOL_PTR so the harness can find them from either one. */
	slot_base[0] = ptr;
	slot_base[1] = ptr + SLOT_BYTES * 8;
	*(unsigned int *)RX_SLOT_A = slot_base[0];
	*(unsigned int *)RX_SLOT_B = slot_base[1];
	/* Plan 2's Task 3: the dictionary follows the slot pair.  It is derived
	 * from the allocation, never published separately -- POOL_PTR is the
	 * record, and the pool layout above fixes the offset.  build-stream.sh
	 * asserts at build time that the request can hold it (dict_slack_check
	 * above); this is the run-time half, for a payload built another way: a
	 * pool too small for the dictionary leaves dict_base 0, every chunk is
	 * refused and nothing is written past the allocation. */
	if (POOL_BYTES >= DICT_OFF + DICT_SIZE) {
		dict_base = ptr + DICT_OFF * 8;
	} else {
		dict_base = 0;
	}
	*(unsigned int *)POOL_PTR = ptr;	/* publish last: a complete record */

	/* Task 6: the status back-channel.  The record starts empty, then the
	 * LANCE is taken over for receive and transmit with interrupts masked --
	 * the OS's drivers were using this chip a moment ago, and a network ISR
	 * running in the middle of the ring setup would see a half-written
	 * descriptor. */
	*(unsigned int *)RX_EOF = 0;
	*(unsigned int *)ST_SENT = 0;
	*(unsigned int *)ST_FRAMES = 0;
	*(unsigned int *)ST_BUF = 0;
	*(unsigned int *)ST_TMD1 = 0;
	*(unsigned int *)ST_INIT = 0;
	*(unsigned int *)ST_TMLAST = 0;

	lance_txslot = 0;
	status_seq = 0;
	status_seen = 0;
	drops_total = 0;
	rx_pos = 0;
	rx_acc = 0;
	rx_nfrag = 0;
	rx_nhold = 0;
	rx_frame_slot = 0;

	vid_dint_();
	lance_ok = lance_start(ptr);
	vid_eint_();
	*(unsigned int *)ST_BUF = status_buf;

	for (;;) {
		*(unsigned int *)VID_ALIVE = *(unsigned int *)VID_ALIVE + 1;
		rx_poll();

		/* Report every STATUS_EVERY accepted frames, and -- once the
		 * harness says the sender has stopped -- for every further
		 * accepted frame, so a finite run ends with a report whose
		 * frames_rx is the final count rather than the last multiple
		 * of 64 (RX_EOF is harness-only; see frame_parse.h).
		 *
		 * Two out-of-cadence reports exist so the host is never left
		 * with silence:
		 *   - the first accepted frame, so a host that is sending
		 *     sees that the payload is running before 64 frames have
		 *     arrived (and gets the dictionary state with it);
		 *   - status_now, set by dict_refuse().  A lost dictionary
		 *     chunk makes every later chunk fail, no frame is ever
		 *     accepted, and the 64-frame rule would never fire: the
		 *     host would report "PAYLOAD NOT RUNNING" instead of
		 *     "dictionary incomplete".  These are one-shot in the
		 *     sense that status_seen advances; further refusals set
		 *     the flag again, which is wanted while an upload is
		 *     broken.
		 */
		frames = *(unsigned int *)RX_OK;
		if (!lance_ok) {
			continue;
		}
		if (frames - status_seen >= STATUS_EVERY
		    || status_now
		    || (frames != 0 && status_seen == 0)
		    || (*(unsigned int *)RX_EOF != 0 && frames != status_seen)) {
			vid_dint_();
			sent = send_status(frames);
			vid_eint_();
			if (sent) {
				status_seen = frames;
				status_now = 0;
				/* "since the last report": reset only after the
				 * frame really went to the chip. */
				decode_worst = 0;
			}
		}
	}
}