vt1200/harness/mame/README.md

VT1200 execution harness (MAME)

A minimal MAME machine that loads the VT1200 firmware into a TMS34010 CPU so we can run it under MAME's debugger: instruction trace, breakpoints, single-stepping, and direct memory watch. It is a visibility tool, not an accurate VT1200 emulation.

Everything in this directory is source-of-truth in this repo; the MAME source tree is disposable and lives outside the project (/home/arti/code/k-space/mame0289).

harness/mame/
├── vt1200.cpp        # the MAME driver (symlinked into the MAME tree)
├── build.sh          # clone-safe build wrapper
├── *.lua             # probe / capture / dump scripts (many; see the reports)
└── roms/vt1200/      # MAME ROM dir (symlink to ../../vt1200_v22.bin)

Reports in this directory:

file	what it records
GATE_CLEANUP_REPORT.md	promoting the working config to the default; retiring the settled env gates
RENDER_REPORT.md	the active-pixel scanline renderer, and the geometry it derived
DUART_REPORT.md	the DUARTs: wiring, the ISR-driven transmit, the datasheet-verified model edits
BRINGUP_LOG.md	the whole bring-up narrative, including withdrawn claims — read it sceptically
ROMPATCH_REPORT.md	runtime ROM patching (debug only — see the caveat)

Address model (the important part)

The TMS34010 is bit-addressed: a register/PC holds a bit address, and a pointer P refers to physical byte P >> 3. MAME models this with a program space of memory_access<32, 1, 3>, i.e. the address-map unit is 1 bit (a 16-bit word spans 16 address units). Source: tms34010.h — note the TMS34020 differs (memory_access<32, 2, 3>), so this is not interchangeable with the 34020. Therefore:

• Address-map entries here are bit addresses, not byte addresses.
• read_word(0x3f200000) returns ROM bytes 0..1 (physical byte 0x07e40000).
• The reset vector is read from logical bit 0xffffffe0, which maps to the last 4 bytes of the image (file offset 0x1bfffc).

The driver maps:

region	bit range	bytes
RAM	0x00000000–0x001fffff	4 MiB at byte 0
ROM	0x3f200000–0x3fffffff	1.75 MiB at byte 0x07e40000
ROM aliases	0x7f…, 0xbf…, 0xff200000–0xffffffff	same ROM (top 2 logical bits not decoded)

The 0xff… alias is what makes the reset-vector page resolve.

What is modelled by default

This changed — see GATE_CLEANUP_REPORT.md. The working configuration used to be a six-variable incantation; it is now the default, and a bare vt1200 vt1200 with no environment runs it.

• Video RAM at bit 0x04000000 — required for the level-1 VIDEO MEMORY test.
• The board-window handshake model (two-tick busy bit).
• MAME's SCN2681 DUART at DUART1 (0x0640000, host/printer), with input port IP4–IP6 held high — that is how the driver reads the VR150 monitor switch (0x70). The level-1 host-port transmit is ISR-driven, so a real DUART is required here, not a convenience.
• A mono palette (pens 0/1 = black/white) and an active-pixel scanline renderer that reads VRAM. See RENDER_REPORT.md.

A bare run progresses through POST and level 1 and stops at HOST PORT - 15, then wedges in the system-board LED loop — a level-1 failure is fatal, not merely reported, so the OS is unreachable while it fails.

What is not modelled

LANCE Ethernet, and DUART2 (0x0620000, keyboard/mouse — opt in with VT1200_DUART2=1). The OS is therefore not usable yet: it is reached, but it does not paint.

Getting past level 1

VT1200_SKIP_LEVEL1=1 drives DUART1's IP3 high, which the firmware takes as a signal to jump to the OS cold-start and skip all 15 level-1 tests. Note what this is: a modelled board input, and the real machine reads IP3 low. So a level-1-skipped run is evidence about the OS, graphics and crash behaviour, and is not evidence about the real terminal's POST.

The skip is deliberately not the default: the real machine runs level 1, so the faithful configuration stays the default and the switch is how we diverge.

Environment knobs (14; down from 33)

All opt-in, all off by default, so a bare run is the working configuration. Instrumentation: VT1200_DUARTLOG, VT1200_BOARD_LOG, VT1200_BOARD_MAP, VT1200_IRQLOG, VT1200_LOG_UNMAPPED, VT1200_DPYLOG (+_FRAMES), VT1200_DUMP_FRAME, VT1200_DUMP_RAM, VT1200_DUMP_VRAM, VT1200_DUMP_DIR, VT1200_NO_RENDER. Behaviour switches: VT1200_SKIP_LEVEL1, VT1200_DUART2.

Regression baseline

progress.lua at 200 frames, no env:

PC=3FDCE400 samples=53 (78 distinct PCs)

This value changed when the working config became the default — previously PC=3FDCE400 samples=199 (a single PC), because a no-env run used to wedge at the hardware-ready gate. The new value is better: a bare run now executes real POST work. It is a change detector, not a specification — if it moves without you meaning it to, find out why.

Build

MAME source is required. A shallow 0.289 checkout is large and slow to fetch.

git clone --depth 1 --branch mame0289 \
    https://github.com/mamedev/mame.git /home/arti/code/k-space/mame0289

harness/mame/build.sh            # symlinks the driver in and runs make

build.sh runs make -j$JOBS SUBTARGET=vt1200 SOURCES=src/mame/dec/vt1200.cpp, which builds a single-driver binary /home/arti/code/k-space/mame0289/vt1200.

The MAME tree is large and the build is memory-hungry: build.sh defaults to -j8, not all cores. Override with JOBS=4 harness/mame/build.sh on a small/low-memory machine. Progress is written to harness/mame/build.log.

Run

Headless execution and inspection use MAME's Lua debugger API from -autoboot_script. (-debugscript only works when a debugger UI drives the console, so it is not usable headless.)

cd /home/arti/code/k-space/vt1200
M=/home/arti/code/k-space/mame0289/vt1200

# trace execution from the reset vector
$M vt1200 -rompath harness/mame/roms -video none -sound none -nothrottle \
   -debug -debugger none \
   -autoboot_script harness/mame/trace.lua -autoboot_delay 0

# demonstrate the pcfReadBitmaps n*4 overflow (OVF_N overrides n, decimal)
$M vt1200 -rompath harness/mame/roms -video none -sound none -nothrottle \
   -debug -debugger none -debuglog -seconds_to_run 5 \
   -autoboot_script harness/mame/overflow.lua -autoboot_delay 0

The Lua API exposes manager.machine.debugger, device.debug (step, go, bpset, wpset), CPU registers (cpu.state['A0'].value), memory (cpu.spaces['program']:read_u32/write_u32), and frame hooks.

Trace from the reset vector (trace.lua)

Prints the reset PC (expect 3F200040), disassembles the power-on code to reset.dasm, and writes an instruction trace of the first two frames to vt1200-reset.trace.

Exercise the pcfReadBitmaps overflow (overflow.lua)

Instead of booting the parser we jump directly into its arithmetic block with a crafted in-memory PCF_CharInfo:

a9  = 0x00040000   ; fake pCS;    pCS->nChars at a9+0x270
a13 = 0x00060000   ; fake arg blk; n (from pcfReadInt32) at a13+0x20
write_u32(0x00040270, 0)        ; nChars == 0
write_u32(0x00060020, n)        ; n
pc = 3f984e70
break at 3f985060               ; just after 'sll 0x2,a11'

This is possible because the font parser's field moves use field 1, which SETF 0,0,1 leaves at 32 bits, so n is a full 32-bit pcfReadInt32 value. See FINDINGS.md for results and the rest of the analysis.