vt1200/harness/mame/BRINGUP_LOG.md

# VT1200 MAME bring-up log

Gate-by-gate boot progress. Companion to `FULL_BOOT_ESTIMATE.md` (the cost estimate)
and `FINDINGS.md` (the original harness + font work).

Goal: get the driver to actually boot the terminal, by discovering each hardware
handshake the firmware waits on and modelling it correctly.

---

## Harness features added (all opt-in via env vars)

| env var | effect |
|---|---|
| `VT1200_BOARD_LOG=<path>` | log every distinct (addr, PC) access to the four undecoded board windows |
| `VT1200_BOARD_MAP=<a=v;...>` | per-address fixed return values, e.g. `0x06200001=1` |
| `VT1200_BOARD_DEFAULT=<v>` | value for addresses with no override |
| `VT1200_BOARD_MODEL=handshake` | write-back registers **plus a self-clearing busy bit** |
| `VT1200_BOARD_BUSY_TICKS=<n>` | busy-bit tick count for `VT1200_BOARD_MODEL=handshake` (the busy bit is fixed to bit 0 of the `+0x10` STATUS register) |
| `VT1200_BOARD_BUSY_W2=<n>` | busy ticks for window `0x06200000` only; applied after `BUSY_TICKS`, so it overrides it for that window |
| `VT1200_BOARD_BUSY_W3=<n>` | busy ticks for window `0x06400000` only; likewise |
| `VT1200_BOARD_DMA_TICKS=<n>` | completion time (ticks) of a command written to the `0x05900000` DMA command port (guessed; unverified); 0 leaves the pair as a plain latch |
| `VT1200_BOARD_SWITCH=<addr>:<mask>:<value>` | monitor switch: bits 4-6 of `0x0640000D` read as `<value>` (raw bits, or a position 0-7) |
| `VT1200_BOARD_RAM=1` | windows behave as plain read/write RAM |
| `VT1200_NO_RENDER=1` | restore the old blank-everything video behaviour |
| `VT1200_DUMP_FRAME=<n>` / `VT1200_DUMP_DIR=<d>` | write that frame as a PPM |

With **no** env vars set the driver behaves exactly as before. Tools: `progress.lua`
(where is it), `stackwalk.lua` (who called it), `coverage.lua` (progress vs loop),
`dpyctl.lua` (display registers).

---

## Gates found, in order

### Gate 1 — hardware-ready poll (SOLVED, real value gate)

```
bcf240: CALLA delay(100)
bcf2a0: MOVE *A9(10h),A11,0     ; read the 0x6200000 window
bcf2c0: BTST 0h,A11
bcf2d0: JREQ BCF200h            ; retry until bit0 set
```
`while (!(read8(0x06200010) & 1)) delay(100);`

Fixed with a single override on the harness's address `0x06200001`. Verified: the
firmware leaves the poll and runs on into bank 6.

### Gate 2 — NOT a gate (a pure delay)

`008880: MOVI 112A880h,A0` / `0088b0: DSJS A0,88B0h` — 18,000,000 iterations inside a
routine that writes `0640000D/E/F` with long settling delays. Passes with time; no
value needed. **Do not confuse a `DSJS` spin for a missing register.**

### Gate 3 — the register is a HANDSHAKE, so constants cannot work

The same register bit is polled with **opposite polarity**:

```
bcf420: BTST 0h,A9   JRNE   -> waits for bit0 SET   (command accepted)
bce670: BTST 0h,A11  JREQ   -> waits for bit0 CLEAR (operation finished)
```

Therefore "the correct value" is not a constant. Implemented a stateful model:
registers are write-back RAM, and the busy bit arms on any write and clears after
`VT1200_BOARD_BUSY_TICKS` reads.

Result: with `VT1200_BOARD_MODEL=handshake VT1200_BOARD_BUSY_TICKS=2` the boot leaves
early POST and reaches the **display/blit** code.

### Gate 4 — graphics/blit-complete poll (needs a stateful model, like gate 3)

In the boot bank, immediately after a TMS34010 graphics operation:

```
004880: JREQ 4960h
004a10: FILL XY                 ; graphics fill
004a20: CMPI 32h,A10 / JRNE 4BA0h
004a60: MOVE @64000D0h,A0,0     ; read the 0x6400000 window
004a90: BTST 2h,A0              ; test BIT 2
004aa0: JAEQ 3F204BA0h          ; bit clear -> branch away
```

The access log corroborates it exactly: `R 0640000D pc=3F204A90`. So the firmware waits on
**bit 2 of the 0x6400000 window right after a graphics fill** — i.e. a
"graphics/blit complete" status bit.

Measured:

| `0640000D` bit 2 | result |
|---|---|
| `0` (stub default) | parked in the display/blit loop, 59 distinct PCs |
| `4` (forced set) | falls back into the POST video/refresh **delay** loop, 10 distinct PCs |

**Neither boots.** Forcing the bit constant simply relocates the wedge — the same lesson
as gate 3. These board status bits are **dynamic handshakes** (busy → done), so a static
value can never satisfy every site that reads them.

### The shape of the problem (worth recording)

Every gate so far has been one of just two kinds:

1. **A pure `DSJS` delay** (gate 2) — passes with time, needs nothing.
2. **A status/handshake bit** (gates 1, 3, 4) — needs *behaviour*, not a value.

That is a much better-scoped problem than "decode an unknown ASIC": the work is to
understand the **semantics of a handful of status bits**, not hundreds of registers.

---

## Where it stands

| stage | stuck at |
|---|---|
| no stubs | `PC=3FDCE400` — one instruction, forever |
| gate 1 fixed | runs into bank 6, then the POST delay loop |
| handshake model | **display/blit code**, ~59–72 distinct PCs, still discovering new ones |
| + 4 MiB RAM | unchanged — size alone is not the blocker |
| + gate 4 bit forced | moves to the POST delay loop — also not it |

`coverage.lua` shows new PCs still appearing at frame 3000 (30 → 72 distinct), so it is
executing, not frozen — but over a very tight footprint, which may be a legitimate
display-refresh loop.

---

## Video status (asked: "are we getting any video?")

**The display controller IS enabled.** Probed live:

```
frame 200  DPYCTL=E008 <ENABLED>  CONTROL=000C  DPYSTRT=FFFC  DPYINT=0000  DPYTAP=0000  DPYADR=E4FC
frame 600  DPYCTL=6008            (bit 15 momentarily clear)
```

Register map used (MAME's tms34010 enum — **MEMORY_MAP.md is correct here**, but note
these are *not* the classic datasheet offsets):

```
0x80 DPYCTL   0x90 DPYSTRT   0xA0 DPYINT   0xB0 CONTROL
0x1B0 DPYTAP  0x1E0 DPYADR   0x160 PSIZE   0x1F0 REFCNT
```

Firmware reference counts (ROM is **little-endian** for instruction operands —
verified against the reset instruction at file 0x54):

```
DPYCTL 45   PSIZE 48   CONTROL 88   DPYADR 4   DPYTAP 2   DPYSTRT 2   DPYINT 8
```

So the display *is* fully configured by the firmware.

**But the rendered frame is black — and that is our harness's fault, not the
firmware's.** MAME's `tms340x0_ind16` only paints the *blanking* regions:

```cpp
if (params.enabled)
    m_scanline_ind16_cb(screen, bitmap, cliprect.min_y, &params);   // active pixels
...
// blank out the blank regions
```

The **active pixels are the driver's job** — every TMS34010 driver in MAME supplies a
scanline renderer (see `atari/harddriv_v.cpp:411 scanline_driver` for the pattern).
Our callback draws nothing, so the active area stays zero.

### Two traps hit while chasing this (recorded so they are not repeated)

1. **A disabled display silences the scanline callback.** `tms340x0_ind16` skips
   the callback when the display is off (DPYCTL bit 15 clear); without the callback
   `tick()` would not run and the busy machines would not clear (and
   `VT1200_DUMP_FRAME` would write nothing). With the display enabled the callback
   *does* fire under `-video none`, and `VT1200_DUMP_FRAME` does write a PPM — so
   `-video none` alone is not the trap. Force rendering with `-aviwrite` only if you
   also need `screen_update`.
2. **`logerror` needs `-log`** (writes `error.log`); `-debuglog` only captures
   *debugger console* output (e.g. the `printf` inside a `bpset` action).
3. **Put frame/RAM dumps in the *scanline* callback, not `screen_update`.** MAME
   calls `screen_update` with small clip regions, and the "region that reaches the
   last line" (`clip_y = 1..999`) only happened **once** in an 80-frame run — so a
   dump guarded on it silently never fires. The scanline callback is invoked once
   per line (`168, 169, 170, …`), so guard on `scanline == screen.visible_area().max_y`.

Both callbacks do fire when rendering is forced, and the scanline callback reports
`enabled=1` — i.e. the display really is on.

### Observed frame dump (frame 60)

```
vt1200-frame-60.ppm   80x901, "79.56% lit"
```

Treat both numbers as **unreliable**: 80x901 is what MAME reconfigured the screen to
from the firmware's HEBLNK/HSBLNK/VTOTAL registers (implausible for a real display —
suggesting the timing registers are not yet properly initialised), and the "lit"
percentage counts uninitialised bitmap data because the active pixels are never drawn.

### RAM dump (frame 200, via the shared RAM pointer)

`vt1200-ram-200.bin` is **262,144 bytes = 256 KiB** — and that size is itself a finding,
because the map range `0..0x1fffff` with 8 address units per byte means **the driver
only models 256 KiB of main RAM**. The real VT1200 has 1 MiB onboard plus up to
3 × 1 MiB SIMMs (service guide), and POST runs a memory test. **Enlarging the RAM
mapping is now a prime suspect and is a cheap experiment.**

Contents of what we do model: full of 32-bit **ROM pointers** (`0x3FD24A90`, `0x3FD2C800`,
…) and resident strings (`$Header: init.c,v 1.3 …`, `DWT TCP/IP Rev 1.0`,
`LATSERVER`/`LANCESERVER`/`MOPSERVER`). 99.6% of the *earlier* low-memory dump was
zeros, but that dump's granularity was wrong; the real picture is tables + pointers.

**No framebuffer appears in main RAM.** Combined with the self-test having a separate
`N - VIDEO MEMORY TEST`, the framebuffer is most likely **separate VRAM** mapped
somewhere not yet identified. That fits the black screen: the display address resolves
into unmapped space, so the renderer reads zeros.

### Hardware facts supplied from the real machine (2026-10-16)

* Output is **always mono black and white** → 1 bit/pixel.
* **1280×1024** → **160 bytes per scanline**, framebuffer **163,840 bytes (0xA0000)**.
* Boot sequence on real hardware: an **ASCII/font table** is printed first, then the
  screen wipes, then the **self-test progress** runs — shown as a large **circle** in
  the middle of the screen.

These give the renderer its geometry **and** a validation target: a correct renderer
should show the character table first.

### RAM-as-bitmap probe

`vt1200-ram-200.bin` rendered at the 160-byte stride (`ram-as-bitmap.png`,
`crop-glyphs.png`) shows a dense band around rows 738–747. Zooming shows it is **not**
glyphs — the hexdump is little-endian 32-bit ROM pointers, i.e. a pointer table.

## What the service guide says (already in `docs/` — read this first next time)

`docs/EK-V1000-SV-002_service-guide.txt` §2.1.1 documents the power-up self-test almost
exactly as observed on the real machine. This is the authoritative boot sequence.

**Level 0 — 8 tests, must complete within 5 seconds:**

```
Simple CPU test                    Video control register test
ROM checksum test                  I/O data path test
RAM test                           Illegal monitor test
Interrupt test                     Ethernet port (LANCE) not reset on power-up test
```

**Level 1 — 15 tests, complete within 25 seconds of power-up.** It "begins by
displaying various patterns on the screen. Next, a **pie graph** display appears to
indicate the progress of level 1 testing. As each test finishes, a segment of the pie
fills in."

```
Video RAM test       Video test          NVR checksum test    Ethernet address ROM test
Option RAM test      Host port test      Printer port test    Image test
DUART1 timer test    Keyboard port test  Mouse port test      DUART2 timer test
Ethernet port (LANCE) test               Mouse test           Keyboard test
```

### Why this matters for our stall

* The user's **"large circle in the middle of the screen" is the level-1 pie graph** —
  and the "ASCII font table" seen first is the level-1 "various patterns". So the real
  machine is well into **level 1** by ~12 s, and finished by ~25 s.
* We have run **22–44 s of emulated time** and are still stuck. Confirmed: blocked, not slow.
* **Level 0 needs hardware we do not model**: an *Interrupt test*, an *I/O data path
  test*, and an *Ethernet port (LANCE) not reset on power-up* test. Our gates
  (status bits on the `0x6400000` window) are almost certainly part of the
  **video control register test** or the **I/O data path test**.

### The monitor selection switch is read at power-up

From §1.1.2: "The terminal searches for the monitor selection switch position **only
during the power-up sequence**." Positions select the monitor model, which sets the
screen size / dots-per-inch:

```
1 = VR150 CRT (15")        2 = VR319 CRT (19")       3 = VR262 CRT (19")
3 = VRE01 EL flat panel    5 = VR315 CRT (15")
```

And level 0 includes an **Illegal monitor test**. So the firmware reads that switch
during power-up, and **our model does not provide it** — a stub returning all-ones or
zero could plausibly fail "illegal monitor". **Finding where that switch is read, and
returning a valid position, is a concrete next target.**

### Video alignment patterns (§2.1.3) — useful validation targets

* **Screen of H's** — fills the active area with uppercase H's (focus)
* **Circle Cross-hatch** — crosshatch plus a large circle in the middle (linearity/aspect)
* **All White** — the whole active area white (active-area adjustment)

### Confirmed hardware facts

* CPU is a **TMS34010FNL-50** → **50 MHz** (the driver currently guesses 40 MHz; the
  users guide lists the -50 part and VCC = max, 50 MHz).
* Display **1280 x 1024 at 72 Hz**, monochrome.
* Memory: 4 MB total — 1 MB on the system logic board plus up to three 1 MB SIMMs.

## Monitor-selection switch — FOUND (2026-10-16)

Located via the ROM string **"Undefined monitor switch setting"** (file `0x019AF4`,
bit `0x3F2CD7A0`) and its reference at file `0x190F8`. The checking code is around bit
`0x3F2C8320` (file ~`0x19060`):

```
3f2c80b0: move @0x62000d0, a11, 0    ; another config bit -> "Present"/"Not Present"
3f2c80e0: btst 5, a11
3f2c8320: move @0x64000d0, a11, 0    ; <<< THE MONITOR SWITCH PORT
3f2c8350: andi 0x70, a11             ; switch position is bits 4-6
3f2c8380: cmpi 0x70, a11   -> "VR150"
3f2c84a0: cmpi 0x50, a11   -> "VR262"
3f2c85c0: cmpi 0x30, a11   -> "VR315"
           (also)          -> "VR319"
           else            -> "Undefined monitor switch setting"
```

| bits 4–6 | monitor |
|---|---|
| `0x70` | VR150 |
| `0x50` | VR262 |
| `0x30` | VR315 |
| other | VR319 / undefined |

The firmware prints a configuration block on screen during power-up:

```
   Display Type              Monochrome
   Monitor Type              VR150
```

All 6,574 ROM strings are now extracted to **`reference/rom-strings.txt`** (file offset
and bit address per line), so these messages are browsable.

### The reason a constant override did NOT unblock the boot

`0x64000D0` is a **bit-mixed port**: bits 4–6 are the switch *input*, while the rest is
a latched *output* that the firmware writes (the reset vector writes 0 to it). Returning
a constant for the whole byte breaks the output bits, so the boot stays stuck.

New harness knob for exactly this: **`VT1200_BOARD_SWITCH=<addr>:<mask>:<value>`** —
bits in `<mask>` read as `<value>`, all other bits stay write-back.

*(Updated 2026-10-16: the model now applies this mix only to the monitor switch port
`0x0640000D` with mask `0x70`, and `<value>` is the raw bits **or** a position 0-7. A
different `<addr>`/`<mask>` is warned about and ignored — see the extraction section at
the end.)*

```
VT1200_BOARD_MODEL=handshake VT1200_BOARD_BUSY_TICKS=2 \
VT1200_BOARD_SWITCH="0640000D:70:70"
```

Note the harness address is the disassembly operand `>> 4` (see the address-unit note at
the end of this file), so the code's `@0x64000D0` is the harness's `0x0640000D`.

**Verified** with `VT1200_BOARD_LOG`: the port reads `0x70` while the firmware's own
write is `0x0000`, i.e. the switch bits are inputs and the rest are latched outputs, as
intended.

**It does not by itself unblock the boot** — `0x00` and `0x70` take different paths
(56 vs 57 distinct PCs, different park points) but both still wedge in the display/blit
code. The remaining read sites are unchanged:

```
06200001  (6 PCs)   06200003  (3 PCs)
0640000D  (6 PCs)   0640000E  (2 PCs)   0640000F  (2 PCs)
```

So the monitor switch was a *correctness* fix (no more "Undefined monitor switch
setting"), not the blocker.

### Harness bug fixed: `build.sh` was hiding compile failures

`make ... | tee "$LOG"` made the script's exit status that of `tee`, so `set -e` never
fired and **the script printed `built:` even when the compile had failed** — which sent
one whole round of experiments against a stale binary. `build.sh` now captures make's
status, tails the log and exits non-zero on failure. Treat a bare `built:` as
insufficient; it now genuinely means success.

## Raster geometry — SOLVED (16 pixels per clock)

The firmware's own raster registers were the key:

```
DPYCTL=E008 (enabled)  HTOTAL=004F(79)  VTOTAL=0384(900)  HSBLNK=004C(76)
DPYINT=0000  VCOUNT=0383
```

MAME computes `width = (HTOTAL+1) * pixels_per_clock`. With the driver's guess of
**1 pixel per clock** that gave an absurd **80-pixel** line. But:

```
80 pixel-clocks per line x 16 pixels/clock = 1280 pixels  <- exactly the display width
72 Hz x 80 x 900 = 5.184 MHz                              <- a sane pixel clock
```

So the TMS34010 fetches **16 pixels per clock**, and the driver now sets:

```cpp
TMS34010(config, m_maincpu, XTAL(50'000'000));   // TMS34010FNL-50
m_maincpu->set_pixel_clock(5'184'000);
m_maincpu->set_pixels_per_clock(16);
m_screen->set_raw(5'184'000, 1280, 0, 1280, 901, 0, 901);
```

Verified: `screen: 1280x901`, effective frame period 0.0142 s (**~70 Hz**, target 72).

**Crash trap:** `pixels_per_clock` must match the SCREEN's raw geometry. With
`pixels_per_clock=16` and an 80-wide bitmap, the renderer computed
`heblnk = 76 * 16 = 1216` and wrote 1216 pixels into an 80-wide bitmap —
`double free or corruption`. **Always keep the screen width consistent with
`(HTOTAL+1) * pixels_per_clock`.**

### The clock comment in the driver was wrong

The original driver said *"the exact value is irrelevant to instruction semantics"*.
That is false: the firmware **calibrates its delays from the video**, and the raster
geometry drives MAME's whole emulated timebase.

---

## Interrupts — where the real blocker is

The self-test error strings sit immediately adjacent to the wedge we spent hours on
(file `0x178F00`-`0x179500`; our stuck loop was `0x179C80`):

```
LEVEL0 ERROR -> no display interrupt error - 28
LEVEL0 ERROR -> display interrupt too slow error - 28
LEVEL0 ERROR -> display interrupt too fast error - 28
LEVEL0 ERROR -> Perpetual (pending) Display Interrupt - 28
LEVEL0 ERROR -> Extraneous Interrupt 1 / 2 Occurrence - 28
LEVEL0 ERROR -> Extraneous Host Interface Interrupt Occurrence - 28
LEVEL0 ERROR -> Extraneous Non-Maskable Interrupt Occurrence - 28
LEVEL0 ERROR -> Trap Interrupt Occurrence - 28
```

**That is level 0's interrupt test, and it is exactly where we were stuck.** Probed live:

```
DPYCTL=E008  INTENB=0000 (interrupts NOT enabled)  INTPEND=0400 (display IRQ pending)
```

So MAME *is* generating the display interrupt, but **INTENB never gets enabled** by the
firmware, so the CPU never takes it and the test can never pass. Either the firmware is
stuck before it enables interrupts, or the enable write is being lost.

`INTENB` (0xC0000110) write sites in the ROM: file `0x000060`, `0x00077C`, `0x000810`,
`0x000EE4` (the reset/POST bank — these are the *disable* at reset), plus `0x06F308`,
`0x09DC54`, `0x09DC60`, `0x09DDAC`. Our wedge is at file `0x17F2xx`, so the firmware has
not reached the later enable sites.

**This is the item to attack next**: find what the firmware is waiting for *before*
enabling interrupts, and model it.

### Watchpoint results (decisive)

`harness/mame/wp.lua` arms debugger watchpoints on the CPU's internal registers (the
driver cannot see those writes). Over 300 frames:

```
watchpoints armed on INTENB / INTPEND / DPYCTL
done (300 frames)          <- NOT ONE write reported
```

And sampling INTENB every frame (`intr.lua`) shows it is **never** written — it is
cleared once in the reset bank and stays `0000` for the whole run:

```
f   1 INTENB CHANGED -> 0000 (DI=0)
f   1 INTPEND CHANGED -> 0000
f   2 INTPEND CHANGED -> 0400        <- display IRQ fires here and stays pending
distinct INTENB values: 0000 (all 600 frames)
```

**Conclusion:** the firmware is stuck *before* any interrupt setup. It is not failing the
interrupt test — it has not reached it. Whatever it is spinning on is a board-level wait
(a status bit in the undecoded windows), not the CPU's interrupt registers.

### What this means for the remaining work

The bring-up has reached the boundary the original estimate predicted: to go further we
need a **modelled interrupt controller / status block** in the `0x5800000`–`0x6400000`
windows, and eventually the DUARTs and the LANCE (level 0 explicitly tests the LANCE, and
level 1 tests both DUARTs, keyboard and mouse).

The difference from the original estimate is that the problem is now **scoped**: we know
the access surface (`VT1200_BOARD_LOG` gives ~18 read sites), we know the gate shapes
(handshake bits, not values), we have the right raster and clocks, and every finding is
recorded here.

## Board register map — current best understanding

From the access log (`VT1200_BOARD_LOG`) and the disassembly. All addresses in the
harness's units (disassembly operand `>> 4`). **Four** windows, not three — `0x5900000`
was only discovered when a disassembly read showed the firmware pairing it with
`0x5800000`.

| window | what we know |
|---|---|
| `05800000` | **command/data pair, low half.** Written with a value; the code at `3fddcbf0` writes a *bit address shifted right by 3* (bit -> byte) into it, implying a DMA-style device that uses **byte** addressing |
| `05900000` | **command/data pair, high half.** Written with command numbers (0, 1, 2, 4, 0x42). Only one access seen in 400 frames, so barely used during our stall |
| `06200001` | polled **6 sites**; bit 0 = a ready/handshake flag (gate 1). Bit 2 tested elsewhere |
| `06200003` | polled **3 sites**, written with 0x11/0x8C/0xE1 in sequence |
| `06200000/02/08/09/0A` | written in a block by the code at file `3FA91xxx` (looks like a register initialisation table) |
| `0640000D` | **bit-mixed port**: bits 4-6 = the **monitor-selection switch**; the rest is latched output. Bit 2 is polled after a `FILL XY` (gate 4) |
| `0640000E/F` | written 1 / 0xFF around the POST video delay routine; read at 2 sites each |

### Tools that did NOT work (so they are not retried)

* `screen_update` for frame dumps — only called in small clip regions; use the **scanline** callback.
* `logerror` for diagnostics — needs `-log`; `-debuglog` only captures debugger console output.
* The `history` debugger command — produces nothing in free-run (`-nothrottle`) mode; it needs
  the debugger to be actively stepping.
* `dis34 -a` (analyze) — mis-detects several code regions here as data (reports
  `0/N words marked as code`). Disassemble those **linearly** with `dis34 -l -b <base>`.

## The windows share a common register layout (key structural finding)

The routine at bit `0x3FA911C0` (file `0x112238`) takes the **window base as an
argument** and drives it generically:

```
3fa911f0: move *a13(0xffe0), a11, 1   ; a11 = window base, passed in
3fa91210: movi 0xca, a9
3fa91240: move a9, *a11(0x20), 0      ; write the COMMAND/DATA register
3fa91260: push 10 / calla 0x3fdce1c0  ; delay(10)
3fa912c0: addk 32, a9                 ; next pattern value
3fa912e0: move a9, *a11(0x20), 0      ; and again
...
```

and the gate-1 poll reads `*[base+0x10]` (`0x06200010`) testing bit 0.

So each window is a **same-shaped controller**:

| offset | role |
|---|---|
| `+0x10` | **STATUS** (bit 0 = ready/complete; bit 2 tested after `FILL XY`) |
| `+0x20` | **COMMAND / DATA** (written in a delayed sequence: 0xCA, 0xEA, 0x3A, 0x4A, 0x5A, 0x1A, ...) |
| `+0xD0` | **mixed I/O**: bits 4-6 = monitor switch (input), rest latched output |

That means the model is **one generic register file instantiated per window**, not four
unrelated blocks — and it explains why the same shapes keep appearing.

## Video memory FOUND — `0x00400000` (2026-10-16)

Found empirically, not by static sweep. A new catch-all mapping
(`VT1200_LOG_UNMAPPED=<path>`) logs every access to memory the driver does **not**
model. With it, the firmware's unmapped traffic collapses to exactly one region:

```
range:  0x00400000 .. 0x0040D7FF   (map units; 1 unit = 1 bit, 8 per byte)
writers: 117,002 W  from pc=3F204A20   (file 0x944 -- the POST blit, right after FILL XY)
readers:   6,409 R  from pc=3FDF9A00   (file 0x17F340 -- the display/blit code,
                                        reading at an 8-byte stride)
also:                pc=3FDF97A0       (file 0x17F2F4)
```

That is the region the spike independently ranked as its VRAM candidate
(`MOVI 0x4000000,B4` in video init). Two independent methods agree.

**Why the screen is black:** we never mapped it, so the blitter's writes vanished and its
reads returned zero.

**Now mapped** under `VT1200_VRAM=1` (`map(0x00400000, 0x0047ffff).ram().share("vram")`,
i.e. 512 K bits = 64 KiB of video memory, gated so the no-env baseline is untouched), with
`VT1200_DUMP_VRAM=<frame>` to dump it.

**Dumped at frames 5, 30, 120 and 400: all zero.** That is *consistent*, not a bug — the
writes are clearing/testing video RAM (a memory test writes and reads back), and the
firmware has not yet drawn anything because it is stuck early. So the blitter region is
identified, but **no picture can appear until the firmware gets far enough to draw one.**

### Method notes worth keeping

- `map()` entries are built at machine-**configuration** time, so a mapping gated on a
  pointer initialised in `machine_start()` is never registered. Gate on `getenv()`.
- The register-first catch-all works: real regions added after it take precedence, which
  is why the firmware still boots unchanged while the catch-all is installed.
- Map units are **bits** (8 per byte) — confirmed by a 0x80000-unit `.ram()` reporting
  `bytes() == 65536`.

## WHAT THE FIRMWARE IS ACTUALLY DOING (2026-10-16) — the key diagnostic

We can now read the firmware's own console output without seeing the screen. The printer
routine entry is at bit `0x3FDE00C0`, and every call site loads the message pointer into
`a9` immediately before `calla` — so a breakpoint at that entry logging `a9` yields the
transcript. `harness/mame/cap.lua` does this.

Captured:

```
'LEVEL1/2 TESTING ** '
'LEVEL1/2 ERROR -> VIDEO MEMORY - 22 '
```

### What this means (this reframes the whole bring-up)

1. **The firmware has already PASSED level 0.** It is in level 1/2 testing, so the level-0
   interrupt test passed. `INTENB` staying `0000` is therefore **not** a level-0 failure.
2. **It fails the level-1 VIDEO MEMORY test** (error 22 — the service guide's level-1
   "Video RAM test") and then **restarts**.
3. The restart is explained: the stall loop entry `0x3F208060` begins with **`DINT`** —
   interrupts disabled — and resets `SP` to `0x53E0`. It is called from **8 sites**, all in
   the reset/POST bank (file `0x0568`, `0x05C0`, `0x0970`, `0x0C86`, `0x0E76`, `0x0ED2`,
   `0x0F48`, `0x1008`). So the firmware loops: run a test -> fail -> restart with
   interrupts off -> run again.

**The causal chain is now complete and consistent:**

> video memory not modelled -> the level-1 video RAM test fails -> error 22 is reported ->
> the firmware restarts with interrupts disabled -> it never reaches the point where it
> draws -> the screen is black, and no renderer can help.

So the fix is **not** the renderer: it is to model video memory well enough that the
level-1 video RAM test passes. That is the next task.

### Resolving the message pointers

Pointers are bit addresses; the image is byte-based at `0x07E40000`, so file offset is
`(P >> 3) - 0x07E40000`.

```
a9=0x3FDCA8E8 -> file 0x17951D : 'LEVEL1/2 TESTING ** '
a9=0x3FDC98F8 -> file 0x17931F : 'LEVEL1/2 ERROR -> VIDEO MEMORY - 22 '
```

The other captured `a9` values (`0x00C0000C`, `0x0100000C`, `0x0140000C`, `0x0180000C`,
`0x01C0000C`) are not ROM pointers — they step by `0x400000` and look like addresses the
video RAM test walks. Worth following up when modelling VRAM: the test appears to walk
memory in `0x400000`-sized steps.

### Method note

This is the single most useful diagnostic we have built: **the firmware narrates its own
self-test through `0x3FDE00C0`.** Any future stall can be identified the same way in one
run, instead of by disassembly archaeology.

## BLOCKER: address-unit confusion + headless debugger limits (2026-10-16)

Two things are now blocking progress, and both are instrumentation problems rather than
missing knowledge. Fix these before further register work.

### 1. Watchpoints DO NOT FIRE in this headless configuration

`harness/mame/cal.lua` watches **three** encodings of the same known-hit address — the
driver-map address `0x0620000D` (which `VT1200_BOARD_LOG` *proves* the firmware hits), the
firmware operand `0x6200010`, and a shifted variant `0x00620000`. Over 200 frames:

```
not a single hit
```

Since the first of those is demonstrably accessed, the watchpoint itself is not firing:
**`wpset` is a silent no-op when the debugger has no UI**, exactly like the `history`
command. `bpset` *does* work (the console capture at `0x3FDE00C0` fires reliably).

**Consequence, and it is a correction:** `wp.lua`'s earlier claim that "nothing writes
INTENB/INTPEND/DPYCTL" rested on watchpoints that were never firing. The *frame-sampled*
part of that conclusion stands (`intr.lua` polls INTENB every frame and sees only `0000`),
but the watchpoint evidence must be treated as void.

**For memory observation, instrument the DRIVER, not the debugger.** `VT1200_BOARD_LOG`,
`VT1200_LOG_UNMAPPED` and the dump functions are all driver-side and all work.

### 2. Four address conventions, still not reconciled

| # | convention | example |
|---|---|---|
| 1 | firmware operands (bit addresses, as disassembly prints) | `@0x64000D0` |
| 2 | the driver's address-map units (`BOARD_BASE` + offset) | `0x0640000D` |
| 3 | the catch-all log's addresses | `0x400000`..`0x40D7FF` |
| 4 | the debugger's units | unusable headless |

The unresolved contradiction: the catch-all logged `0x400000..0x40D7FF` as **unmapped**,
yet the main-RAM map (`0x00000000..0x01FFFFFF`, 4 MiB) contains those addresses. Either the
catch-all shadows RAM (but then the firmware would be running on zeros and would not reach
level 1), or its `off` is not the space address. **The video-memory conclusion is therefore
provisional until this is settled.**

Concrete calibration plan (uses only driver-side logging, which works):

1. Add a logging mapping over a slice of RAM (`0x00000000..0x0000FFFF`) registered AFTER
   the RAM entry. If it logs, map precedence is last-wins and handlers can be layered; if
   not, first-wins.
2. Add the same over each candidate encoding of the video region and see which one logs the
   blitter's traffic.
3. That single run fixes the units and the precedence, and every address in this document
   can then be normalised.

---

## Next steps

FIX THE INSTRUMENTATION FIRST (see above), then model video memory.