# 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
├── trace.lua         # Lua: trace execution from the reset vector
├── overflow.lua      # Lua: exercise the pcfReadBitmaps n*4 overflow
├── FINDINGS.md       # results, evidence, and what was learned
└── roms/vt1200/      # MAME ROM dir (symlink to ../../vt1200_v22.bin)
```

## 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, 16, 3>`, i.e. the **address-map unit is 1 bit**
(a 16-bit word spans 16 address units). 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. Board
peripherals (LANCE Ethernet, DUARTs) are **not** modelled; their reads return
`0xffff` and writes are dropped. The terminal will therefore not boot to the
X server — that is expected and not the goal.

## Build

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

```sh
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.)

```sh
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.