SDD
f49f9984bb
Closed cross-check (single run): the catch-all and VT1200_CAL proxies are directly comparable (both base 0) and resolve identically; last-wins holds for same- and mismatched-width overlaps. The apparent conflict was a units error: a .rw(FUNC(offs_t)) handler receives (bitaddr-base)>>4 (tms34010 Width=1, AddrShift=3), so the catch-all logs bitaddr>>4. Its 0x400000..0x40D7FF is bit 0x04000000..0x040D7FF0 (bytes 0x800000..0x81AFFE), outside the declared 4 MiB RAM -- genuinely unmapped. The old 0x00400000 proxy was 16x too low. Catch-all reproduces the committed log byte-for-byte and does not perturb coverage (56 / PC=3F208950 with and without). Regression holds: PC=3FDCE400 samples=199. Report: harness/mame/XCROSS_REPORT.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
├── 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 offset0x1bfffc).
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.
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.