The harness forced the VR150 monitor switch, so the firmware wrote the OS
screen at 1024 px / 128-byte rows. VT1200_VRAM_REAL then re-read that
framebuffer at the VR319 pitch, which sheared the OS and wrapped the right
quarter of every row.
Model VR319 (switch 0x60) by default -- the real unit's mode -- so the
firmware itself programs 1280x1024 with a 256-byte row pitch (CONVDP=0x14).
The renderer's pitch follows VT1200_MONITOR, and the destination span now
comes from HEBLNK..HSBLNK instead of x=0, placing the active 1280 px at
x=336 in MAME's 1696-wide raster.
Verified: clean Terminal Manager at frame 600, legible faithful diagnostics
menu and level-1 error screens, tile mode PASS with tile_check 1280/1280
tiles, tablet/mouse pixel-identical. VT1200_MONITOR=VR150 reproduces the
old geometry; the regression baseline moves 198 -> 199 (2 PCs) and is
recorded in harness/mame/README.md.
The 1987 TI K&R toolchain already compiled and linked a fib test, but nothing
had ever been executed on a VT1200. This is that program: hello.c draws HELLO
into video RAM (1 bpp, bit 0x04000000, no X11) and is loaded through the COFF
loader with hostload.py, on the real unit and in MAME.
Two runtime traps shaped the design, both recorded in the README:
* boot.obj's exit() is `for(;;)` and _c_int00 ends in `reti`. That is a
standalone-ROM runtime, not a process: the child spun forever, and because
the VT1200 OS is cooperative it took the whole terminal down (illegal
opcode on hardware). entry.asm replaces boot.obj -- set STK/FP, call
_main, `rets` back to the create_process trampoline, the same return
xhello uses -- and the image carries no .cinit (tables are built in code).
* Pointers are bit addresses and pointer arithmetic scales by object size,
so p + n on a char * is n bytes. The pre-fix frame in evidence/ is that
bug: 32 one-byte bars spread across a row.
Framebuffer geometry is not portable: the real unit is 1280 px wide at a
256-byte row pitch, MAME's canvas is 1024 px at 128. build.sh builds both
(ROW_BYTES), and the README records how the real pitch was measured from a
real-unit frame rather than taken from MAME.
Evidence: real-unit frames in harness/c-hello/evidence/ (the pre-fix stripes
and the working HELLO), and dwt/loader/evidence/real-unit-xhello/ for the X
window that ran on the unit earlier.
arti