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arti
41b0979aef
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.
27 lines
893 B
NASM
27 lines
893 B
NASM
; entry.asm -- process entry for the C hello world.
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;
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; The C runtime's boot.obj is designed for a ROM system: _c_int00 sets SP (the
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; *system* stack), calls main, calls exit (which is `for(;;)`) and ends in
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; `reti`. None of that fits a process made by the VT1200's create_process:
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; the spin never yields, and `reti` returns from an interrupt that never
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; happened (it pops a garbage PC -> illegal opcode).
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;
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; So we enter here instead: set the C program stack (STK = A14, FP = A13), call
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; main, and `rets` back to the create_process trampoline -- the same return the
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; xhello payload uses. SP (the OS's interrupt stack) is left alone.
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;
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; _cstack is 4096 bytes; the compiler grows the program stack upward from STK.
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.text
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.globl _start
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.globl _main
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_start:
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setf 32, 0, 1
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movi _cstack, a14
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move a14, a13
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calla _main
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rets
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.bss _cstack, 32768, 16 ; 32768 bits = 4096 bytes
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.end
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