DOS sample

A tiny 16-bit program for testing GameFrame's DOS tools. It is our own code: no game is involved.

• sample.s: routines in the style of a Borland Pascal program (FAR, Pascal calling convention, DS = data segment): a Turbo-Pascal-style random generator, a loop that draws numbers, a checksum over a far pointer, a selector fixup, an imported call (KERNEL.3) and a DOS call (INT 21h), and main_loop: eight frames of a tiny main loop for tools/emu/fragments.py (DPMI blocks, the VGA display, a file read each frame).
• make_exe.py OUT: assembles it with zig cc (LLVM, .code16) and links it with Python into sample.exe (NE, 16-bit protected mode), sample-td.exe (the same with Turbo Debugger symbols appended) and sample-mz.exe (real-mode MZ).

tests/run.sh builds it and checks dosinfo, readseg, tdinfo, dos_emu, Ghidra and DOSBox-X against it. The first random draw from the built-in seed 0x1234 with range 100 is 23: the emulator, the program's exit code under DOSBox-X and the debugger channel must all agree.

Pitfall found while writing it: LLVM's Intel syntax assembles retf 2 in .code16 as a 32-bit return (66 CA 02 00). The source uses AT&T syntax (lretw $2).