0.26.0–0.30.0 — The 16-Bit Adventures

Five versions turned --bits 16 from a rejected mode (0.22.0) into a mode with a real, if intentionally narrow, instruction subset.

0.26.0 — MOV/ADD/SUB on AX, CX, DX

d3a6cac — Bump version to 0.26.0; add initial 16-bit MOV/ADD/SUB support introduced operand-width as a property tracked on each statement, so layout and encoding could agree on instruction length without guessing from the target mode alone:

Validation here was unusually thorough for a “0.x-in-development” milestone: all 32 registered tests passed under WSL2/GCC in addition to Windows, and the GCC build caught a missing <string.h> include that MSVC had silently tolerated — a concrete payoff for keeping a second toolchain in CI since the project’s earliest days.

0.27.0 — One memory operand, deliberately narrow

0fcd4a2 — Bump version to 0.27.0; add 16-bit indirect MOV support from [bx] to AX/CX/DX added exactly one addressing form: mov ax, [bx]; → 8B 07. The README is explicit that this is intentionally limited to [bx] — no other memory operand is supported. Scoping a feature to the single simplest case, with a name that makes the limitation obvious, is a recurring way Kasm avoids implying more capability than it has.

0.28.0 — A boot-sector-shaped fixture

19b5d52 — Bump project version to 0.28.0 combined 16-bit instructions, location-aware TIMES padding, and binary literals into a 512-byte image fixture ending in the 55 AA boot signature. The README is careful to call this a layout fixture, not a bootable program — there’s no defined entry convention, segment state, stack, or BIOS-service usage yet. It looks like a boot sector and is exactly the right size, but running it in a real (or emulated) BIOS boot path is future work.

0.29.0 — Segment registers

d314108 — Update project version to 0.29.0; add support for segment- register push/pop added push/pop for es, cs, ss, ds, fs, and gs. The legacy four use single-byte opcodes; fs/gs need the 0F two-byte escape. pop cs is deliberately not encoded — there is no valid modern x86 form for it, so the omission is a correctness decision, not a gap.

0.30.0 — Subtract with borrow

sbb joined the 16-bit immediate forms for ax/cx/dx, using the same 81 /3 ModR/M family as the earlier sub support. The README notes plainly that sbb consumes the processor carry flag as part of its semantics, but Kasm neither executes code nor otherwise establishes that flag — the encoding honors the contract; nothing in Kasm currently fulfills it.

Across this whole arc, the 16-bit instruction set stayed deliberately narrow — a handful of registers, one memory form, no general addressing modes — while still being real enough to produce a byte-exact, test-covered boot-sector-shaped image. That restraint is a design choice as much as anything encoded in opcodes.