0.15.0–0.20.0 — Growing the Arithmetic ISA

This stretch of versions is the most repetitive-looking part of Kasm’s history, and that repetition is the point: once the pattern of “parser rule → semantic check → encoder entry → decoder entry → permanent test” was established, adding an instruction became a mechanical, low-risk process.

The 0.18.1 regression fix is worth reading closely

103547d — Update project version to 0.18.1, add INC and JZ instruction support, and include regression tests for decoding is a bug-fix release inserted into the middle of otherwise linear feature growth. It closed the decoder gap noted in the previous chapter: inc eax (FF C0) and jz (0F 84) had valid encoders but an incomplete decoder, so a valid program would encode correctly, write its outputs, and then have the CLI exit with unknown or truncated encoding while printing the decoded listing. The fix added the missing decoder cases and a permanent end-to-end regression covering both instructions’ byte output and their decoded listing together, exactly the kind of test the 0.14.0 decoder made possible.

Registers beyond eax

Growing from eax-only forms to ecx and edx forms (folded into this era via 6809007 and 1384ac9) introduced the “second byte of the 0x81 family selects the operation and register” pattern documented in the project’s opcode lookup guide — 81 C1 is add ecx, imm32, 81 E9 is sub ecx, imm32. This is the first time Kasm had to disambiguate more than one instruction sharing an opcode prefix, which is also why register codes started being tracked explicitly on statements rather than inferred at encode time.

By 0.20.0, Kasm had a genuinely useful arithmetic and bitwise instruction set for eax. What it didn’t have yet was any explicit acknowledgment that 16-bit and 32-bit targets even existed as distinct concepts — that correction came next.