velxio/test/test_intel/autosearch/16_8086_reference_implementations.md

8086 Reference Implementations (permissive only)

GPL/LGPL implementations (DOSBox, MAME, QEMU, Fake86) are excluded per the project's permissive-only license policy. The three references below are all MIT or BSD/Apache.

1. 8086tiny — Adrian Cable

• URL: https://github.com/adriancable/8086tiny
• License: MIT (verified from header of 8086tiny.c revision 1.25: "Copyright 2013-14, Adrian Cable… This work is licensed under the MIT License. See included LICENSE.TXT.").
• LOC: ~600 lines of single-file C (the README's "fully commented source under 25 KB" claim).
• Test ROM coverage: ships with custom XT-compatible BIOS (bios_source/bios.asm) and a FreeDOS-class disk image; documented to run DOS 3.3, MS-DOS, Windows 3.0, AutoCAD, MS Flight Simulator, GW-BASIC, Alley Cat. No formal per-instruction test suite — correctness validated empirically by software-runs-or-doesn't.

2. MartyPC — Daniel Balsom

• URL: https://github.com/dbalsom/martypc
• License: MIT (verified, LICENSE reads "Copyright 2022-2025 Daniel Balsom — Permission is hereby granted, free of charge…" — standard MIT).
• LOC: not declared on the project page; the repo is multi-crate Rust, on the order of tens of thousands of lines (estimate, not verified — flag as uncertain).
• Test ROM coverage: the MartyPC CPU achieves 99.9997 % cycle accuracy against the SingleStepTests 8088 V2 suite (https://github.com/SingleStepTests/8088: 10 000 tests per opcode, hardware-recorded on a real AMD D8088, MIT-licensed, also covers prefetch-queue state). Validated by physical cycle-by-cycle comparison against a real 8088 driven by an Arduino Mega (https://github.com/dbalsom/arduino_8088). All NEC V20 native instructions also tested vs. hardware. Runs the historically demanding 8088MPH and Area 5150 demos.

3. YJDoc2/8086-Emulator — Yashodhan Joshi

• URL: https://github.com/YJDoc2/8086-Emulator
• License: Dual Apache-2.0 / MIT (the Rust-ecosystem default).
• LOC: small (a few thousand LOC of Rust); a Rust/web interpreter rather than a system emulator.
• Test ROM coverage: none. README explicitly disclaims "does not allow jumps to memory positions, does not support ISRs, no external devices." Useful for instruction-decoder cross-checking only — not a fidelity reference.

Cross-checked edge cases

The three implementations agree on the easy stuff. Disagreements worth recording for Velxio:

• AF on ADD/SUB: All three set AF = 1 iff carry out of bit 3 (add) or borrow into bit 3 (sub). Matches manual p.2-35 (PDF p.57).
• DAA pseudocode: 8086tiny and MartyPC both implement the manual's algorithm verbatim (PDF p.58, see 15_8086_authoritative_spec.md §9). OF is left unchanged by 8086tiny (pragmatic choice) but written unconditionally to a defined value by MartyPC matching the SingleStepTests reference recordings (so OF is "undefined per Intel" but actually deterministic in silicon — the test suite captures the silicon state).
• MUL/DIV flag undefinedness: Manual p.2-51 (PDF p.73) marks SF/ZF/AF/PF undefined and OF/CF defined for MUL (= 1 if upper-half nonzero). MartyPC matches the silicon's actual values (which are NOT random — SF mirrors bit 15/31 of the result, ZF reflects the full result on real hardware according to SingleStepTests recordings). 8086tiny treats them as truly-don't-care and leaves them at the previous values. This is a visible behavioral split — Velxio should follow MartyPC/SingleStepTests for accuracy.
• DIV by zero / DIV overflow: All three correctly raise INT 0; the saved-IP value points at the next instruction (post-DIV), per manual p.2-25 (PDF p.47). Some early documentation suggested IP points at the DIV itself; this is a documented errata corrected in the 1981 manual.
• REP + string-op exact sequence: All three follow the algorithm in 15_8086_authoritative_spec.md §10. SingleStepTests covers REP with CX = 0 (immediate fall-through), CX = 1, and mid-iteration interrupt with both LOCK and segment-override prefixes — the latter is where 8086tiny is known to drop the segment-override on resume (matching the manual's documented quirk on PDF p.64).
• Undocumented opcodes: 0x0F (POP CS — works on 8086 only, hangs the 8088), 0x60-0x6F aliases, 0xC0-0xC1 aliases of 0xC2-0xC3, 0xD6 (SALC), 0xF1 (alias of INT 1). MartyPC handles all per silicon; 8086tiny handles the common ones (POP CS, SALC). YJDoc2 handles none.