Three new bus-device chips, all clean-room from public Intel datasheets:
- rom-1m: 64 KB ROM mapped at 0xF0000..0xFFFFF for 8086 boot. 20-bit
address bus watch with out-of-range tristate. 16-byte known signature
pre-loaded at the reset vector 0xFFFF0.
- 8255 PPI: Mode-0 (basic I/O) only; three 8-bit ports with split
upper/lower port C. Control word parsing for direction setup. Bit
set/reset and Modes 1/2 deferred.
- 8251 USART: async-mode UART using vx_uart_attach for bit-timing;
mode word + command word + status interface; TxRDY/RxRDY/TxEMPTY
status pins; DTR/RTS pass-through. Internal-reset honoured.
Deferred to a follow-up Phase C+:
- 4001 ROM and 4002 RAM (multi-phase 4-bit bus timing requires either
an external clock-gen chip or Bus4004-equivalent host coordination
that's only available in the JS test harness today).
- 8253 PIT (6 modes, countdown logic).
- 8259 PIC (ICW init state machine + cascade + INTA cycle).
These are flagged in autosearch/18_complete_emulation_plan.md as
deferred — Phase D (4004/4040 I/O) and the 8259 work depend on them
landing first.
Tests: test_buses 17 → 30 passing (+13). Total test_intel 73 → 86
passing, 0 failed, 17 todo. Master plan doc updated.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Companion chip for 8086 minimum-mode boards that demultiplexes
AD0..AD15 → A0..A15 under control of ALE. ~80 LOC clean-room from
the public Intel 8282/8283 datasheet.
Pin contract (20-pin DIP): DI0..7 in, DO0..7 out, STB strobe, OE̅
output enable, VCC, GND. Behaviour:
STB=1, OE̅=0 → DO follows DI (transparent)
STB falling → latch held while STB=0
OE̅=1 → DO pins released (modelled as VX_INPUT)
Tests: 4/4 passing (pin contract, transparent mode, latch hold,
output enable). Brings test_intel total to 47 passing.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
David Montero