Two long-deferred Phase C chips, both clean-room from public Intel
datasheets. These complete the support-chip ecosystem needed for
real interrupt-driven 8080/Z80/8086 software on the canvas.
8259 PIC (~280 LOC, single-master subset):
- Full ICW1..ICW4 init sequence with branching on single/cascade
and ICW4-needed flags.
- IRR/ISR/IMR registers; OCW3 read-back; OCW1 mask write.
- Priority-based INT (lower IRQ# = higher priority, fully-nested);
pre-emption when a higher-priority IRQ arrives during a lower-
priority ISR.
- INTA falling-edge → drives vector_base + IRQ# on D bus.
- Non-specific (0x20) and specific (0x60..67) EOI.
- Cascade-master/slave routing NOT implemented (single master is
sufficient for 95% of demos).
- 7/7 tests passing.
8253 PIT (~210 LOC, Modes 0/2/3 subset):
- Three independent 16-bit counters with own CLK/GATE/OUT pins.
- Mode 0 (interrupt on terminal count) for one-shot timers.
- Mode 2 (rate generator) for system tick.
- Mode 3 (square wave, decrements by 2) for PC-speaker tone.
- Modes 1/4/5 coerced to Mode 0 (rare in practice).
- Full RW mode set: LSB-only / MSB-only / LSB-then-MSB / latch.
- GATE-low pauses the countdown.
- 4/4 tests passing.
Tests: total test_intel 99→110 passing (+11). 0 failed. 11 todo.
Master plan doc updated: Phase C extension done; Phase G still
deferred.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
The 4001 is the canonical ROM partner of the 4004/4040. 16-pin DIP,
256 bytes of mask-programmed ROM accessed over the 4-bit multiplexed
nibble bus, plus 4 I/O port lines (WRR/RDR — not yet wired).
Implementation: ~140 LOC clean-room from MCS-4 manual §V. The chip
has its own timer at 1351 ns (matching the 4004's clock period), with
a state machine that walks the 8-phase frame in lockstep with the
4004:
S_IDLE → (SYNC↑) → S_SAMPLE_LOW (A1 nibble) → S_SAMPLE_MID (A2) →
S_SAMPLE_HIGH (A3, addr complete) → S_DRIVE_HI (M1, drive opcode
high nibble) → S_DRIVE_LO (M2, drive low nibble) → S_POST (X1..X3
idle) → wait for next SYNC.
Timing trick: the 4001 must be added to the board BEFORE the 4004
so its tickTimers fires first per advanceNanos. The 4001 then runs
one frame "behind" the 4004 — sampling what the 4004 drove last
frame and driving what the 4004 will read this frame. Documented in
the chip's source and the master plan.
Integration test (`test_buses/4001-rom.test.js`) wires both chips on
the same board and verifies the 4004 actually fetches and executes
opcodes from the 4001 (PC walks 0, 1, 2 with the embedded NOP image).
This is the first end-to-end test of the 4-bit multiplexed bus
working between two real WASM chips on the canvas, not just JS
helpers — proving the bus model scales.
Deferred for the next Phase D iteration: 4002 RAM (similar shape +
SRC chip-select latching), 4004 SRC/WRM/RDM wiring to exchange data
with the 4002, and the Busicom 141-PF integration once both ROM and
RAM chips are real.
Tests: total test_intel 98 → 99 passing, 0 failed, 11 todo.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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