First test wiring the 8086 CPU to a real 8259 PIC chip on the same
board and proving hardware-interrupt routing works end-to-end:
IRQ0 input → PIC asserts INT → CPU's INTR pin → CPU runs INTA
cycle → PIC drives vector 0x40 on AD bus → CPU does do_int(0x40)
→ fetches CS:IP from IVT entry at 0x100 → ISR runs → IRET → main
resumes from HLT.
Two related chip fixes required to make this work:
1. 8086 INTA cycle no longer drives AD itself.
Real 8086 INTA bus cycle has the PIC drive the data lines, not
the CPU. My earlier code did `bus_read_byte(0, false)` which
first drove AD with addr=0, overwriting whatever the PIC had
driven. Fix: release_ad → INTA̅ low → sample AD (PIC's INTA
watcher fires synchronously and drives) → INTA̅ high.
2. 8086 HLT now interruptible.
on_clock previously early-returned on G.halted, so step()
never ran and the INTR check never executed. Real 8086 HLT
wakes on INTR/NMI. Fix: remove the early return; step()'s
own halted check (later in the function) only no-ops if no
pending interrupt.
Tests: total test_intel 110 → 111 passing (+1, the integration
test). 0 failed. 11 todo. test_8086 now 11→12 passing.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Root cause of the deferred CALL/RET test: my chip was missing the
0xC6 / 0xC7 (Group 11 — MOV r/m, imm) opcodes. Bytes like the
test's "MOV byte [0x8002], 0x55" (0xC6 0x06 0x02 0x80 0x55) fell
through to the default NOP, then the chip decoded the residual
0x06 0x02 0x80 0x55 as PUSH ES + ADD r/m + ... taking SP into
unpredictable territory.
Implementation:
- 0xC6 (8-bit) and 0xC7 (16-bit) variants added.
- The encoding is opcode + modrm + disp + imm. Critically the disp
bytes (consumed by calc_ea) come BEFORE the immediate, so we
compute EA first, then fetch imm. Earlier draft had imm fetched
before disp — that had imm winning the disp slot and disp becoming
the next instruction's bytes. Caught only after wiring CALL+RET.
Tests: 8086 10→11 passing. Total test_intel: 93→94 passing,
0 failed, 11 todo. CALL/RET integration test now active and green.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
David Montero