FA-89590 / Instruction set emulation / Member archive

Return from interrupt pops pc before flags · case 05

rti restores the saved flag into pc and the saved pc into I.

Member previewVariant 5 · 3 implementations · 7 checks per implementation

Case contract

Input [pc, i, steps] with steps [irq, nmi, instr]. Each step: an NMI 0->1 edge (the line starts low) is taken first; otherwise a high IRQ line is taken when the I flag is 0; otherwise the instruction runs (nop/cli/sei advance pc by 1, rti pops I then pc if the stack is non-empty). Taking an interrupt pushes pc then I, sets I=1 and jumps to 0x9000 (NMI) or 0x8000 (IRQ). The NMI edge detector samples the line every step. Return [event, pc, I] per step.

Why this case matters

Interrupt controllers in CPU emulators must separate edge- and level-triggered sources, masking and nesting to reproduce handler timing.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
masked irq waits for cli	`[["nop", 257, 1], ["cli", 258, 0], ["irq", 32768, 1], ["rti", 0, 258], ["nop", 1, 258], ["nop", 2, 258], ["nop", 3, 258], ["nop", 4, 258], ["nop", 5, 258]]`	`[["nop", 257, 1], ["cli", 258, 0], ["irq", 32768, 1], ["rti", 258, 0], ["nop", 259, 0], ["nop", 260, 0], ["nop", 261, 0], ["nop", 262, 0], ["nop", 263, 0]]`	Failed

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