FA-89344 / Instruction set emulation / Member archive

SBC treats the carry flag as a borrow flag · case 04

Subtraction with carry set comes out one too small and with carry clear one too large.

Member previewVariant 4 · 3 implementations · 9 checks per implementation

Case contract

Input [a, c, ops] for an 8-bit accumulator with carry c; V starts 0. adc m: s = a+m+c, A = s & 0xFF, C = s > 0xFF, V = signed overflow of the 8-bit add. sbc m is adc of (m xor 0xFF) with the same carry (carry set means no borrow). cmp m: C = a >= m (unsigned), N/Z from (a-m) & 0xFF, A and V unchanged. After each op report [A, N, V, Z, C] where N is bit 7 and Z is result == 0.

Why this case matters

CPU emulators live or die on flag semantics; carry-as-inverted-borrow and overflow are the classic 8-bit emulation bugs.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
sbc with carry set (no borrow)	`[[31, 0, 0, 0, 1], [110, 0, 0, 0, 0], [243, 1, 0, 0, 0]]`	`[[32, 0, 0, 0, 1], [112, 0, 0, 0, 0], [244, 1, 0, 0, 0]]`	Failed

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