FA-89440 / Instruction set emulation / Member archive

Absolute Y-indexed mode adds the X register · case 05

abs,Y accesses the address offset by X, so table lookups indexed by Y read the wrong element.

Member previewVariant 5 · 3 implementations · 13 checks per implementation

Case contract

Input [mode, operand, x, y, mem] with mem mapping decimal-string addresses to bytes (missing = 0). zp: operand & 0xFF. zpx: (operand + x) & 0xFF. abs: operand. absx: (operand + x) & 0xFFFF. absy: (operand + y) & 0xFFFF. ind: little-endian pointer at operand, where the high byte is fetched from the same page ((operand & 0xFF00) | ((operand + 1) & 0xFF)). indx: pointer at zero-page (operand + x) & 0xFF, high byte from the next zero-page byte (wrapping). indy: pointer at zero-page operand (high byte wrapping in zero page), plus y, wrapped to 16 bits.

Why this case matters

Addressing-mode decoding is the core of any 8-bit CPU emulator; page-wrap quirks are observable program behaviour.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
absolute Y with X set	`8214`	`8197`	Failed

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