REFORGE
Atari 2600 / F262
BEING VERIFIED

ZERO-PAGE RMW WRITES THE OLD BYTE BEFORE THE MODIFIED BYTE

STATUS
BEING VERIFIED
AREA
Memory / cartridge
RECORDED
2026-07-16
PUBLIC SOURCE
Research dossier / finding 262
RECORD ID
atari2600-f262
SNAPSHOT
f114355884d03a2c

MOS manual section 5.6 fixes zero-page address formation: opcode at PC, address operand at PC+1, then the effective $00,ADL memory access. Pinned Stella revision 90d01ecb309b9c3317ca762078d4df40681e111b makes the remaining NMOS bus behavior explicit for ASL/ROL/LSR/ROR/DEC/INC and stable SLO/RLA/SRE/RRA/DCP/ ISC: read the operand, write the same unmodified value back, perform the operation, then write the modified value.

Raw traces, ROM material, local paths, and personal data are not part of this public record.