December 5, 2025
Press X to Hex
Extra Instructions of the 65XX Series CPU
Hidden 6502 ‘cheat codes’ are back—retro fans cheer, purists clutch pearls
TLDR: A revived 1990s list exposes hidden 6502 chip instructions once used as clever tricks on vintage machines. Commenters hype their role in copy protection and share a guide, while others warn they’re unpredictable—great for retro tinkering, risky for serious work, and a reminder of old-school ingenuity.
Adam Vardy’s resurrected 1995–96 guide to the 6502’s secret, “illegal” opcodes reads like a treasure map for old chips—the kind inside icons like the Commodore 64. These are the hidden combo moves: do one thing, get two results. The community lit up. rossjudson swears these tricks were weaponized for copy protection, and even laughs about “mystery page boundary overflows” (translation: when data crosses a memory line and weirdness ensues). Veterans call them the CPU’s cheat codes; newbies ask, “Wait, is this safe?”
Then comes the how‑to crowd: ruk_booze drops a hands-on walkthrough, No More Secrets, and the thread turns into a retro cooking show—mix two opcodes, bake eight cycles, serve hot. Strongest opinion: undocumented doesn’t mean useless; it means powerful, but unpredictable. Cue the drama: nostalgia vs caution. Some are grinning at the clever hacks, others clutch manuals and warn these opcodes can break your game at the worst moment. Jokes fly—“press X to hex,” “the chip had secret superpowers”—and a few memes crown these opcodes as the original boss fights. Whether you’re here for the lore or the loopholes, the crowd agrees: cracking open these hidden moves is equal parts brain candy and controlled chaos.
Key Points
- •Out of 256 possible 6502 one-byte opcodes, 151 are official; the article catalogs the remaining 105 undocumented opcodes for the 65XX/85XX series.
- •The document provides hexadecimal opcode values, addressing mode notation (ab, cd), and execution cycle counts alongside examples.
- •Many undocumented opcodes combine two standard 6502 instructions; readers should consult 6502 documentation for flag effects.
- •Detailed entries cover ASO/SLO, RLA, LSE/SRE, and RRA, each with supported addressing modes and equivalent sequences using standard instructions.
- •AXS/SAX ANDs A and X and stores the result to memory without affecting flags; the LAX opcode is introduced as loading both A and X.