* The ROR instruction didn't exist in the very earliest (pre-1977) chips. See: [https://www.pagetable.com/?p=406 Measuring the ROR Bug in the Early MOS 6502]
* The 6502 core used inside the [[NES]] is missing the Decimal Mode feature. [https://archive.org/details/nes-programmers-reference-guide-by-electronic-arts-1989/ NES programmer's reference guide] [https://www.nesdev.org/NESDoc.pdf NESDoc] [https://www.nesdev.org/wiki/Mapper NES mappers] [https://problemkaputt.de/everynes.htm Noca$h's Everynes] [https://www.nesdev.org/wiki/Emulator_tests NES emulator tests] [https://tcrf.net/Category:Nintendo_Console_Testing_Software Official Nintendo testing software]
* The 6507 CPU, used in the [[Atari VCS]], has only 13 address lines. So it can only address 8KB instead of 64KB. It also lacks the IRQ and NMI interrupt lines. [https://youtu.be/qvpwf50a48E Atari VCS: The Ultimate Talk] [https://cdn.hackaday.io/files/1646277043401568/stella.pdf Stella programmer's guide] [https://www.atarimania.com/documents/stella_system_training_manual.pdf Stella system training manual] [https://problemkaputt.de/2k6specs.htm Noca$h's 2k6specs]