A bug in the KIM-1 monitor. I continued my pages on the KIM Explained and found a bug in the KIM-1 monitor: KIM 0200 A9 ;; 00BB 00 :: 00AA 00 ???? FFFF 1C The KIM-1 accepts ':'– '?', $3A .. $3F as valid hex characters A ..F! The answer lies in the PACK routine, where an incomplete check for a valid hex character is made. Read more about this in one of the many pages in KIM-1 Explained . http://retro.hansotten.nl/6502-sbc/kim-1-manuals-and-software/the-kim-1-monitor-explained/getbyt-and-pack/