The KIM-1 explained

Hans Otten

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Jul 13, 2026, 8:37:58 AMJul 13

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The KIM-1 explained

The hardware and the software in the KIM-1 work closely together. The tiny program, less than 2KB in the two ROMs, together with the simple hardware is very clever designed.

The KIM-1 is a complete computer with two user interfaces and data storage with a simple namespace. It is one of the first 6502 computers, and many clones or derived 6502 SBC systems are designed with more or less KIM-1 copied parts of the software and hardware. On this site you can find many examples!

http://retro.hansotten.nl/6502-sbc/kim-1-manuals-and-software/the-kim-1-monitor-explained/

On these pages I will try to explain how all this is working together. It will not be a rewrite of the KIM-1 user manual, , but more a deeper personal dive into the software and hardware of what makes the KIM-1 tick. It also expects a basic knowledge of the 6502 and digital electronics.

Based upon my experience and study of the KIM-1 since 1979. The first years to learn the 6502 CPU and the 6530 RRIOTs. The next time when I wrote the KIM-1 Simulator (2015 to now). Many notes on paper and files everywhere. Now consolidated in a lot of pages.

Work in Progress July 6-13, 2026

Adolfo Fernandez

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Jul 20, 2026, 5:27:56 AMJul 20

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Your site is amazing, Hans. So instructive and detailed. Great work

Hans Otten

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Jul 23, 2026, 8:41:08 AMJul 23

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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 and incomplete check for a valid hex character is made. Read more about this in one of the many pages in KIM-1 Explained .