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!

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

The KIM-1 monitor software exists of two separate parts. The 6530-002 RRIOT ROM, called KIM as separate IC, RRIOT and the 6530-003 RRIOT ROM.
The two are not written as one program, the 6530-002 routines do not need the 6530-003.
One could speculate the 6530-002 software was developed together with the LED/keyboard display and TTY interface hardware. And when that design was done,
the need for data storage for the user arose and the 6530-003 was added.

The 6530-003 only has audio tape read and write routines and uses the ports of the 6530-002 for the audio bit streams.
The 6530-002 can be used to build a standalone computer, the 6530-003 is an addon for a 6530-002 based system.
Besides sharing RAM locations in the zeropage and the RAM area in the 6530-002 RRIOT the two do not use each others routines. The 6530-003 routines only know the address of the 6530-002 START routine.

Pages to follow:

– architecture
CPU
RAM
RRIOTs
memory decoding
expendability and external access
– memory layout
– KIM-1 startup
– RESET routine
– TTY/KB selection
– the two user interfaces of the KIM-1
– TTY
hardware
routines
CLI
– LED display and keyboard
hardware
routines
CLI
– the KIM-1 file system and namespace
– the audio tape read
– the audio tape write
– missed opportunities in the KIM-1 design
– integration of main CLIs and tape routines
– tape read and write not callable as subroutines

Work in Progress July 6, 2026

Many sources of the KIM-1 monitor are floating around. All typed in from the user manual, most are not identical to the original text. None deliver the real KIM-1 ROMs, they fail with filling unused bytes with FF. And the text and comments are incomplete, wrong or made up.

So in June 2026 I have typed in the source of the KIM-1 monitor as it is listed in the User manual. Every line has the same line number, all text including comments is as in the paper listing. The listing is also paged as the original. Converted to a modern MOS Technology assembler format (I used TAS32), that delivers the exact ROM binary (including those 00 filler bytes!) as dumped from the real 6530-002 and -003 by me.

KIM-1 Monitor source, listing and binaries and TASM32 archive

More on the KIM-1 ROMs here.

MOStermind is an implementation of the Mastermind board game for the KIM-1 computer.
Developed by HKZ.

It’s written in 6502 assembly using DASM assembler and MAME as developing tools.

You can download it here

How to play

The game follows the basic rules of mastermind: the codemaster generates a hidden code and you have to guess it in a limited number of tries, while the codemaster gives you a report on how much you got right for each guess.

The codes are be composed by 4 symbols, each symbol can be one of the following 8 characters: `A`, `B`, `C`, `D`, `E`, `F`, `G`, `H`. Note that a character appearing multiple times in a code is perfectly valid.

After you type in the 4 character code and press `RETURN`, the codemaster will analyze your guess and report the result using 4 symbols:

– `O` indicates that one symbol is both the right character and at the correct spot
– `*` indicates that a symbol is a character that appears in the code, but is in the wrong place
– `.` Indicates that a character is not present at all in the code

Note that these are **NON POSITIONAL**, and they’re just shown in order of precedence: first `O`, then `*`, then `.`. They bear no information on which of the characters is right. You’ll have to find that yourself.

In case you want to cancel a guess before submitting it (to avoid wasting a try), you can press `BACKSPACE`. The guess submission will be aborted, no analysis will be performed and you’ll be able to submit a new guess.

Does it run on the KIM-1 Simulator? YES!

You only have to set the FOCAL_V3D break setting in the Settings.

Again Eduardo Casino has enhanced the KIM-1 Simulator V2.3.0 after adding the K-1103 and Corsham SD-Shield with CODOS and CP/M-65 operating system in V2.0:

Version 2.2 and 2.3:

– The Console has now a complete VT100 emulation with color and the DEC Special Character set
– The console supports cut and paste text with the mouse (very handy to dump a papertape into the KIM-1)
– The K-1008 Visable Memory emulation has become blazing fast
– All settings are saved between sessions

A KIM-3B is a 8K bytes memory expansion made by MOS Technology (already part of Commodore then) for the KIM-4 motherboard.

I recently acquired a good looking KIM-3B myself. Nothing new, all is already known about it. Now with photos made by myself!