On the keyboard the GO key is used to start the program (line 138, via a JMP to GOEXEC), on the TTY CLI the G command handler at line 292.

0072 00EF PCL .BLOCK 1 ; PROGRAM CNT LOW
0073 00F0 PCH .BLOCK 1 ; PROGRAM CNT HI
0074 00F1 PREG .BLOCK 1 ; CURRENT STATUS REG
0075 00F2 SPUSER .BLOCK 1 ; CURRENT STACK POINTER
0076 00F3 ACC .BLOCK 1 ; ACCUMULATOR
0077 00F4 YREG .BLOCK 1 ; Y INDEX
0078 00F5 XREG .BLOCK 1 ; X INDEX

GOEXEC LDX SPUSER At 1Dc8, LINE 841
TXS
LDA POINTH ; PROGRAM RUNS FROM
PHA ; OPEN CELL ADDRESS
LDA POINTL
PHA
LDA PREG
PHA
LDX XREG ; RESTORE REGS
LDY YREG
LDA ACC
RTI

Program counter, Stack pointer SP, Process register PS, X, Y, ACC are initialized from the zeropage locations and the RTI transfers the CPU execution.
The RTI (return from Interrupt) restores all registers from the stack, including the Program counter.
The user program starts at the current selected address (in POINTL, POINTH).

While this way of starting a program has the advantage that the program may start with user supplied values for the CPU registers, there is no guarantee that these zeropage locations contain meningfull values. In fact, after a RESET the contents of RAM and therefore these start values are random. That can create a malfunction program if it does not itself initialize registers.
And not all do, like Microsoft Basic: the decimal flag is not cleared via CLD. And the program crashes if by chance the CPU is in decimal mode.

So before using the G command it is good practice to clear location 00F1, the Processor Status register to clear the decimal flag and 00F2, the stack pointer to FF.
A well behaving program should initialize stack and do a CLD just to be sure.

Returning to the KIM monitor can be done with a JMP START (1C4F).

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, please 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, the 6530 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:

• Execute a program: GOEXEC
• GETCH read a character from TTY
• Startup of the KIM-1: RESET
• Sigma, KB/TTY selection
• Suppress the KIM-1 echo
• Load papertape format

– 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

RST     LDX   #$FF       ; KIM ENTRY VIA RST at $1C22
        TXS   
        STX   SPUSER
        JSR   INITS
  
        subroutine INITS and INIT1 at 1E88

       INITS   LDX   #$01       ; SET KB MODE TO ADDR
               STX   MODE        
       ;
       ; the Port A and B data direction registers are setup here
       ; Port a is all input
       ; Port B PB0-PB5 is output
       ; PB1 and PB2 are 1, other PBx 0
       ;
       INIT1   LDX   #$00        
               STX   PADD       ; FOR SIGMA USE SAD
               LDX   #$3F        
               STX   PBDD       ; FOR SIGMA USE SBDD
               LDX   #$07       ; ENABLE DATA IN    
	       STX   SBD        ; OUTPUT
               CLD
               SEI
               RTS

here code to determine baudrate, see the TTY section

START   JSR   INIT1                              
        LDA   #$01
        BIT   SAD
        BNE   TTYKB

Read here how the TTY/LED selection is made with an external switch
[/code]
fall into TTY interface

TTYKB do the LED display and keyboard scanning

[/code]

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, 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.

Work in Progress July 6, 2026

Problems with the KIM-1 TTY character input

1. The KIM-1 hardware is hardware echoing incoming serial characters to the output, no echo in software involved, so you cannot influence what appears on screen. Very annoying!
2. The KIM-1 GETCH routine is blocking, no way to check for a character coming in, like a Break. waiting.
   Also quite annoying if porting other software to the KIM-1 or you want the program interruptable.
3. While a program is running something CPU intensive and you type something the program is not really waiting for, the characters appear on screen. Because the KIM-1 does hardware echoing of TTY input, this is unavoidable it seems

Here I present solutions for these problems in software, made possible by the genius hardware design of the KIM-1 TTY I/O.
You can have serial input wihout echo, non-blocking and even make the TTY input deaf for unwanted input.

Are they perfect? Maybe not, it is still bitbanging the incoming serial signal. It can miss the correct starting point for the incoming character bit stream.
If you want a perfect solution, you will need interrupt driven ringbuffered serial I/O with a dedicated IC like the 6850, 6511 etc.
Without this extra hardware you can achieve acceptable results with these routines.

Settings 2.3.1

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.

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.

I ordered one and indeed a 40 pin IC appeared. So this is not a 6503!
Then I searched the internet for the 338-6503 printed on the IC and found references to it being a R65C02.
And indeed, the 6502 testers recognized it as working 65C02. Weird!
The ‘6503’ was found in Apple IIe and Apple IIc computers.

Rockwell R6500/1 family

The R6500/1 is a 6502 , ROM (2K) 64 bytes RAM ,4×8 bit I/O, a 16 bit timer with I/O pin.
The ROM contents are factory mask-programmed, not programmable by the user.

This makes it a standalone microcomputer, no extra RAM/ROM or decode logic required.

There are no external address lines or data lines, the 6502 is all internal except the interrupt lines and clock.

Rockwell R6500/1EC

To develop applications with the R6500/1 Rockwell developed the R6500/1EC. This IC has the same pins as the R6500/1 but adds address and data lines and relies on an external ROM.

Downloads

	R6500/1 One-Chip Microcomputer
	R6500/1E Microprocessor Emulator Device
	R6500/1 Microprocessor Programming Reference Card

Rockwell R6500/1 family

The R6500/1 is a 6502 , ROM (2K) 64 bytes RAM ,4×8 bit I/O, a 16 bit timer with I/O pin.
The ROM contents are factory mask-programmed, not programmable by the user.

This makes it a standalone microcomputer, no extra RAM/ROM or decode logic required.

There are no external address lines or data lines, the 6502 is all internal except the interrupt lines and clock.

Rockwell R6500/1EC

To develop applications with the R6500/1 Rockwell developed the R6500/1EC. This IC has the same pins as the R6500/1 but adds address and data lines and relies on an external ROM.

Downloads

	R6500/1 One-Chip Microcomputer
	R6500/1E Microprocessor Emulator Device
	R6500/1 Microprocessor Programming Reference Card

The 6500/1 has been used for example in the AMIGA 1520 Plotter. I found this image on ebay: