Build your own KIM-1

Build your own KIM-1, Commodores first computer.


  • All names with a copyright are acknowledged.
  • If the reader uses information from this document to write software or build hardware, then it is on his own account. I cannot be held responsible for blowing up your computer, mother-in-law or whatever; it will always be your own fault.
  • I'm not a sexist, with 'he' and 'him' I also mean 'she' and 'her'.


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What is the KIM-1?
If you don't know that by now then read this document about the KIM-1 first.

The trouble-shooter: 6530
Anybody who is a little bit familiar with the hardware market can tell you that you cannot buy the 6530 anymore. Happily enough there is another IC available which you could call its brother: the 6532.
The 6532 has 16 I/O-lines, an internal timer and 128 bytes of RAM onboard. And no ROM. But we (should) know by now that the internal ROM of the 6530 could be selected unindependently from the I/O. So for this project we'll use an external EPROM as replacement. The pinout of the 6532 is completely different but that should not be a problem. The next difference is the fact that the 6532 has a separate IRQ and PB7 line. As we will see, the functionality of both lines is the same as with the 6530. To create the same circumstances we only have to connect them together. The third difference is the availability of PB6 with a 6532. See it as a bonus as I haven't found any reason how it could jeopardise our project. The fourth difference is that it is possible to generate an interrupt depending the behaviour of PA7. But this is an option, which is out of function by default after a reset.
The last and major difference however lays in the way the registers are selected:

function:       RS:  A6:  A5:  A4:  A3:  A2:  A1:  A0:  R/W: 
RAM              0    x    x    x    x    x    x    x    x   
DRA              1    x    x    x    x    0    0    0    x     A
DDRA             1    x    x    x    x    0    0    1    x     B
DRB              1    x    x    x    x    0    1    0    x     C
DDRB             1    x    x    x    x    0    1    1    x     D
PA7, IRQ off,                                               
      neg edge   1    x    x    0    x    1    0    0    0     F
PA7, IRQ off,                                               
      pos edge   1    x    x    0    x    1    0    1    0     G
PA7, IRQ on,                                                
      neg edge   1    x    x    0    x    1    1    0    0     H
PA7, IRQ on,                                                
      pos edge   1    x    x    0    x    1    1    1    0     I
read interrupt                                              
       flag      1    x    x    x    x    1    x    1    1     E
read timer,                                                 
       IRQ off   1    x    x    x    0    1    x    0    1     J
read timer,                                                 
       IRQ on    1    x    x    x    1    1    x    0    1     K
Clock / 1,                                                  
       IRQ off   1    x    x    1    0    1    0    0    0     L
Clock / 8,                                                  
       IRQ off   1    x    x    1    0    1    0    1    0     M
Clock / 64,                                                 
       IRQ off   1    x    x    1    0    1    1    0    0     N
Clock / 1024,                                               
       IRQ off   1    x    x    1    0    1    1    1    0     O
Clock / 1,                                                  
       IRQ on    1    x    x    1    1    1    0    0    0     P
Clock / 8,                                                  
       IRQ on    1    x    x    1    1    1    0    1    0     R
Clock / 64,                                                 
       IRQ on    1    x    x    1    1    1    1    0    0     S
Clock / 1024,                                               
       IRQ on    1    x    x    1    1    1    1    1    0     T
In total 5 addresslines are used, meaning 32 registers. But 11 of the 19 registers have one or more mirrors.
Read:          J E J E         K E K E         J E J E         K E K E 
Write:         F G H I         F G H I         L M N O         P R S T 
R/W:   A B C D         A B C D         A B C D         A B C D         
As we can see, the last 16 registers equal the 16 of the 6530 itself. So now we have to develop some logic which will do the following:
  • The 6532 is only visible within a range of 128 bytes
  • The first 16 bytes represent register 16 to 31
  • The next 48 bytes are mirrors of the first 16
  • The last 64 bytes appear as RAM
  • Input A6 won't be used and can be tied to GND
  • Input A4 is connected to addressline A4 of the 6502 via an inverter.
  • An 74LS138/74LS08 construction or equivalent enables the RS- and CS-lines at the right moment.

Here we have a luxury problem. We only need 2K of (EP)ROM like the 2716. The problem is that the 2716 is hard to find and more expansive then the 2764 or its bigger brothers. When we use a bigger EPROM we only have to tie the unused addresslines to GND. The same problem occurs with the RAM.
If we have to use bigger RAMs or EPROM's anyway, it is quite easy to use other parts of that chip by OR-wiring the CS-line with more Kx-outputs of the main 74145. (Don't forget the addresslines!) In case of the EPROM we also can tie switches to the surplus addresslines and have the advantage of a multi-KERNAL system.

Source codes of the ROM
Available at this page. Should be 100% OK.

Schematics of the new KIM-1
Available here as a GIF-file.
What are the major differences with the original SCH?

  • Replacement of the 6530s by 6532s.
  • Replacement of the 6108 RAM-ICs by one 6264 or equivalent 8K*8 SRAM.
  • Adding an EPROM.
  • Adding a 74LS138 to decode the RAM and I/O of the 6532s.
  • Combining K6 and K7 to one line, dropping the resistor for K6.
  • Adding jumpers to enable combining other K-lines as well.
I didn't change things, which are more or less obvious like replacing the clockcircuit by a module or replacing all different TTL-ICs by LS or HCT types.

You can email me here.