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Super Simple CPU - Logisim

This is a super simple CPU design implemented in Logisim-evolution for study.

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This is a super simple CPU design implemented in Logisim-evolution for study.
This was inspired by https://hackaday.io/project/191290-the-simplest-4-bit-risc-cpu and the CHUMP design https://github.com/kara-abdelaziz/CHUMP-processor/blob/master/A%20Simple%20and%20Affordable%20TTL%20Processor%20for%20the%20Classroom.pdf

Just like other projects of this type, I think this is a great simple starting point to understand a CPU. It is one step simpler than the CHUMP design for example. Though not really practical for any real use, but shows the basic parts. If this were built using TTL parts the ROM could be a simple diode array, perhaps in a socket board so the programs could be changed easily by moving the diodes around. 

Read Simplest 4 bit RISC CPU for details on the instructions, just note, my OPCODEs are slightly different and are listed on the Logisim design  at the top of the page.

cylon.hex

Hex file program to load into ROM

x-hex - 76.00 bytes - 10/30/2023 at 18:16

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simple_CPU_1.circ

The Logisim design

circ - 14.68 kB - 10/30/2023 at 18:15

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  • 1
    To run the Super Simple CPU

    Just install the current  version of Logisim-evolution (I used v3.8.0)

    Then open the simple_CPU1.circ file.

    I recommend this Video as a quick way to get up to speed using Logisim

    The .circ file includes a simple program that is in the ROM that counts by 2 and repeats.

    0010 XXXX CLR     ;Clear
0001 0010 LDA 2   ;Load Accumulator with 2 and Add
1000 0100 JMPC 4  ;Jump if Carry set
0100 0001 JMP 1   ;Jump unconditional to location 1
0010 XXXX CLR     ;Clear
0001 1111 LDA F   ;Load Accumulator with F and Add
0100 0000 JMP 0   ;Jump unconditional to location 0

    There is another program cylon.hex as another example

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