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SAP-1 (Simple as Possible)

A simple 8-bit micro-processor using mostly discrete logic chips.

This project is mostly based on the 8-bit Breadboard computer by Ben Eater

Image of the Processor Board

Banner Specs

  • ~400 Hz Clock
  • 8-bit Data Bus
  • 16-bit Control Word
  • 5 Microsteps per Instruction
  • 16 byte RAM


Check the projects Pages for the documentation of the ISA and Hardware modules.


  • Clock with adjustable frequency and single-step button
  • 2 8-bit Data Registers (A & B)
  • ALU implementing sum and difference between Registers A & B, carry and zero flag
  • Flag Register to save the ALU flags between instructions
  • 4-bit Instruction Counter with load (jump)
  • Output module to display a byte as positive decimal or 2s-complement with data latch
  • Random Access Memory with 16 Bytes for instructions and data
  • 4-bit Memory Address Register to address the 16 bytes of RAM
  • 8-bit Instruction Register with the upper nibble representing the opcode, the lower nibble can be used for instruction parameters
  • Instruction Decoder to run the microcode of the 16 different instructions with 5 microinstuctions each. Uses a 16-bit control word to control the other modules

Project Structure

Files Description
Board/Board.* Autodesk Eagle project woth the Schematic and a fully routed Boardlayout
Board/Board_*.zip precreated Gerbers for Production
Board/BOM.xlsx a BOM with Mouser partnumbers
Spec/ a Python module that defines and documents the Instruction Set Architecture of the Processor.
Spec/ script to create the microcode LUT binary for the ID EEPROMS. Uses
Spec/ script to create the LUT binary for the output decode EEPROM
LUTs/*.bin precreated binaries for the EEPROMS
Example-Programs/*.s Some Example Programs in SAP-1 assembly
Tools/sap-asm Assembler for SAP-1 assembly listings. Outputs binaries or prints programming instructions.
Simulations/ iCircuit simulation files for some parts of the architecture

Writing Code

The documentation has an extensive section on the Instruction Set Architecture you can use to get started writing programs.


In the Tools directory an assembler is available that can either output binary files or, if it detects stdout to be a TTY, prints programming instructions for you to manually program the RAM using the DIP-Switches.

$ cat Example-Programs/Counter.s 
0xF: 0x1

JMP loop

$ Tools/sap-asm Example-Programs/Counter.s
○○○○: ○○○●●●●●
○○○●: ●●●○○○○○
○○●○: ○○●○●●●●
○○●●: ○●●○○○○●
●●●●: ○○○○○○○●

$ Tools/sap-asm Example-Programs/Counter.s | hexdump 
0000000 1f e0 2f 61 00 00 00 00 00 00 00 00 00 00 00 01

Programming the EEPROM LUTs

The provided Makefile offers some convenient rules to create the LUTs and program them to an EEPROM. To program the LUTs you can use an Arduino based programmer with only 2 external chips (+EEPROM) which is the officially supported method using the eepro CLI.

However, you can also use any other programmer to flash the binary files created by the scripts.

If you want to use eepro, make sure the package is installed

pip3 install -r requirements.txt

Then you can programm the needed 3 EEPROMS (1x Output Decoder, 2x Microcode) using

make program_lut file=LUTs/microcode.bin        # < for the microcode EEPROMS
make program_lut file=LUTs/outputdecoder.bin    # < for the Output Decoder EEPROM

The rule makes sure the LUTs are created or updated if the creating script has changed since last creation.

If you want to use any other programmer, just run

make luts

to create the binaries in the LUTs/ directory.


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