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8bitProgrammableComputer.circ
ALU.png
Computer.png
README.md
ROMCounterInput.png

README.md

8-bit Programmable computer

Finished Circuit

About this computer

This is an awesome experience, about implementation of 8-bit programmable computer. This computer is made up of a ROM and an ALU. Programs should be converted to HEX code and then be uploaded on ROM, it'll start to execute them. In this README.md file, you'll find out how this computer is designed and how it exactly works!

Instruction Set Architecture

This computer, includes 4 ALU options, An, Bn, S1 and S0. S1 and S0 pins are connected to a multiplexer and they do selection of results. An & Bn pins, do invertion on operands A and B. They are used for implementing NOR, NAND and SUB functions.

An Bn S1 S0 Function
0 0 0 0 AND A, B
0 0 0 1 OR A, B
0 0 1 0 ADD A, B
0 1 1 0 SUB A, B
1 0 1 0 SUB B, A
1 1 0 0 NOR A, B
1 1 0 1 NAND A,B

The HEX code

It's very very easy to make hex code for an operation. What you need to consider is this table :

ALUCode Operand A Operand B
4 bits 8 bits 8 bits
bits 16-19 bits 8-15 bits 0-7

That's all, a 20 bit binary number, or 5-digit hexadecimal expression. Consider this operation :

ADD 15, 2
ALUCode Operand A Operand B
0010 00001111 00000010
2 0f 02

The HEX code we got is 20f02. We can upload this to our ROM now, and see the result!

The ALU

ALU

This ALU is made up of :

  1. 3 multilpexers ( One 8-bit 4x1 mux for selecting result of operations, two 1-bit 2x1 mux for swtiching between A or ~A (B or ~B))
  2. 8-bit AND gate
  3. 8-bit OR gate
  4. 1-bit OR gate, which can make 2's complement of B for us.
  5. 2 8-bit NOT gates (for making NOR and NAND logics, and implemetation of SUB function).

ROM

This is the ROM presented by Logisim but, as input of address bus, a 4-bit counter is designed. This is schematics of the counter : ROM counter

TODO List

  1. Adding B-A operation (currently, the ALU only supports A-B). - DONE!
  2. Adding a simple register file