4 Most minimal extension of friend's 'CPU In a Week' in a day
4 : Most minimal extension of friend's 'CPU In a Week' in a day
- Author: Gregory Kollmer
- Description: 8-bit Single-Cycle CPU
- GitHub repository
- Clock: 0 Hz
How it works
This project is the most minimal extension (add CLR & MUL to ISA) of Ramyad Hadidi's 8-bit CPU that details the design and implementation of an 8-bit single-cycle microprocessor. The processor includes a register file and an Arithmetic Logic Unit (ALU). The design was crafted to handle a simple instruction set architecture (ISA) that supports basic ALU operations, load/store operations, and status checks for the ALU carry -- all within less than a week. While the current version lacks a program counter and external memory, thus omitting any form of jump operations, it provides a solid foundation for understanding basic computational operations within a custom CPU architecture.
ISCA Overview
The ISA is straightforward and is primarily focused on register operations and basic arithmetic/logic functions. Below is the breakdown of the instruction set:
// ISA --------------------------------------------------------------
//-- R level
`define MVR 4'b0000 // Move Register
`define LDB 4'b0001 // Load Byte into Regsiter
`define STB 4'b0010 // Store Byte from Regsiter
`define RDS 4'b0011 // Read (store) processor status
// 1'b0100 NOP
// 1'b0101 NOP
// 1'b0110 NOP
// 1'b0111 NOP
//-- Arithmatics
`define NOT {1'b1, `ALU_NOT}
`define AND {1'b1, `ALU_AND}
`define ORA {1'b1, `ALU_ORA}
`define ADD {1'b1, `ALU_ADD}
`define SUB {1'b1, `ALU_SUB}
`define XOR {1'b1, `ALU_XOR}
`define INC {1'b1, `ALU_INC}
`define MUL {1'b1, `ALU_MUL}
// 1'b1111 NOP
How to test
The processor has been tested through a suite of 12 testbenches, each designed to validate a specific functionality or operation. These testbenches cover basic ALU operations, data movement between registers, and the load/store functionalities. Although basic operational tests are passing, timing interactions between instructions have not been exhaustively verified, and it is anticipated that a sophisticated compiler would handle these timing considerations effectively, reminiscent of approaches taken in historical computing systems. [ADD TESTS FOR MUL EXTENSION]
External hardware
Currently, the processor does not interface with any external hardware components. It operates entirely within a simulated environment where all inputs and outputs are managed through testbenches. This setup is ideal for educational purposes or for foundational experimentation in CPU design.
IO
| # | Input | Output | Bidirectional |
|---|---|---|---|
| 0 | Register 1 (R1) Address bit 0 | Data out bit 0 (either register data / Processor stat) | Data in bit 0 / Register 3 (R3) Address bit 0 |
| 1 | Register 1 (R1) Address bit 1 | Data out bit 1 (either register data / 0) | Data in bit 1 / Register 3 (R3) Address bit 1 |
| 2 | Register 1 (R1) Address bit 2 | Data out bit 2 (either register data / 0) | Data in bit 2 / Register 3 (R3) Address bit 2 |
| 3 | Register 1 (R1) Address bit 3 | Data out bit 3 (either register data / 0) | Data in bit 3 / Register 3 (R3) Address bit 3 |
| 4 | Instruction ISA Opcode bit 0 | Data out bit 4 (either register data / 0) | Data in bit 4 / Register 2 (R2) Address bit 0 |
| 5 | Instruction ISA Opcode bit 1 | Data out bit 5 (either register data / 0) | Data in bit 5 / Register 2 (R2) Address bit 1 |
| 6 | Instruction ISA Opcode bit 2 | Data out bit 6 (either register data / 0) | Data in bit 6 / Register 2 (R2) Address bit 2 |
| 7 | Instruction ISA Opcode bit 3 | Data out bit 7 (either register data / 0) | Data in bit 7 / Register 2 (R2) Address bit 3 |