131 Basic8 CPU
131 : Basic8 CPU
- Author: SalihOmerOngun
- Description: 8-bit minimal CPU with 8 registers, ALU, branches and JAL
- GitHub repository
- Open in 3D viewer
- Clock: 10000000 Hz
How it works
Basic8 is a minimal 8-bit CPU with the following features:
- 8 general purpose 8-bit registers (r0 hardwired to 0)
- 8-bit program counter (PC)
- 16-bit instruction format
- ALU operations: ADD, SUB, OR, XOR, AND, SLL, SRL, SRA
- Immediate operations: ADDI, ANDI, ORI, XORI (6-bit immediate)
- Branch instructions: BEQ, BNE (PC-relative, 6-bit offset)
- Jump and link: JAL (PC-relative, saves return address)
Instructions are 16-bit and loaded in two cycles via ui_in:
- Cycle 1: uio_in[0]=1 — ui_in high byte is latched into internal register
- Cycle 2: uio_in[1]=1 — ui_in low byte is combined with the latched high byte to form the full 16-bit instruction, which is then decoded and executed
The program counter (PC) is output on uo_out, allowing an external controller to feed instructions from a ROM.
How to test
Connect a ROM (instruction memory) externally. On each instruction:
- Read PC from uo_out
- Send high byte of ROM[PC] via ui_in with uio_in[0]=1
- Send low byte of ROM[PC] via ui_in with uio_in[1]=1
- Repeat
See test/test.py for a cocotb testbench with full instruction set coverage.
External hardware
None required. The CPU is driven entirely through the ui_in and uio_in pins.
IO
| # | Input | Output | Bidirectional |
|---|---|---|---|
| 0 | instruction byte (high or low) | PC bit 0 | load_hi - latch high byte of instruction |
| 1 | instruction byte (high or low) | PC bit 1 | exec - execute instruction |
| 2 | instruction byte (high or low) | PC bit 2 | |
| 3 | instruction byte (high or low) | PC bit 3 | |
| 4 | instruction byte (high or low) | PC bit 4 | |
| 5 | instruction byte (high or low) | PC bit 5 | |
| 6 | instruction byte (high or low) | PC bit 6 | |
| 7 | instruction byte (high or low) | PC bit 7 |