8-Bit CPU

A fully custom, hand-crafted 8-bit CPU designed from first principles in Logisim-Evolution — no prebuilt RAM, no cheat components, just raw logic, microcode, and plenty of stubborn curiosity.

This processor features its own Instruction Set Architecture (ISA), a microcoded control unit, and a gate-level RAM module implemented entirely with flip-flops and decoders.

Project Highlights

• 8-bit Datapath, 4-bit Address Bus, and 16 bytes of hand-built RAM.
• Microcoded Control Unit with ROM-driven sequencing (T-states + flags → control signals).
• Conditional Execution using Zero and Carry flags wired directly into the microcode address lines.
• Manual Program Loader — flip-switch your own machine code straight into RAM.
• Step-by-Step Execution using a 4-stage microcycle counter (T0 → T4).

Architecture Overview

Datapath Components

• Program Counter
• Instruction Register
• Register A (Accumulator)
• Register B
• ALU (supports Add/Sub, sets flags)
• Output Register (BCD display)
• MAR (Memory Address Register)
• RAM (16×8, built from D-FFs)

Control Components

• 8× wide microcode ROM
• 4-bit micro-step counter
• Flag register (Zero + Carry)
• Conditional microcode branching

Instruction Set Architecture (ISA)

All instructions follow the standard Fetch → Decode → Execute cycle.

Opcode	Mnemonic	Meaning
0000	NOP	Do nothing (literally)
0001	LDA	Load A ← RAM[address]
0010	ADD	A ← A + B
0011	SUB	A ← A − B
0100	STA	RAM[address] ← A
0101	LDI	A ← immediate 8-bit value
0110	JMP	PC ← address
0111	JC	Jump if Carry flag = 1
1000	JZ	Jump if Zero flag = 1
1110	OUT	Output A to the display
1111	HLT	Halt clock; freeze CPU

Microcode & Control Logic

Each instruction expands into up to four micro-steps (T0–T4), generating control signals for:

• Register enabling/disabling
• ALU mode selection
• RAM read/write
• PC increment/load
• Conditional jumps using flags

The ROM address format:

[ T-state (3 bits) | Opcode (4 bits) | Flags (2 bits) ]

Project Structure

/
├── cpu.circ               # Main Logisim-Evolution file
├── assets/
│   ├── cpu.png            # Datapath overview
│   └── control-logic.jpeg # Handwritten microcode planning
├── README.md              # You're reading it!
└── rom/                   # ROM hex logic

Running the CPU

1. Install Logisim-Evolution.
2. Open cpu.circ.
3. Select the main circuit.
4. Toggle the step clock manually… or hit Ctrl + K for auto-clocking.

Want to write programs? Flip the PROGRAM_MODE switch and manually poke bytes into RAM like it’s 1971.