🖥️ 4-Bit CPU — CircuitVerse

A simple 4-bit processor built from scratch in CircuitVerse, featuring an ALU, register file, instruction memory, data memory, and control unit.

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📐 Architecture

┌─────────────────────────────────────────────────────┐
│                    4-bit CPU                        │
│                                                     │
│  ┌──────┐    ┌──────────────┐    ┌───────────────┐  │
│  │  PC  │───▶│  Instruction │───▶│ Control Unit  │  │
│  │      │    │    Memory    │    │               │  │
│  └──────┘    └──────────────┘    └───────┬───────┘  │
│     ▲                                    │          │
│     │              ┌─────────────────────▼──────┐   │
│     │              │   Register File (R0–R3)    │   │
│     │              └──────────┬─────────────────┘   │
│     │                        │                      │
│     │              ┌─────────▼──────────┐           │
│     │              │        ALU         │           │
│     │              │  (ADD, AND, OR...) │           │
│     │              └──────────┬─────────┘           │
│     │                        │                      │
│     │              ┌─────────▼──────────┐           │
│     └──────────────│   Data Memory      │           │
│                    │   (16×4b RAM)      │           │
│                    └────────────────────┘           │
└─────────────────────────────────────────────────────┘

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🧩 Components

Component	Description
PC (Program Counter)	4-bit register holding the address of the current instruction
Instruction Memory	ROM storing the program instructions, or a manual 8-bit input for testing
Register File	4 general-purpose registers: R0, R1, R2, R3 (4 bits each)
ALU	4-bit arithmetic/logic unit (ADD, AND, OR, SUB)
Control Unit	Decodes instructions and generates control signals
Data Memory	16×4-bit RAM for reading and writing data

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📋 Instruction Set Architecture (ISA)

Instructions are 8 bits wide, but only the lower 4 bits are used for the opcode and operand:

[ 7 ]   = Reset flag — set to 1 to asynchronously reset all registers
[ 6:4 ] = Unused
[ 3:2 ] = Opcode  (2 bits)
[ 1:0 ] = Operand (destination register / address)

Opcode	Instruction	Description
00	ALU OP	Arithmetic/logic operation between registers
01	STORE	Write register value to Data Memory
10	LOAD	Read value from Data Memory into register
11	JUMP	Jump to specified address

Control Signals

Signal	Function
ALU_OP0	ALU operation bit 0
ALU_OP1	ALU operation bit 1
RegWrite	Enable write to register file
MemWrite	Enable write to data memory
MemRead	Enable read from data memory
Jump	Enable PC jump

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🛠️ How to Run

1. Open CircuitVerse
2. Import the file cpu_4bit.cv via File → Import
3. Load instructions into the ROM or connect a manual 8-bit input for quick testing
4. Press Play or enable the clock
5. Watch register and memory values update each clock cycle

Resetting the CPU

• Via ROM (bit 7): Set bit 7 of any instruction to 1 — this triggers an asynchronous reset on all registers instantly during program execution
• Manually: Use the dedicated Reset button connected to the Asynchronous Reset pin of all flip-flops in the register file

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📁 Project Structure

cpu_4bit/
├── cpu_4bit.cv    # Main CircuitVerse project file
└── README.md      # Documentation

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🔧 Example ROM Program

Address | Value | Instruction
--------|-------|------------
0x0     | 0001  | ALU: R1 ← R0 op R1
0x1     | 0100  | STORE R0 → mem[0]
0x2     | 1000  | LOAD  R0 ← mem[0]
0x3     | 1100  | JUMP  → 0x0

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⚙️ Technical Details

• Data width: 4 bits
• Address width: 4 bits (16 addressable locations)
• Register type: Synchronous D Flip-Flop with Enable
• Clock: Synchronous, positive edge-triggered
• Instruction memory: ROM (read-only)
• Data memory: 16×4-bit RAM
• Simulator: CircuitVerse (browser-based)

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👤 Author

Fagadeanu Rares-Stefan Built as a computer architecture learning project.

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📜 License

MIT — free for educational use.