4-bit-ALU/README.md

4-Bit Arithmetic Logic Unit (ALU) & 8-Bit Multiplication

• This project was originally designed for TTL-based implementation but was estimated to require approximately 100 TTL chips.
• Consequently, it is now being revisited for FPGA implementation using the Tang Primer 20k. The circuit also includes a 7-segment display and a 4511 TTL chip for BCD-to-7SD encoding.
• Additionally, old and unused Verilog and Logisim files are being included in the repository.
• Gowin Education v1.9.9.03 & Logisim Evolution v3.9.0

Circuit Design

• All bits are unsigned.
• The circuit uses an 8-input DIP switch for A and B, a 3-input opcode selector, and a 1-bit CarryIn/BorrowIn input (depending on the opcode).
• The BCD module employs a double dabble algorithm, but the 3rd and 4th bits in the 100s position are not enabled. As a result, the integer "400" cannot be displayed on the 7-segment display.
• The opcode uses a 3-bit decoder with one-hot encoding for selection.
• CarryOut/BorrowOut and Overflow are displayed using LEDs.

Inputs:

• 2-Bit Selector: Determines which data is displayed on the screen.
• 3-Bit OpCode: Specifies the operation to be performed.
• 4-Bit Input A: The first operand.
• 4-Bit Input B: The second operand.
• 1-Bit CarryIn/BorrowIn: Input carry for arithmetic operations.

Outputs:

• 11-Bit Result: The ALU converts the binary result to BCD. This feature can be easily disabled if binary output is preferred.
• 1-Bit CarryOut/BorrowOut: Indicates carry or borrow in arithmetic operations.
• 1-Bit Overflow: Indicates an overflow condition during addition.

Preview

Click to expand and view media of the real-life circuit.

opCode

A&B Input

A input

B input

Logic Unit

AND Gate

OR Gate

XOR Gate

Arithmetic Unit

Addition & Addition with CarryIN

Addition & Overflow and CarryOut led

Subtraction & BorrowOut Led

8-2 with Borrow In

Multiplication

Screen Selector Table

The 2-bit screen selector determines what is displayed on the screen:

Screen Selector	Displayed Data
00	Input A
01	Input B
10	opCode
11	Result

Opcode Table

The table below describes the operations performed by the ALU based on the 3-bit opcode:

Opcode	Operation	Result Bits	Carry/Borrow In/Out Usage
000	Addition	4	✅
001	Subtraction	4	✅
010	Multiplication	8	❌
100	AND	4	❌
101	OR	4	❌
110	XOR	4	❌
011	❌	❌	❌
111	❌	❌	❌

Important NOTE!

You are free to use this repository as you wish, but please note that I am not actively maintaining it. This repository was created for a specific project, and I would like to preserve the original code.

Credit

• BUR4KBEY: Honorable mention for resolving DIP switch assignments in the FloorPlanner(Gowin). Also for this readme template.