205 4-Bit ALU
205 : 4-Bit ALU
- Author: Ilir Shala
- Description: Mini 4-Bit ALU
- GitHub repository
- Open in 3D viewer
- Clock: 50000000 Hz
How it works
This design implements a 4-bit ALU with an integrated 8×4-bit register file.
Inputs are mapped as follows:
signal function ui_in[3:0] operand A (4-bit) ui_in[7:4] operand B (4-bit) / register address uio[3:0] opcode (4-bit) clk TinyTapeout clock rst_n active-low reset
The ALU supports basic arithmetic and logical operations as well as register-file access:
opcode function 0000 A + B 0001 A − B 0010 A AND B 0011 A OR B 0100 A XOR B 0101 A << B[1:0] 0110 A >> B[1:0] 0111 pass B 1000 write A → reg[B] 1001 read reg[B] 1010 A + reg[B] 1011 A − reg[B]
The output uo_out[7:0] contains the result and flags:
bit meaning 7 ZERO flag (result=0) 6 SIGN flag (MSB of result) 5 OVERFLOW flag (signed add/sub) 4 CARRY flag (unsigned add/sub) 3:0 result (4-bit)
How to test
Use DIP switches or microcontroller GPIO lines to drive ui_in and uio pins.
Examples:
write value 7 into register 3 ui_in = { B=3 , A=7 }, opcode=1000
read register 3 ui_in = { B=3 , A=don’t care }, opcode=1001
add register 3 + A (A on ui_in lower nibble) opcode=1010
You can observe the result and flags on the 8 output pins.
External hardware
None required.
For demo you can use:
8 DIP switches for ui_in 4 DIP switches for opcode 8 LEDs for uo_out
A simple MCU board (Arduino / Pico) makes more comfortable testing, but is optional.
IO
| # | Input | Output | Bidirectional |
|---|---|---|---|
| 0 | ui_in[0] | uo_out[0] | uio_in[0] |
| 1 | ui_in[1] | uo_out[1] | uio_in[1] |
| 2 | ui_in[2] | uo_out[2] | uio_in[2] |
| 3 | ui_in[3] | uo_out[3] | uio_in[3] |
| 4 | ui_in[4] | uo_out[4] | |
| 5 | ui_in[5] | uo_out[5] | |
| 6 | ui_in[6] | uo_out[6] | |
| 7 | ui_in[7] | uo_out[7] |