# Digital design 4-bit ALU. Submission for Tiny Tapeout 03.

This is a digital design for a 4-bit ALU inspired by classical Z80, 6502 and 8085 processors.

I wanted design to be visually easy to understand and closer to perhaps how it would've been done back in the days,
so I chose to design logic circuit in Wokwi instead of HDL. 

## How does it work?

Each clock cycles ALU performs one of the 8 possible operations and stores result in the 4-bit accumulator register.
  ```
  accumulator <4 bit> = accumulator <4 bit> (operation) operand <4 bit>
  ```

### Supported operations
  ```
000xxxx   lda imm   ::  imm -> accumulator
001xxxx   neg imm   ::  0x0F - imm -> accumulator
010....   shr       ::  accumulator / 2 -> accumulator
011xxxx   sub imm   ::  accumulator - imm -> accumulator
100xxxx   and imm   ::  accumulator & imm -> accumulator
101xxxx   xor imm   ::  accumulator ^ imm -> accumulator
110xxxx   or  imm   ::  accumulator | imm -> accumulator
111xxxx   add imm   ::  accumulator + imm -> accumulator
  ```
### Input
```
IN0       clk
IN1..IN3  operation type [3 bit] (see below)
IN4..IN7  operand value [4 bit]
```
### Output
```
OUT0..OUT3  value stored in the accumulator [4 bit]
OUT4..OUT6  unused (todo: zero, overflow flags)
OUT7        carry flag
```
### Control signals
Map between operation type (opcode) and control signals
```
        mux1 mux2 mux3 AtoX negX setC outC invC
000 lda   -    -    1    0    0    -    0    -
001 neg   -    -    1    0    1    -    0    -
010 shr   -    -    1    1    0    -    0    -
011 sub   1    1    0    0    1    1    1    1
100 and   0    0    0    0    0    -    0    -
101 xor   0    1    0    0    0    -    0    -
110 or    1    0    0    0    0    -    0    -
111 add   1    1    0    0    0    0    1    0
```