Arithmetic Logic Unit
The Arithmetic Logic Unit (ALU) is the centrepiece of any modern day computer
- ALUs implement a fixed set of functions
- Each function can take at most two multi-bit arguments
- The ALU outputs the result of the boolean function and a set of flags
- The instruction is a set of control bits which select which function the ALU should output
- And
- Adder
- Pre/post processing on inputs/outputs
Our specific ALU accepts
- Two 16-bit wide arguments
- A 6-bit wide instruction
And outputs - a 16-bit wide output
- A bit to indicate if the output is 0
- A bit to indicate o the output is less than 0
White a 6-bit wide instruction gives the ALU a possible
- We are only interested in 18 of them
Control Bits
The ALU has the choice between two basic functions, logical And and Addition
- All operations are multi-bit operations
| Bit | Description |
|---|---|
| za | Zero input A |
| na | Negate input A |
| zb | Zero input B |
| nb | Negate input B |
| f | Selection between & and + |
| no | Negate output |
Example
| za | na | zb | nb | f | no | output | |
|---|---|---|---|---|---|---|---|
| 0 | 0 | 1 | 1 | 1 | 0 | a-1 | |
| a | a | a | a | a | a-1 | a-1 | a-1 |
| b | b | b | 0 | -1 |
^ | ^ | ^ |
Implementation
Notice that the ALU is composed of
- A logic circuit that can negate or zero an input (twice)
- Logic circuits for Addition and And
- A logic circuit to select between them
- A logic circuit to negate the output
- A logic circuit to handle flags