reveal.js

## Constructing an
# ALU

---

CS 130 // 2021-11-10

## Administrivia
- Assignment 4
    + Due tomorrow night (due to my error)
- 
 Assignment 5 is released
    + Will talk about it at the end of class today

# Questions
## ...about anything?

# ALU

## The ALU
- What does ALU stand for?
    + Arithmetic Logic Unit
- 
 What does an ALU do?
    + Does all the logical and arithmetic operations of the CPU

## Basic ALU
- The simplest ALU conceivable is one that only operates on two 1-bit inputs and can perform only two operations: **AND** and **OR**
    ```py
    def basic_alu(a, b, op):
        if op == 0:
            return (a and b)
        else:
            return (a or b)
    ```
- 
 Let's implement this in Logisim!

## Including Addition as an Op
- For Assignment 4, you were implementing a full adder like the one below:

![Full Adder Diagram](/teaching/2021f/cs130/assets/images/COD/full-adder.png)

## Including Addition as an Op
- We can change our ALU so that when `op = 2`, we will do addition instead of AND or OR
    ```py
    def basic_alu(a, b, c_in, op):
        if op == 0:
            return (a and b)
        elif op == 1:
            return (a or b)
        elif op == 2:
            return (a + b + c_in) # 2-bits, c_out and result
    ```
- 
 Let's implement this in Logisim!

# Multi-Bit ALU

## Including Other Operations
- How can we include **subtraction** as an op?
- 
 How can we include **NAND** as an op?

## Including Other Operations
- This version of the 1-bit ALU has inputs:
    + `$a$`, `$b$`, `$a_\text{inv}$`, `$b_\text{inv}$`, `$c_\text{in}$`, and `$\text{op}$`
- 
 What are each of the inputs if I want to compute:
    + 
 `$a$` plus `$b$`
    + 
 `$a$` minus `$b$`
    + 
 `$a$` AND `$b$`
    + 
 `$a$` NAND `$b$`
    + 
 NOT `$a$`

# `slt`