# lec9 

This lecture has a corresponding activity found in `lab/` it is called `combinational-logic.md`.
It is more useful to practice combinational logic as opposed to read about it so the sub section here will be minimal in information.
It's recommended that you try as many of the problems in the activity until you understand the concept, _don't bother doing them all_.

## Combinational Logic

### OR

`a+b` is equivalent to saying `a` or `b`.

### AND

`ab` is equivalent to saying `a` and `b`.

Note that this syntax is simlar to multiplication so `a*b` is equivalent to the above.

### NOT

`!a` is equivalent to saying not `a`.
We can also denote it with a bar over the expression we want to _not_.

![Figure-Not](../img/not.png)

## Decoders

Here we'll learn by doing 

```
Selector = 2 Bits
Output = 4 Bits
```
As a challenge you can try using the combinational logic gates from above to try and tackle this yourself
s1 |s2 |o3 |o2 |o1 |o0
 0 | 0 | 0 | 0 | 0 | 1
 0 | 1 | 0 | 0 | 1 | 0
 1 | 0 | 0 | 1 | 0 | 0
 1 | 1 | 1 | 0 | 0 | 0

 
## Multiplexor

Typically we'll refer to multiplexors by their size.

> what does it do?

It takes a signal as `2^n` inputs and out puts out `n` signals as output.

Example: We have a selector(s0), two inputs[in0 & in1], and one output `out`. 
The selector will select an input and we will generate some output in `out`.

s0 | i0 | i1 | out
0  | 0  | 0  | 0
0  | 0  | 1  | 1
0  | 1  | 0  | 0
0  | 1  | 1  | 1
1  | 0  | 0  | 0
1  | 0  | 1  | 0
1  | 1  | 0  | 1
1  | 1  | 1  | 1


This ultimately lets us pick data out of memory given some address.

## Half Adder

For now we'll take two inputs and get 1 output, with a carry-output.

Let's add 2 bits

ab	|out
00	|0
01	|1
10	|1
11	|0

What about the carry bit however? What would _it_ look like given the preivous operations?

ab	|carryout
00	|0
01	|0
10	|0
11	|1

Before what this implies note that the result of the carryout resembles

## Full Adder

Two inputs, One output, One carry-out, One carry-in

Here we'll add up `a & b`(inputs) and `c` carry-in

cab		|output
000		|0
001		|1
010		|1	
011		|0
100		|1
101		|0
110		|0
111		|1