diff --git a/cst337/lec/lec9.md b/cst337/lec/lec9.md
new file mode 100644
index 0000000..121b23c
--- /dev/null
+++ b/cst337/lec/lec9.md
@@ -0,0 +1,115 @@
+# 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)
+
+### Big AND 
+
+Behavior is the same as an `and` but instead of two inputs we can have many more inputs.
+It will only ever return a 1 if all inputs are 1.
+
+### Big OR 
+
+Again we are mimicing the behvior of the normal or gate but this time we can have multiple inputs as opposed to just two.
+If only one of the many inputs is 1 then we return a 1 for the output of the Big OR.
+
+## 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 | i1 | i0 | 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