asm: base for lec11

cst337/lec/lec11.md

@@ -1,3 +1,41 @@
 # lec11
 
+At this point I'l mention that just reading isn't going to get you anywhere, you have to try things, and give it a real earnest attempt.
 
+__ALU:__ Arithmetic Logic Unit
+
+## Building a 1-bit ALU 
+
+![fig0](../img/alu.png)
+
+First we'll create an example _ALU_ which implements choosing between an `and`, `or`, `xor`, or `add`.
+Whether or not our amazing _ALU_ is useful doesn't matter so we'll go one function at a time(besides `and/or`).
+
+First recognize that we need to choose between `and` or `or` against our two inputs A/B.
+This means we have two inputs and/or, and we need to select between them.
+_Try to do this on your own first!_
+
+![fig1](../mg/fig1llec11.png)
+
+Next we'll add on the `xor`.
+AGAIN: try to do this on your own, the main hint I'll give here is: the current mux needs to be changed.
+
+![fig2](../img/fig2lec11.png)
+
+Finally we'll add the ability to add and subtract. 
+You may have also noted that we can subtract two things to see if they are the same dhowever, we can also `not` the result of the `xor` and get the same result.
+
+![fig3](../img/fig3lec11.png)
+
+At this point our _ALU_ can `and`, `or`, `xor`, and `add`/`sub`.
+The mux will choose one which logic block to use; the carry-in line will tell the `add` logic block whether to add or subtract.
+Finally the A-invert and B-invert line allow us to determine if we want to invert either A or B (inputs).
+
+## N-bit ALU
+
+For sanity we'll use the following block for our new ALU.
+
+![fig4](../img/fig4lec11.png)
+
+Note that we are chaining the carry-in's to the carry-out's just like a ripple adder.
+also each ALU just works with `1` bit from our given 4-bit input.