nearly done with lecture 1 rewrie

337/lec/lec1.md

@@ -4,13 +4,152 @@
 
 The first lecture has bee 50% syllabus 25% videos, 25% simple terminology; expect nothing interesting for this section
 
-## Performace Options 
+## General Performance Improvements in software
+
+
 In general we have a few options to increase performace in software; pipelining, parallelism, prediction.
 
-Parallelism/Pipelining 
+1. Parallelism
 
-* I'll just assume you know what this entail; one does many things at once; the other is like queues for processessing.
+If we have multiple tasks to accomplish or multiple sources of data we might instead find it better to work on multiple things at once[e.g. multi-threading, multi-core rendering]
 
-* Prediction 
+2. Pipelining
 
-Yes this means interpreting potential outcomes/inputs/outputs etc. __BRANCHING__. We try to predict potentiality and account for it ahead of time.
+Here we are somehow taking _data_ and serializing it into a linear form.
+We do things like this because it could make sense to things linearly[e.g. taking data from a website response and forming it into a struct/class instance in C++/Java et al.].
+
+3. Prediction 
+
+If we can predict an outcome to avoid a bunch of computation then it could be worth to take our prediction and proceed with that instead of the former.
+This happens **a lot** in cpu's where they use what's called [branch prediction](https://danluu.com/branch-prediction/) to run even faster.
+
+## Cost of Such Improvements
+
+As the saying goes: every decision you make as an engineer ultimately has a cost, let's look at the cost of these improvements.
+
+1. Parallelism
+
+If we have a data set which has some form of inter-dependencies between its members then we could easily run into the issue of waiting on other things to finish.
+
+Contrived Example: 
+
+```
+Premise: output file contents -> search lines for some text -> sort the resulting lines
+
+We have to do the following processes:
+print my-file.data 
+search file
+sort results of the search
+
+In bash we might do: cat my-file.data | grep 'Text to search for' | sort
+```
+
+Parallelism doesn't make sense here for one reason: this series of proccesses don't benefit from parallelism because the 2nd and 3rd tasks _must_ wait until the previous ones finish first.
+
+2. Pipelining
+
+Let's say we want to do the following:
+
+```
+Search file1 for some text : [search file1] 
+Feed the results of the search into a sorting program [sort]
+
+Search file2 for some text  [search file2]
+Feed the results of the search into a reverse sorting program [reverse sort]
+
+The resulting Directed Acyclic Graph looks like
+
+[search file1] => [sort]
+
+[search file2] => [reverse sort]
+```
+
+Making the above linear means we effectively have to:
+
+```
+[search file1] => [sort] [search file2] => [reverse sort]
+| proc2 waiting........| 
+```
+
+Which wastes a lot of time if the previous process is going to take a long time.
+Bonus points if process 2 is extremely short.
+
+
+3. Prediction
+
+Ok two things up front:
+
+* First: prediction's fault is that we could be wrong and have to end up doing hard computations.
+* Second: _this course never covers branch prediction(something that pretty much every cpu in the last 20 years out there does)_ so I'm gonna cover it here; ready, let's go.
+
+For starters let's say a basic cpu takes instructions sequentially in memory: `A B C D`.
+However this is kinda slow because there is _time_ between getting instructions, decoding it to know what instruction it is and finally executing it proper.
+For this reason modern CPU's actually fetch, decode, and execute(and more!) instructions all at the same time.
+
+Instead of getting instructions like this:
+
+
+```
+0
+ AA
+   BB
+     CC
+       DD 
+```
+
+We actually do something more like this
+
+```
+A
+ AB
+   BC
+     CD
+	   D0
+```
+
+If it doesn't seem like much remember this is half an instruction on a chip that is likely going to process thousands/millions of instructions so the savings scales really well.
+
+
+This scheme is fine if our instructions are all coming one after the other in memory, but if we need to branch then we likely need to jump to a new location like so.
+
+```
+ABCDEFGHIJKL
+^^^*     ^
+   |-----|
+```
+
+Now say we have the following code:
+
+```
+if (x == 123) {
+	main_call();
+}
+else {
+	alternate_call();
+}
+```
+
+The (psuedo)assembly might look like
+
+```asm
+	cmp x, 123 
+	je second
+main_branch:	; pointless label but nice for reading
+	call main_call
+	jmp end
+second:
+	call alternate_call
+end:
+	; something to do here
+```
+
+Our problem comes when we hit the je. 
+Once we've loaded that instruction and can start executing it, we have to make a decision, load the `call main_call` instruction or the `call alternate_call`?
+Chances are that if we guess we have a 50% change of saving time and 50% chance of tossing out our guess and starting the whole _get instruction => decode etc._ process over again from scratch.
+
+Solution 1: 
+
+Try do determine what branches are taken prior to running the program and just always guess the more likely branches.
+If we find that the above branch calls `main_branch` more often then we should load that branch always; knowing that the loss from being wrong is offset by the gain from the statistically more often correct branches.
+
+...

337/lec/lec10.md

@@ -18,12 +18,12 @@ _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.
+Try doing this on your own but as far as hints go: don't be afraid to make changes to the mux.
 
 ![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.
+You may have also noted that we can subtract two things to see if they are the same however, we can also `not` the result of the `xor` and get the same result.
 
 ![fig3](../img/fig3lec11.png)