Added initial noise stuff

cs132/part4.md

@@ -5,6 +5,8 @@ math: true
 title: Memory Systems
 ---
 
+#### _All diagrams in this page belong to Akram- please contact me directly for the original resources so that you can modify them or distribute them as you need._
+
 # Memory Systems
 ## Memory hierarchy
 When deciding on a memory technology, you must consider the following factors:
@@ -91,3 +93,50 @@ In our **address decoder**, we have $$ log_{2} (W) $$ many control pins, where $
 
 > We are trying to avoid long, narrow arrays when we design our memory cell arrays. We want to **maximise space for memory cells** and minimise space taken up by IO.
 
+# Detecting and Correcting Errors
+
+> Although this topic is within the memory systems lecture, it is fundamental to error detection on the whole and hence has its own section here.
+
+Broadly speaking, there are two types of errors:
+- Errors that occur **within a system**, e.g. in a memory system.
+- Errors that occur in the **communication between systems**, e.g., in the transmission of messages or data between systems. This is what we will focus on.
+
+## Noise
+
+- We typically **send information through channels**- when these channels become affected by **unwanted information**, they become **noisy**.
+- Noise will arise from the **physical properties** of devices:
+  - Thermal noise
+  - Noise of electronic components
+  - Noise of transmission circuits
+- **Magnetic media** will also have a "classic form of noise" due to the "random alignment of magnetic fields".
+
+> Noise is **always present**. If it doesn't come from the components themselves, it'll come from external sources such as radiation. Noise is hence one of the **limiting factors** in computer systems.
+> In magnetic stores, when we have **decreased area** to store a bit, **noise gets worse** which increases the likelihood of errors.
+
+### Digital logic devices
+
+> We choose binary systems for our number systems as it provides us a **high degree of noise immunity**.
+> We also need to consider the **tolerances of the components we use**
+
+#### Illustrating noise immunity, a trademarked Akram Analogy™
+
+_If you are comfortable with the idea of noise immunity and transistor-transistor logic voltage levels, you probably won't need to read this._
+
+To illustrate the first point, consider the following thought experiment:
+- You and your friend have found a massive tunnel (assuming CS students step outdoors). **The tunnel has water dripping and some other ambiguous sounds.**
+- You both stand at either end of the tunnel, and you realise now you want to say something to your friend. You have two choices:
+  - You can choose to simply clap your hands to get their attention (a binary communication system), OR
+  - You can choose to say a magic password that only they will respond to (a base-26 communication system).
+
+Given the ambiguous sounds in the tunnel, which do you think your friend will be able to distinguish better? Would they be able to distinguish a clap above a specific volume? Or would they be able to distinguish the spoken magic password? How do you know when a sound is finally loud enough to constitute you communicating with one another?
+
+This idea of a small window where we do not consider a signal high or low is widest when we use a binary system- if we had any more possible values, we would need to find even more ranges which we consider 'nothing' (_**i.e. neither 0 nor anything else)**_. 
+
+> Using binary means that we only focus on two logical values.
+
+In the image below, you can see the illustrated example for the above analogy, with annotated TTL voltage levels for context.
+
+<img src="part4res/4-5.png" alt="Memory cell word diagram" style="zoom: 50%;"/>
+
+<img src="part4res/4-6.png" alt="Memory cell word diagram" style="zoom: 50%;"/>
+