Created CPU (markdown)

CPU.md

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+# Central processing unit (CPU)
+Work in progress
+
+## CPU Architecture
+
+Work in progress
+
+
+## Random-access memory
+
+Work in progress
+
+
+## General purpose registers
+The registers are 16-bits locations available to the CPU and are used in some instructions and system calls. Read [Basic Assembly Tutorial](https://github.com/simon987/Much-Assembly-Required-3/wiki/Basic-Assembly-tutorial) for more information.
+
+| Register | Purpose |
+| -------- | ------- |
+| **A** | General |
+| **B** | General |
+| **C** | General |
+| **D** | General |
+| **X** | General |
+| **Y** | General |
+| **BP** | Base pointer |
+| **SP** | Stack Pointer |
+
+Read [Basic Assembly Tutorial](https://github.com/simon987/Much-Assembly-Required/wiki/Basic-Assembly-tutorial) for more information about the BP & SP registers and the stack.
+
+## The FLAGS register
+The FLAGS register holds the **status** - information about the last executed instruction - of the CPU. The FLAGS register cannot be read or written directly. There are 5 flags that can modified or read by instructions (Read [Instruction set](https://github.com/simon987/Much-Assembly-Required/wiki/Instruction-set) to learn which instructions modifies the FLAGS register):
+
+| Flag | Description |
+|:--------- |:----------- |
+| **CF** (Carry flag) | Relevent for *unsigned* operations. Indicates that there was a carry out of the most significant (leftmost) bit   |
+| **ZF** (Zero flag) | Indicates that the result of an operation is 0 |
+| **SF** (Sign flag) | Indicates that the most significant (leftmost) bit of the result of an operation is set |
+| **OF** (Overflow flag) | Relevent for *signed* operations. Indicates that the sign of the result of an signed operation is wrong (See exemples below) |
+| **BF** (Break flag) | Tells the CPU to stop the execution. This flag is set with the **BRK** instruction. |
+
+### Carry Flag
+The carry flag is set when there is a carry out of the destination's most significant bit
+
+   For example: 
+```
+ + 1111 1111 1111 1111   # Signed:-1    Unsigned: 65535
+   0000 0000 0000 0001   # Signed: 1    Unsigned: 1
+   -------------------
+   0000 0000 0000 0000
+```
+Note that the result of the result of the previous operation is perfectly valid for signed numbers (See [Basic Assembly Tutorial](https://github.com/simon987/Much-Assembly-Required/wiki/Basic-Assembly-tutorial)) but it is invalid for unsigned operations.  
+
+
+### Sign flag
+The sign flag is set when the most significant (leftmost) bit of the result of an operation is set.
+
+For example:
+```
+ + 0100 0000 0000 0000   # Signed: 16384   Unsigned: 16384
+   0100 0000 0000 0000   # Signed: 16384   Unsigned: 16384
+   -------------------
+   1000 0000 0000 0000   # Signed:-32768   Unsigned: 32768
+   ^
+   Sign flag is set
+```
+
+### Overflow flag
+The carry flag is set when the signed two's complement result of an operation does not fit in the destination, or more specifically, when there is a carry/borrow **into** or **out** of the most significant (leftmost) bit, **but not both**.  
+The overflow flag is set when the result of an addition of two positive numbers is negative or when the result of an addition of two negative numbers is positive.
+
+For example:
+```
+   There is a carry out of the most significant (leftmost) bit, OF is set.
+   v        
+ + 1111 1111 1111 1111   # Signed:-1       Unsigned: 65535
+   1000 0000 0000 0000   # Signed:-32768   Unsigned: 32768
+   -------------------
+   0111 1111 1111 1111   # Signed: 32768   Unsigned: 32768
+
+  There is a carry out of the most significant bit
+  \ There is also a carry into the most significant bit, OF is not set
+   VV
+ + 1111 1111 1111 1111
+   1100 0000 0000 0000
+   -------------------
+   1111 1111 1111 1111
+
+    There is also a carry into the most significant bit, OF is set
+    V
+ + 0100 0000 0000 0000
+   0100 0000 0000 0000
+   -------------------
+   1000 0000 0000 0000
+
+```