Let's Explore Machine Code

This is way too early.
- We will be back to this in chapter 4.
- With a different computer.
- But I want to explore machine/assembly/higher level languages just a bit.
Don't try to memorize this computer.
- It is something that I made up.
But keep the ideas in mind.

Copy this down

PC: 
IR:
ACC:
Input: 32
Output:

MEMORY
Address
     0 
     1 
     2 
     3 
     4 
     5 
     6 
     7

The hardware
- We need four "things"
- A Program Counter or PC hold the number of the next instruction we will follow.
  - It starts at 0.
  - We will usually change this by 1.
  - Each time we change it, cross the old value out and write the new one next to it.
```
PC: 0 1 2
```
- A instruction register or IR. This will have program values,
  - We will put different values here, but do the same as the pc.
- An accumulator or ACC. This is the place we will do most math. Again, just cross out values and write new ones.
- Memory
  - We have 8 memory locations.
  - Write any memory values in the proper location
  - Cross off and write down if we need to.

The program

The program will be written in Wombat Assembly
We will learn this as we go.
The important part is an instraction has two parts.
- The first letter is an opCode. It tells us what to do.
  - Thse are the digits 0-9 and the letters A-E
- The second part is an operand. It gives us infomation on data.
  - These mean different thinigs.
  - Sometimes they are a 0 because nothing is needed.
A program is a series of these instructions.
1. 30
2. C10
3. 40
4. 00
Above is a very simple program.

Let's trace it.

When the computer starts

The PC is 0
The ACC is 0
All memry is 0

Write this down.

PC: 0
IR:
ACC: 0
Input:
Output:

MEMORY
Address   0   1   2   3   4   5   6   7  
  0   0   0   0   0   0   0   0

The first thing that happens is

the instruction that matches the PC is copied to the IR

One is added to the PC

PC: 0 1
IR: 30
ACC: 0
Input:
Output:

MEMORY
Address   0   1   2   3   4   5   6   7  
  0   0   0   0   0   0   0   0

Next we look at the instruction,

The opCode is 3
According to the table this means do input.
Get a number from the user and write the value in the ACC.

Assume the value is 32.

PC: 0 1
IR: 30
ACC: 32
Input: 32
Output:

MEMORY
Address   0   1   2   3   4   5   6   7  
  0   0   0   0   0   0   0   0

We have finished the first instruction, so we start over.

the instruction that matches the PC is copied to the IR
Since the PC is 1, the instruction is C10

Increment the PC

PC: 0 1 2
IR: 30 C10 
ACC: 32
Input: 32
Output:

MEMORY
Address   0   1   2   3   4   5   6   7  
  0   0   0   0   0   0   0   0

An opCode of C means add the operand (10) to the value in the accumulator (32) and store the result in the accumulator.

PC: 0 1 2
IR: 30 C10 
ACC: 32 42
Input: 32
Output:

MEMORY
Address   0   1   2   3   4   5   6   7  
  0   0   0   0   0   0   0   0

The next instruction

4 means write the value of the accumulator to the output.

PC: 0 1 2 3
IR: 30 C10  40
ACC: 32 42
Input: 32
Output: 40

MEMORY
Address   0   1   2   3   4   5   6   7  
  0   0   0   0   0   0   0   0

Finally an opcode of 0 means stop. The computation is over.

Let's try a math trick without the computer.
- Think of a number. Write this down as n.
- Subtract one from your number
- Add four to your number.
- Add five to your number.
- Subtract the orignial number and think about the result.
- I can read your mind, the final number is 8
- (n - 1 + 4 + 5 - n) = 8
Here is a wombat program that will do this trick:
1. 30
2. 21
3. C-1
4. C5
5. C4
6. 61
7. 40
8. 00
Let's trace it.
If you want, I wrote a simple simulator to do this.
We can try the other programs on the page.
If we have time, could we write a program that would count from 10 to 0?