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# Registers, Instructions, and System Calls

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Note to self: talk about binary
  introduce arrays and have them investigate what they look like in memory
  practice loading items from different places in the array

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CS 130 // 2024-09-03

# Review

## Recap of last week

- MIPS: RISC architecture
  + 32, 32-bit registers
- We wrote some a simple MIPS program in Mars
  + watched data change in registers and memory
- Data and instructions get turned into binary and placed in memory

- Covered `add`, `lw`, `sw`

```mips
.data

a: 9
b: 3
result: 0

.text

lw $t0, a
lw $t1, b
add $t2, $t0, $t1
sw $t2, result
```

# More on Registers and Instructions

## MIPS Register Conventions

<div class="twocolumn">
<div>
<table style="font-size: 60%">
  <tbody>
    <tr> <th>Name</th> <th>Reg #</th> <th>Usage</th></tr>
    <tr><td>$zero</td> <td>0</td> <td>Constant 0</td></tr>
    <tr><td>$v0,...</td><td>2-3</td> <td>Values</td></tr>
    <tr><td>$a0,...</td><td>4-7</td> <td>Arguments</td></tr>
    <tr><td>$t0,...</td><td>8-15</td> <td>Temporaries</td></tr>
    <tr><td>$s0,...</td><td>16-23</td> <td>Saved (variables)</td></tr>
    <tr><td>$t8,...</td><td>24-25</td> <td>More temporaries</td></tr>
    <tr><td>$gp</td><td>28</td> <td>global pointer</td></tr>
    <tr><td>$sp</td><td>29</td> <td>stack pointer</td></tr>
    <tr><td>$fp</td><td>30</td> <td>frame pointer</td></tr>
    <tr><td>$ra</td><td>31</td> <td>return address</td></tr>
  </tbody>
</table>
</div>
<div>
<ul>
    <li class="fragment"> Register 1 is reserved for the assembler
    <li class="fragment"> Registers 26-27 are reserved for the OS
    <li class="fragment"> Only 8 $s registers (0..7)
    <li class="fragment"> Only 10 $t registers (0..9)
</ul>
</div>
</div>

## Exercise

- Write a MIPS program that does the equivalent of this high-level line of code
```python
pay = (salary + bonus) - (health_premium + taxes)
```

## Immediate values

- A literal value like 3 in the example below is called an **immediate value**
- `li` means _load immedate_

```mips
.data

x: 5

.text

lw $t0, x
li $t1, 3 #load 3 directly into $t1
add $t0, $t1, $t0
```

## I-type instructions

- There's also a variation of `add` where the second operand is replaced with an immediate value
- `addi` means *add immediate* 
  - it's an **I-type instruction**

```mips
.data

x: 5

.text

lw $t0, x
addi $t0, $t0, 3
```

## R-Type instructions

- The add instruction that uses only registers is an **R-type instruction**

```mips
add $t0, $t1, $t0
```

## System Calls

- We can make **system calls** to have the system perform things like input and output
- Put the system call code in `$v0`
- Put argument in `$a0` (and maybe `$a1` if needed)

#### Output Exercise

- Run this in MARS and discuss what happens with your neighbors
- What do you think `.asciiz` does?

```mips
.data

message: .asciiz "Hello!"

.text

li $v0, 4 #4 is the code for printing a string
la $a0, message #the argument for the syscall
syscall
```

#### Input Exercise

- Run this in MARS and discuss what happens with your neighbors
- Which register does the user's input go into?

```mips
.data

prompt: .asciiz "Enter an integer:"

.text

li $v0, 4 #4 is the code for printing a string
la $a0, prompt #the argument for the syscall
syscall

li $v0, 5 #5 is the code for reading an integer
syscall
```

#### Assignment: Interactive Program

You now know enough to do the first assignment

- [Assignment 1](../../assignments/assignment-1/): Write a program that interacts with the user and performs some kind of computation based on their input
- Find other syscall codes on page B-44 of the textbook

# Binary Numbers

## Let's talk about how counting works

![Odometer rollover](/Fall2024/CS130/assets/images/Odometer_rollover.jpg)

How do you count if you only have two digits?