## Assembly Language Syntax

---

### Learning Goals:

1. Develop a simple mental model of a computer.
   
1. Understand how to read/write assembly language syntax.
   
1. Learn some X86 assembly language statements.

---

### Computer Architecture

---

### Main Components:

<ul>
  <li class="fragment" data-fragment-index="0">Central Processing Unit</li>
  <ul>
    <li class="fragment" data-fragment-index="1">Registers (Fast Memory)</li>
    <li class="fragment" data-fragment-index="2">Arithmetic & Logic Unit (ALU)</li>
  </ul>
  <li class="fragment" data-fragment-index="3">Main Memory (RAM)</li>
  <span class="fragment strike semi-fade-out" data-fragment-index="6">
  <li class="fragment" data-fragment-index="4">Input/Output Devices (Peripherals)</li>
  <li class="fragment" data-fragment-index="5">Data Bus</li>
  </span>
</ul>

---

[Image Source](https://en.wikipedia.org/wiki/Computer_architecture)

---

### Central Processing Unit (CPU):

- Controls the flow of data to and from memory and I/O devices.
  
- The CPU performs arithmetic and logical operations on the data. 
  
- The CPU can decide the order of operations based on the results of arithmetic
  and logic operations.
  
- It contains a small amount of very fast memory (registers).

---

### Random Access Memory (RAM):

Provides storage that is readily accessible to the CPU and I/O devices.

### Input/Output Devices (Peripherals):

Communicates with the outside world and with mass storage devices.

---

---

## <img data-src='https://patshaughnessy.net/assets/2016/11/26/register.png' width=800px />

## Assembly Language Syntax

---

<div>

C Code:

```c
int hacs408e() {
    return 408;
}
```

</div>

<table style="min-width: var(--slide-width)" class="fragment">
<thead>
<tr>
<th>Intel</th>
<th>AT&T</th>
</tr>
</thead>
<tr>
<td>

```x86asm [1:]
hacs408e():
        mov     eax, 408
        ret
```

</td>
<td>

```x86asmatt [1:]
hacs408e():
        movl    $408, %eax
        ret
```

</td>
</tr>
<table>

---

### Prefixes:

- AT&T:
  
  - Registers are prefixed with '%'
    
  - Numbers are prefixed with a '$'
    
  - Intel does not use prefixes
    
- Intel:
  
  - Hexadecimal data is suffixed with 'h'
    
  - Binary values are suffixed with 'b'
    
  - AT&T does not use suffixes

---

<table style="min-width: var(--slide-width)">
<thead>
<tr>
<th>Intel</th>
<th>AT&T</th>
</tr>
</thead>
<tr>
<td>

```x86asm [1:]
mov     eax,1
mov     ebx,0ffh
int     80h
```

</td>
<td>

```x86asmatt [1:]
movl    $1,%eax
movl    $0xff,%ebx
int     $0x80
```

</td>
</tr>
<table>

---

### Direction of Operands:

<table style="min-width: var(--slide-width)">
<thead>
<tr>
<th>Intel</th>
<th>AT&T</th>
</tr>
</thead>
<tr>
<td>

```x86asm
dest <-- source
```

- Left most operand

</td>
<td>

```x86asmatt
source --> dest
```

- Right most operand

</td>
</tr>
<table>

---

<table style="min-width: var(--slide-width)">
<thead>
<tr>
<th>Intel</th>
<th>AT&T</th>
</tr>
</thead>
<tbody>
<tr>
<td>

```x86asm [1:]
; instr   dest,source
  mov     eax ,ecx
```

</td>
<td>

```x86asmatt [1:]
# instr   source,dest
  movl    %ecx,%eax
```

</td>
</tr>
</tbody>
<table>

---

### Memory Operands:

- In Intel syntax the base register is enclosed in '[' and ']'
  
- AT&T syntax it is enclosed in '(' and ')'.

---

<table style="min-width: var(--slide-width)">
<thead>
<tr>
<th>Intel</th>
<th>AT&T</th>
</tr>
</thead>
<tbody>
<tr>
<td>

```x86asm [1:]
mov     eax,[ebx]
mov     eax,[ebx+3]
```

</td>
<td>

```x86asmatt [1:]
movl    (%ebx),%eax
movl    3(%ebx),%eax
```

</td>
</tr>
</tbody>
<table>

---

### Suffixes:

- Some AT&T instructions have a suffix
  
- They are used to denote operand size
  
  - 'l' is for long (4 bytes)
    
  - 'w' is for word (2 bytes)
    
  - 'b' is for byte (1 byte)
    
- Intel only uses these labels for memory operands
  
  - 'dword ptr' for long
    
  - 'word ptr' for word
    
  - 'byte ptr' for byte

---

<table style="min-width: var(--slide-width)">
<thead>
<tr>
<th>Intel</th>
<th>AT&T</th>
</tr>
</thead>
<tbody>
<tr>
<td>

```x86asm [1:]
mov     al,bl
mov     ax,bx
mov     eax,ebx
mov     eax, dword ptr [ebx]
```

</td>
<td>

```x86asmatt [1:]
movb    %bl,%al
movw    %bx,%ax
movl    %ebx,%eax
movl    (%ebx),%eax
```

</td>
</tr>
</tbody>
<table>

---

### END