Chapter 2: Assembly Language Directives
### 2.1 Introduction
Assembly language directives are instructions to the assembler that provide information about how to process the code. In this chapter, we will explore common assembly language directives that are used to define data, sections, and other essential aspects of a program.
### 2.2 Data Definition Directives
#### 2.2.1 `db` - Define Byte
- **Purpose:**
- Reserves one or more bytes of storage and initializes them with specified values.
- **Example:**
```assembly
variable db 42 ; Defines a byte with the value 42
message db 'Hello' ; Defines a byte array with the ASCII values of 'H', 'e', 'l', 'l', 'o'
```
#### 2.2.2 `dw` - Define Word
- **Purpose:**
- Reserves one or more words (16 bits) of storage and initializes them.
- **Example:**
```assembly
number dw 1000 ; Defines a word with the value 1000
array dw 1, 2, 3, 4 ; Defines an array of words
```
#### 2.2.3 `dd` - Define Doubleword
- **Purpose:**
- Reserves one or more doublewords (32 bits) of storage and initializes them.
- **Example:**
```assembly
value dd 123456 ; Defines a doubleword with the value 123456
list dd 10, 20, 30 ; Defines a list of doublewords
```
### 2.3 Section Directives
#### 2.3.1 `section` - Section Declaration
- **Purpose:**
- Defines a section of the program, such as the code section or data section.
- **Example:**
```assembly
section .data ; Data section
variable db 42
section .text ; Code section
global _start
```
#### 2.3.2 `global` - Global Declaration
- **Purpose:**
- Declares a label as globally accessible, allowing it to be used in other files.
- **Example:**
```assembly
global _start
_start:
; Code here
```
#### 2.3.3 `extern` - External Declaration
- **Purpose:**
- Informs the assembler that a label is defined in another file.
- **Example:**
```assembly
extern function ; Declares that 'function' is defined in another file
_start:
; Code here
```
### 2.4 Reserving Storage
#### 2.4.1 `resb`, `resw`, `resd` - Reserve Bytes, Words, Doublewords
- **Purpose:**
- Reserves a specified number of bytes, words, or doublewords without initializing them.
- **Example:**
```assembly
buffer resb 100 ; Reserves 100 bytes of storage for a buffer
array resd 10 ; Reserves space for an array of 10 doublewords
```
### 2.5 Equating Values
#### 2.5.1 `equ` - Equate
- **Purpose:**
- Assigns a constant value to a symbol.
- **Example:**
```assembly
size equ 100 ; Defines a constant 'size' with the value 100
```
### 2.6 Including Files
#### 2.6.1 `include` - Include File
- **Purpose:**
- Inserts the content of an external file into the source code.
- **Example:**
```assembly
include 'macros.inc' ; Includes the content of 'macros.inc' in the source code
```
### 2.7 Conclusion
Assembly language directives play a crucial role in providing structure and information to the assembler during the compilation process. As you advance in assembly programming, a solid understanding of these directives will empower you to effectively organize and manage your code.
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