Représentation et types des données

In assembly language, we work directly with raw bits and bytes. But how do we know if a sequence of bytes represents a number, a character, or something else?

This is where data types come in! They give meaning to the raw data, helping both you and the CPU understand how to interpret and manipulate information.

x86 assembly defines standard sizes for data. These directly correspond to how much memory space a piece of data occupies:

• BYTE: 8 bits
• WORD: 16 bits (2 bytes)
• DWORD: Double Word, 32 bits (4 bytes)
• QWORD: Quad Word, 64 bits (8 bytes)

These sizes are fundamental for declaring variables and working with registers.

To store data in memory, we use data definition directives. These tell the assembler to reserve space and optionally initialize it with a value.

• DB: Define Byte (8-bit)
• DW: Define Word (16-bit)
• DD: Define Doubleword (32-bit)
• DQ: Define Quadword (64-bit)

You'll see these often when creating variables in your programs.

An unsigned integer is a number that is always positive or zero. All of its bits are used to represent the magnitude of the number.

For example, an 8-bit unsigned byte can hold values from 0 to 255. A 16-bit unsigned word can hold values from 0 to 65,535.

When you don't need negative numbers, unsigned types are perfect and give you a larger positive range.

Signed integers can represent both positive and negative values. One bit, usually the Most Significant Bit (MSB), is used to indicate the sign (0 for positive, 1 for negative).

Negative numbers are typically represented using Two's Complement. This system makes arithmetic operations work seamlessly for both positive and negative values.

An 8-bit signed byte ranges from -128 to +127.

Characters like 'A', 'b', or '7' are also stored as numbers! The most common standard for this is ASCII (American Standard Code for Information Interchange).

Each character is assigned a unique 8-bit (1-byte) numerical value. For example, the character 'A' is represented by the decimal value 65 (or hexadecimal 0x41).

You can define single characters or entire strings using the DB directive.

This example shows how to define different data types and then load their values into CPU registers. This demonstrates how assembly treats these named memory locations.

Data alignment means placing data in memory at an address that is a multiple of its size. For example, a DWORD (4 bytes) might be aligned to an address ending in 0, 4, 8, or C (hex).

While not strictly required by all CPUs, proper alignment can significantly improve performance. The CPU can fetch aligned data more efficiently, often in a single memory access, avoiding extra work.

Assemblers sometimes provide directives like ALIGN to help ensure proper alignment.

Understanding data types is crucial because it dictates:

• Memory Usage: How much space your data consumes.
• Instruction Choice: Which assembly instructions (e.g., ADD, MOV) are appropriate for the data size.
• Interpretation: Whether the CPU treats 0xFF as 255 (unsigned) or -1 (signed).

Careful type selection prevents errors and ensures your programs behave as expected at the lowest level.

You've learned about common data sizes and how they're defined. Let's test your knowledge!

Great job! You've explored the fundamental concepts of data representation in x86 assembly.

• We define data using directives like DB, DW, DD, and DQ for various sizes.
• Integers can be signed (positive/negative) or unsigned (positive only).
• Characters are stored using the ASCII standard, where each character has a numerical value.
• Understanding data alignment can help optimize performance.

Next, we'll continue building on this knowledge to perform more complex operations!