Differences in Other Syntaxes and Terminology

The syntax for assembly language used in this book is known at the AT&T syntax. It is the one supported by the GNU tool chain that comes standard with every Linux distribution. However, the official syntax for x86 assembly language (known as the Intel syntax) is different. It is the same assembly language for the same platform, but it looks different. Some of the differences include:

• In Intel syntax, the operands of instructions are often reversed. The destination operand is listed before the source operand.

• In Intel syntax, registers are not prefixed with the percent sign (%).

• In Intel syntax, a dollar-sign ($) is not required to do immediate-mode addressing. Instead, non-immediate addressing is accomplished by surrounding the address with brackets ([]).

• In Intel syntax, the instruction name does not include the size of data being moved. If that is ambiguous, it is explicitly stated as BYTE, WORD, Or DWORD immediately after the instruction name.

• The way that memory addresses are represented in Intel assembly language is much different (shown below).

• Because the x86 processor line originally started out as a 16-bit processor, most literature about x86 processors refer to words as 16-bit values, and call 32-bit values double words. However, we use the term "word" to refer to the standard register size on a processor, which is 32 bits on an x86 processor. The syntax also keeps this naming convention - DWORD stands for "double word" in Intel syntax and is used for standard-sized registers, which we would call simply a "word".

• Intel assembly language has the ability to address memory as a segment/offset pair. We do not mention this because Linux does not support segmented memory, and is therefore irrelevant to normal Linux programming.

Other differences exist, but they are small in comparison. To show some of the differences, consider the following instruction:

 movl %eax, 8(%ebx,%edi,4) 

In Intel syntax, this would be written as:

 mov  [8 + %ebx + 1 * edi], eax 

The memory reference is a bit easier to read than its AT&T counterpart because it spells out exactly how the address will be computed. However, but the order of operands in Intel syntax can be confusing.