24.1 Basic Assembler Syntax

The assembler used by GNAT and gcc is based not on the Intel assembly language, but rather on a language that descends from the AT&T Unix assembler as (and which is often referred to as “AT&T syntax”). The following table summarizes the main features of as syntax and points out the differences from the Intel conventions. See the gcc as and gas (an as macro pre-processor) documentation for further information.

Register names

gcc / as: Prefix with “%”; for example %eax
Intel: No extra punctuation; for example eax

Immediate operand

gcc / as: Prefix with “$”; for example $4
Intel: No extra punctuation; for example 4

Address

gcc / as: Prefix with “$”; for example $loc
Intel: No extra punctuation; for example loc

Memory contents

gcc / as: No extra punctuation; for example loc
Intel: Square brackets; for example [loc]

Register contents

gcc / as: Parentheses; for example (%eax)
Intel: Square brackets; for example [eax]

Hexadecimal numbers

gcc / as: Leading “0x” (C language syntax); for example 0xA0
Intel: Trailing “h”; for example A0h

Operand size

gcc / as: Explicit in op code; for example movw to move a 16-bit word
Intel: Implicit, deduced by assembler; for example mov

Instruction repetition

gcc / as: Split into two lines; for example
rep
stosl
Intel: Keep on one line; for example rep stosl

Order of operands

gcc / as: Source first; for example movw $4, %eax
Intel: Destination first; for example mov eax, 4