GNAT User’s Guide for Native Platforms: Basic Assembler Syntax

10.1 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