GNU as

The GNU Assembler ("GAS", executable named 'as') is part of the binutils package. If you are using GCC, you are also using GAS - basically, GCC generates Assembler source which is turned into machine code by a subsequent (automatic) call to gas. Moreover, when you are using inline assembler in GCC code, you are also using GAS.

Actually, GAS is not one assembler, but a collection of assemblers - one for each of the platforms supported by GCC - which are very similar in their available options, their macro language etc.

Up until v2.10 of binutils, GAS supported only AT&T syntax, which differs significantly from the Intel syntax used by virtually every other assembler. Today, GAS supports both syntax sets (.intel_syntax and the default .att_syntax), and even allows disabling the otherwise mandatory '%' register prefix (..._syntax noprefix).

There are some pitfalls - several FP opcodes suffer from a reverse operand ordering that is bound to stay in there for compatibility reasons, .intel_syntax generates less optimized opcodes on occassion (try mov'ing to %si...). But if you can learn to live with them (like, reading the manual and our helpful HowAtandtSyntaxDiffersFromNasm), GAS provides you with a very solid and well-supported assembler that is fully integrated with the other GNU tools (like, implicit make rules, inline calling from C/C++ sources, and the ability to be called as gcc -o myfile.o myfile.s).

AT&T versions of some of the assembly examples around the Wiki can be found on the GasAllInOne page.

GAS defaults to 32bit code, but can be switched to .code16. This requires some additional attention - you will want to check the documentation on that matter.