AT&T syntax is mainly about operand order, but there are assembler directives and constructs specific to AT&T as found in as(1) on any traditional UNIX, including illumos based ones. That every assembler has his own syntax is nothing new, compare and contrast Master SEKA with ASM-One on Amiga, or MASM, TASM or nasm, for example. AT&T as(1) syntax has nothing to do with x86. Any code written for SVR4 as(1) will be using AT&T syntax, irrespective of processor architecture. ISA used with AT&T as(1) will still be that of the processor, of course, but instead of things like a0 or d0, it'll be %a0 and %d0, for example.