Bharata B. Rao offers a guide to the overall use and structure of inline assembly for x86 on the Linux platform. He covers the basics of inline assembly and its various usages, gives some basic inline assembly coding guidelines, and explains the instances of inline assembly code in the Linux kernel.
If you’re a Linux kernel developer, you probably find yourself coding highly architecture-dependent functions or optimizing a code path pretty often. And you probably do this by inserting assembly language instructions into the middle of C statements (a method otherwise known as inline assembly). Let’s take a look at the specific usage of inline assembly in Linux. (We’ll limit our discussion to the IA32 assembly.)
GNU assembler syntax in brief
Let’s first look at the basic assembler syntax used in Linux. GCC, the GNU C Compiler for Linux, uses AT&T assembly syntax. Some of the basic rules of this syntax are listed below. (The list is by no means complete; I’ve included only those rules pertinent to inline assembly.)
Register naming
Register names are prefixed by %. That is, if eax has to be used, it should be used as %eax.
Source and destination ordering
In any instruction, source comes first and destination follows. This differs from Intel syntax, where source comes after destination.