The asm keyword allows you to embed assembler instructions
within C code. GCC provides two forms of inline asm
statements. A basic asm statement is one with no
operands (see Basic Asm — Assembler Instructions Without Operands), while an extended asm
statement (see Extended Asm - Assembler Instructions with C Expression Operands) includes one or more operands.
The extended form is preferred for mixing C and assembly language
within a function, but to include assembly language at
top level you must use basic asm.
You can also use the asm keyword to override the assembler name
for a C symbol, or to place a C variable in a specific register.