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), while an extended asm
statement (see Extended Asm) 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.