While as normally writes only “pure” 32-bit i386 code
or 64-bit x86-64 code depending on the default configuration,
it also supports writing code to run in real mode or in 16-bit protected
mode code segments. To do this, put a .code16 or
.code16gcc directive before the assembly language instructions to
be run in 16-bit mode. You can switch as back to writing
normal 32-bit code with the .code32 directive.
.code16gcc provides experimental support for generating 16-bit code from gcc, and differs from .code16 in that call, ret, enter, leave, push, pop, pusha, popa, pushf, and popf instructions default to 32-bit size. This is so that the stack pointer is manipulated in the same way over function calls, allowing access to function parameters at the same stack offsets as in 32-bit mode. .code16gcc also automatically adds address size prefixes where necessary to use the 32-bit addressing modes that gcc generates.
The code which as generates in 16-bit mode will not
necessarily run on a 16-bit pre-80386 processor. To write code that
runs on such a processor, you must refrain from using any 32-bit
constructs which require as to output address or operand
size prefixes.
Note that writing 16-bit code instructions by explicitly specifying a prefix or an instruction mnemonic suffix within a 32-bit code section generates different machine instructions than those generated for a 16-bit code segment. In a 32-bit code section, the following code generates the machine opcode bytes 66 6a 04, which pushes the value 4 onto the stack, decrementing %esp by 2.
pushw $4
The same code in a 16-bit code section would generate the machine opcode bytes 6a 04 (i.e., without the operand size prefix), which is correct since the processor default operand size is assumed to be 16 bits in a 16-bit code section.