Writing 16-bit Code
While `as' normally writes only "pure" 32-bit i386 code, 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
`.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.
The same code in a 16-bit code section would generate the machine
opcode bytes `6a 04' (ie. 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.
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