How to tell GCC to generate 16-bit code for real mode

Question

I am writing real mode function, which should be normal function with stackframes and so, but it should use %sp instead of %esp. Is there some way to do it?

Answer 1

As far as I know, GCC does not support generation of code for 16-bit x86. For legacy bootloaders and similar purposes, you should write a small stub in assembly language to put the cpu in 32-bit mode and pass off execution to 32-bit code. For other purposes you really shouldn't be writing 16-bit code.

Answer 2

Firstly, gcc could build 16bit code, because the linux kernel is go through realmode to protectmode, so it could even build 16bit c code.

Then, -m16 option is supported by GCC >= 4.9 and clang >= 3.5

gcc will ignore asm(".code16"),you can see it by -S output the assembly code surround by #APP #NO_APP

the linux kernel do the trick to compile 16bit c with a code16gcc.h(only have .code16gcc) pass to gcc compile params directly. see Build 16-bit code with -m16 where possible, also see the linux kernel build Makefile

if you direct put the asm(".code16gcc"), see Writing 16-bit Code, it's not real 16bit code, call, ret, enter, leave, push, pop, pusha, popa, pushf, and popf instructions default to 32-bit size

Answer 3

GCC 5.2.0 (and possible earlier versions) support 16-bit code generation with the -m16 flag. However, the code will almost certainly rely on 32-bit processor features (such as 32-bit wide registers), so you should check the generated assembly carefully.

From the man pages:

The -m16 option is the same as -m32, except for that it outputs the ".code16gcc" assembly directive at the beginning of the assembly output so that the binary can run in 16-bit mode.

Answer 4

GCC does not produce 8086 code. The GNU AS directive .code16gcc can be used to assemble the output of GCC to run in a 16-bit mode, put asm(".code16gcc") at the start of your C source, your program will be limited to 64Kibytes.

On modern GCC versions you can pass the -m16 argument to gcc which will produce code to run in a 16-bit mode. It still requires a 386 or later.