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?

Bocanegra

What OS is is supposed to run under? GCC doesn't support MS-DOS. – Noncooperation

Here is a minimal runnable example that prints hello world with the BIOS in 16-bit mode from C: github.com/cirosantilli/x86-bare-metal-examples/tree/… – Verdha

Answer 1

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.

Cyanamide

But don't forget what Michael Petch mentioned at: #19056147 that this only uses the 16 bit prefix versions of instructions, not the bare 16 code. – Verdha

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

Sanitarium

Answer 3

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.

Flor

Documented at: sourceware.org/binutils/docs/as/… – Verdha

Using .code16gcc (I'm still not sure one can call it a stable feature) will produce code that runs in 16 bit real mode, however it uses an instruction prefix that is only available on 386+ . This means if you are targeting 8086/8088 (real hardware or emulator) then the code will likely fail to work properly. If you intend to bootstrap entry into 32-bit protected mode then write a small 16-bit assembler stub that sets everything up, enters protected mode and then calls to a 32 bit "C" function that is the start of your program. – Pubescent

You can of course hand code 16-bit assembler in GCC as inline assembly using asm directive. – Pubescent

Can you combine -m16 with -mtune=i8086 to get 16 bit Real Mode 8086 code? – Airworthy

Answer 4

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.

Hautboy

Would you please explain much more. What about -m16 option? – Estragon