Returning long long value in i386 assembly?

[clementttttttttt]

Currently making a memory size detecting function in i386 assembly(C wont work because of compiler optimization), need to use long long values.

[kzinti]

I believe the usual convention is to return the value in edx:eax.

edx holds the 32 most significant bits and eax the 32 least significant ones.

[SKC]

I believe the convention is to return the value in edx:eax.

edx holds the 32 most significant bits and eax the 32 least significant ones.

Yes. As the wiki says: "The return value is stored in the eax register, or if it is a 64-bit value, then the higher 32-bits go in edx".

Btw, if you want to disable optimizations in GCC, you can use the 'optimize' attribute:

Code: Select all

__attribute__((optimize("-O0"))) void func();

You can read this to learn more about optimizations in GCC.

[iansjack]

Currently making a memory size detecting function in i386 assembly.

Be very careful about reading/writing random memory addresses. This might have undesired side effects. UEFI and GRUB both provide memory maps; it may be better to use them.

[Octocontrabass]

Currently making a memory size detecting function in i386 assembly

It's not possible to detect installed memory. You must use the memory map provided by the firmware.

(C wont work because of compiler optimization)

If your code behaves differently depending on compiler optimization, the problem is your code, not the compiler optimization.

need to use long long values.

This is documented in the i386 psABI.