VGA Memory Byte Order

[songziming]

I just managed to link asm and c together, and I want to print a char from C

Code: Select all

void kmain(void) {
    unsigned char *video = (unsigned char *)0xb8000;
    video[0] = 'K';
    video[1] = 0x07;
}

But what I got is


Can any one tell me why?

EDIT:
if I do

Code: Select all

video[1] = 'K';
video[0] = 0x07;

I can get the right result, But afaik the higher byte holds the attribute and the lower byte holds the char, why I got opposite after entering C.

[Combuster]

My crystal ball says you missed the Posting Checklist, and especially the gcc/ld part.

[sortie]

This is probably because how little endian works. I'll admit, this does seem the opposite of what I would expect, too. If changing the order works, just do that instead, though.

[songziming]

My crystal ball says you missed the Posting Checklist, and especially the gcc/ld part.

I use clang as the compiler

[Combuster]

I use clang as the compiler

That implies a number of things:
1) You did not use a tutorial (the shady and worse ones use gcc, the better ones a cross-compiler, but none that I know use clang)
2) Therefore, you have to have invented your own command line invocations
3) All the things regarding improper gcc use can equally apply to clang.

Point is, the shifting of bytes is a signature indication of running code of a different architecture - 64 bit code run in 32-bit mode gives a lot of off-by-one errors due to encoding differences.

[alexfru]

If the data segment's base address is off by 1 in the GDT, you may see such behavior.

[songziming]

It seems to be the problem of x86-64. If I pass the -m32 option to the compiler, it would get the correct result.