C Inline ASM：将整数传递给寄存器

Question

I'm trying to make a simple kernel but I'm stucked on a cursor screen movement. My function, for move the cursor, is written in Assembly, but the rest of the kernel is in C, so I have to make a C function for call that function. And that's the C function:

void setCursor(unsigned int x, unsigned int y) {
    asm("mov (%0), %%bl" :: "r" (y) : "%bl");
    asm("mov (%0), %%bh" :: "r" (x) : "%bh");
    asm("call set_cursor");
}

In this function, I try to move the var unsigned int x to register bh and the other, unsigned int y , to register bl , and then call the function to move the cursor.

The problem is when I call this function, the cursor disappear from the screen. I think the values x and y and registers values bl and bh are not the same. For give you an idea of that clue, I tested to pass values directly in the asm directive and it works:

void setCursor(unsigned int x, unsigned int y) {
    asm("mov $1, %bl");
    asm("mov $2, %bh");
    asm("call set_cursor");
}

Any help is welcome. And thanks in advance. :D

Assembly output from compiler Ubuntu 12.04 64bit compiling in 32bit

setCursor:
.LFB12:
    .cfi_startproc
    pushl   %ebp
    .cfi_def_cfa_offset 8
    .cfi_offset 5, -8
    movl    %esp, %ebp
    .cfi_def_cfa_register 5
    movl    12(%ebp), %eax
    pushl   %ebx
    .cfi_offset 3, -12
#APP
# 131 "./kernel/screen.c" 1
    mov (%eax), %bl
# 0 "" 2
#NO_APP
    movl    8(%ebp), %eax
#APP
# 132 "./kernel/screen.c" 1
    mov (%eax), %bh
# 0 "" 2
# 133 "./kernel/screen.c" 1
    call set_cursor
# 0 "" 2
#NO_APP
    popl    %ebx
    .cfi_restore 3
    popl    %ebp
    .cfi_def_cfa 4, 4
    .cfi_restore 5
    ret
    .cfi_endproc

Answer 1

Write the asm set_cursor function to use the CDECL calling convention and you can avoid the inline asm entirely. Just use extern void set_cursor(int x, int y) and you can call it like any other C function. I use this for all asm in my own "kernel" (LGDT/IDT/etc). The only exception being setting cr3 for paging, since it's so simple.

Assuming Intel syntax, you could add:

set_cursor:
   mov     ecx, [esp + 4] # x in ecx (args are pushed right to left)
   mov     edx, [esp + 8] # y in edx

   push    ebx            # ebx has to be preserved in CDECL

   mov     bh, cl
   mov     bl, dl

   # Your original code...

   pop     ebx
   ret

Answer 2

In your code, you seem to be accessing the wrong variable with %0 , since the position 0 of variables has been skipped, and usualy, you must set = to the destination variable.

asm("mov (%1), %%bh" : : "r" (x) : "=bh");
                       |
                    %0 skipped

Also you can try to change the way you access registers:

register unsigned int *p0 asm ("r0") = x;

Read more here .