c引导加载程序传递函数参数不起作用

Question

I have a basic bootloader in c, but when I try to pass a parameter to a function, it doesn't work quite right. The following code doesn't work:

__asm__(".code16\n");
__asm__("call main\n");

void main(){
    putchar('X');
    while(1){}
}

void putchar(char c){
    __asm__("movb %0, %%al\n" : : "r"(c));
    __asm__("movb $0x0e, %ah\n");
    __asm__("int $0x10\n");
}

It should print out an X, but instead it just moves the cursor forward as if printing a space.

The following code works because it doesn't pass the character as a parameter:

__asm__(".code16\n");
__asm__("call main\n");

char c;

void main(){
    c = 'X';
    putchar();
    while(1){}
}

void putchar(){
    __asm__("movb %0, %%al\n" : : "r"(c));
    __asm__("movb $0x0e, %ah\n");
    __asm__("int $0x10\n");
}

Can someone explain why it isn't working?

Answer 1

The problem wasn't the fact that it was being passed as a parameter, but rather, I was using the assembly wrongly. The following code works:

__asm__(".code16\n");
__asm__("call main\n");

void main(){
    putchar('!');
    while(1){}
}

void putchar(char c){
    __asm__ __volatile__("int $0x10" : : "a"(0x0e00 | c), "b"(0x0007));
}