/* needs to stay the first line */
asm(".code16gcc\njmp $0, $main");

/* space for additional code */

char WRITE_CHARACTER_TTY = 0x0E;

// Syscall found here http://www.ctyme.com/intr/rb-0106.htm
void putc(char c) {
    short command = WRITE_CHARACTER_TTY << 8 | c;
    // volatile because there is no output, so the function might get optimized
    // away
    // clang-format off
    asm volatile(
        "int $0x10;"
        ::"a"(command)
    );
    // clang-format on
}

void print(char const *const str) {
    for (int i = 0; str[i] != '\0'; ++i) {
        putc(str[i]);
    }
}

// Syscall found here http://www.ctyme.com/intr/rb-1754.htm
char getc() {
    char ret;
    asm volatile(
        ".no_key:"
        "xor %%ax, %%ax;"
        "mov $0x0100, %%ax;" // check for keystroke:
        "int $0x16;"         // http://www.ctyme.com/intr/rb-1755.htm
        "jz .no_key;"        // wait for a key
        ".done:"
        "xor %%ax, %%ax;"
        "mov $0x1000, %%ax;" // get keystroke: http://www.ctyme.com/intr/rb-1754.htm
        "int $0x16;"
        "mov %%al, %%bl;"  // temp store char in bl
        "mov $0400, %%ax;" // clear keyboard buffer:
        "int $0x16;"       // http://www.ctyme.com/intr/rb-1759.htm
        : "=b"(ret)
        :
        : "ax");
    return ret;
}

void sleep(short ms) {
    long ys = ms * 1000;
    short cx = ys >> 16;
    short dx = ys;

    // http://www.ctyme.com/intr/rb-1525.htm : Int 15/AH=86h bios wait
    char ah = 0x86;
    char al = 0x00;
    short command = ah << 8 | al;

    asm volatile("int $0x15;" : : "a"(command), "cx"(cx), "dx"(dx));
}

void main(void) {
    print("Hello!");
    print("\n\r > ");
    while (1) {
        putc(getc());
    }
}