Cannot read sectors from disk in C

Question

I'm using an i686-elf-gcc cross compiler to generate code to run in real-mode.

I'm trying to read a sector from my kernel. I know this is the location where my kernel is located, in the second sector, 0th drive, 0th track. It builds fine, but after I call read, sectors_read is still 0.

u8 status;
u8 sectors_read;
read(1, 0, 1, 0, 0, status, sectors_read);
kprint("STATUS: ");
hex_to_ascii(status, num_str_buffer);
kprint(num_str_buffer);
kprint("\nSECTORS READ: ");
num_str_buffer[0] = '\0';
hex_to_ascii(sectors_read, num_str_buffer);
kprint(num_str_buffer);

void read(u8 sector_size, u8 track, u8 sector, u8 head, u8 drive, u8 status, u8 sectors_read)
{
    asm volatile("mov $2, %AH");
    asm volatile("mov %0, %%AL" : : "r"(sector_size) : );
    asm volatile("mov %0, %%CH" : : "r"(track) : );
    asm volatile("mov %0, %%CL" : : "r"(sector) : );
    asm volatile("mov %0, %%DH" : : "r"(head));
    asm volatile("mov %0, %%DL" : : "r"(drive));
    asm volatile("int $0x13");
    asm volatile("mov %%AH, %0":"=r"(status) : );
    asm volatile("mov %%AL, %0":"=r"(sectors_read) : );
}

Answer 1

There's a bunch of stuff wrong with your read function. As you have not posted a reproducible example or even enough details to know what toolchain you are programming for, I cannot give a definite answer to your problems. Here are a number of things that are wrong:

• in C, all function arguments are passed by value. Assigning to status and sectors_read in read will not affect the arguments you pass in the callee. Pass a pointer instead.
• you do not tell the compiler that you clobber ax, cx, and dx or that the contents of these registers are of importance. The compiler might thus use them for whatever purpose it likes, interfering with the semantics you intend. To fix this, use register constraints instead of mov instructions to place the arguments into the right registers.
• You do not seem to set es:bx to the address of a buffer to read data into. Thus, the BIOS will read data into whatever address es:bx points to, essentially overwriting random memory.

Here is an example how you could fix these issues in the read function. Note that there are likely more changes needed depending on your situation.

typedef unsigned char u8;
typedef unsigned short u16;

void read(u8 sector_size, u8 track, u8 sector, u8 head, u8 drive,
    u8 *buffer, u8 *status, u8 *sectors_read)
{
    u16 result;

    asm volatile("push %%cs; pop %%es; int $0x13"
        : "=a"(result)
        : "a"(0x200|sector_size), "b"(buffer),
          "c"(track<<8|sector), "d"(head<<8|drive)
        : "es", "memory");

    *status = result >> 8;
    *sectors_read = result >> 0;
}

The code above assumes ia16-gcc. For i686-gcc, use

    asm volatile("int $0x13"
        : "=a"(result)
        : "a"(0x200|sector_size), "b"(buffer),
          "c"(track<<8|sector), "d"(head<<8|drive)
        : "memory");

instead.

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