cfenollosa_os-tutorial/07-bootsector-disk

Concepts you may want to Google beforehand: hard disk, cylinder, head, sector, carry bit

Goal: Let the bootsector load data from disk in order to boot the kernel

Our OS won't fit inside the bootsector 512 bytes, so we need to read data from a disk in order to run the kernel.

Thankfully, we don't have to deal with turning spinning platters on and off, we can just call some BIOS routines, like we did to print characters on the screen. To do so, we set al to 0x02 (and other registers with the required cylinder, head and sector) and raise int 0x13

You can access a detailed int 13h guide here

On this lesson we will use for the first time the carry bit, which is an extra bit present on each register which stores when an operation has overflowed its current capacity:

mov ax, 0xFFFF
add ax, 1 ; ax = 0x0000 and carry = 1

The carry isn't accessed directly but used as a control structure by other operators, like jc (jump if the carry bit is set)

The BIOS also sets al to the number of sectors read, so always compare it to the expected number.

Code

Open and examine boot_sect_disk.asm for the complete routine that reads from disk.

boot_sect_main.asm prepares the parameters for disk read and calls disk_load. Notice how we write some extra data which does not actually belong to the boot sector, since it is outside the 512 bits mark.

The boot sector is actually sector 1 (the first one, sectors start at 1) of cylinder 0 of head 0 of hdd 0.

Thus, any bytes after byte 512 correspond to sector 2 of cylinder 0 of head 0 of hdd 0

The main routine will fill it with sample data and then let the bootsector read it.

Note: if you keep getting errors and your code seems fine, make sure that qemu is booting from the right drive and set the drive on dl accordingly

The BIOS sets dl to the drive number before calling the bootloader. However, I found some problems with qemu when booting from the hdd.

There are two quick options:

1. Try the flag -fda for example, qemu -fda boot_sect_main.bin which will set dl as 0x00, it seems to work fine then.
2. Explicitly use the flag -boot, e.g. qemu boot_sect_main.bin -boot c which automatically sets dl as 0x80 and lets the bootloader read data