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64 lines
2.3 KiB
Markdown
64 lines
2.3 KiB
Markdown
*Concepts you may want to Google beforehand: hard disk, cylinder, head, sector,
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carry bit*
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**Goal: Let the bootsector load data from disk in order to boot the kernel**
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Our OS won't fit inside the bootsector 512 bytes, so we need to read data from
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a disk in order to run the kernel.
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Thankfully, we don't have to deal with turning spinning platters on and off,
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we can just call some BIOS routines, like we did to print characters on the screen.
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To do so, we set `al` to `0x02` (and other registers with the required cylinder, head
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and sector) and raise `int 0x13`
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You can access [a detailed int 13h guide here](http://stanislavs.org/helppc/int_13-2.html)
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On this lesson we will use for the first time the *carry bit*, which is an extra bit
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present on each register which stores when an operation has overflowed its current
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capacity:
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```nasm
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mov ax, 0xFFFF
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add ax, 1 ; ax = 0x0000 and carry = 1
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```
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The carry isn't accessed directly but used as a control structure by other operators,
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like `jc` (jump if the carry bit is set)
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The BIOS also sets `al` to the number of sectors read, so always compare it
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to the expected number.
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Code
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----
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Open and examine `boot_sect_disk.asm` for the complete routine that
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reads from disk.
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`boot_sect_main.asm` prepares the parameters for disk read and calls `disk_load`.
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Notice how we write some extra data which does not actually belong to the boot
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sector, since it is outside the 512 bits mark.
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The boot sector is actually sector 1 (the first one, sectors start at 1)
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of cylinder 0 of head 0 of hdd 0.
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Thus, any bytes after byte 512 correspond to sector 2 of cylinder 0 of head 0 of hdd 0
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The main routine will fill it with sample data and then let the bootsector
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read it.
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**Note: if you keep getting errors and your code seems fine, make sure that qemu
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is booting from the right drive and set the drive on `dl` accordingly**
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The BIOS sets `dl` to the drive number before calling the bootloader. However,
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I found some problems with qemu when booting from the hdd.
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There are two quick options:
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1. Try the flag `-fda` for example, `qemu -fda boot_sect_main.bin` which will set `dl`
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as `0x00`, it seems to work fine then.
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2. Explicitly use the flag `-boot`, e.g. `qemu boot_sect_main.bin -boot c` which
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automatically sets `dl` as `0x80` and lets the bootloader read data
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