Unless it yields souce code to peek at,
search engines hardly help.
The
OSDEV wiki has a complete information regarding this matter, if you can follow them in order. So I'm trying to give a favor here(for everyone who keep asking this question over and over again).
All you need before you begin,
1.
El-Torito
2.Int 13h Extension (See
Ralf Brown's Interrupt List)
3.
ISO_9660 Filesystem
As mentioned in the EL-Torito standard, a bootable-CD can be emulated as a
Floppy Disk Image, a
Hard Drive Image or a
Non-emulated Disk. If you prefer to emulate the CD as a Floppy Image, the BIOS emulates the CD as a Floppy Disk and hence, you can read sectors(512 bytes) as if it were actual Floppy Disk. The same applies for the Hard Disk Image. The rest of these post focuses on the
Non-emulation Disk.
To read a sector from the CD(non-emulated), you must use INT13h Extensions(unless you've written a driver). The tricky part on reading the sector is that, the sector size is 2048 bytes long. So you've to locate the kernel in terms of
Blocks. Basically, you've two options: Search the Path Table(ISO 9660 Filesystem) or, hard code the Block value. One advantage of preferring the CD as a boot media is that, each CD Block(sector) is 2048 bytes long meaning that, it gives a lot of room to add extra stuffs to the bootloader itself, which also allows you to add the code for parsing the ISO 9660 Filesystem.
Before you execute Int 13h extension commands, check for the availabilty:
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; Just a code snippet. Refer to 'Ralf Brown's Interrupt List' regarding these fields.
mov ah,0x41
mov bh,0x55
mov bl,0xAA
mov dl,[bootdrv] ; DL contains the drive value provide by the BIOS
int 0x13
cmp bx,0xAA55
jz no_int13h_extension ; Int 13h Extensions not present
Once you confirm the availability of Int 13h Extensions, you can start reading sectors,
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; Just a code snippet. Refer to 'Ralf Brown's Interrupt List' regarding these fields.
mov ah,0x42 ; Extended Read
mov si,DAP_cd ; DS:SI -> Disk Address Packet
mov dl,[bootdrv]
int 0x13
jc Failed ; CF set on Error
DAP_cd:
db 0x10 ; Size of Disk Address Packet
db 0x00 ; Reserved
dw 0 ; No. of blocks to transfer (depends upon the size of the kernel)
dw 0x0000 ; Transfer buffer (Offset)
dw 0x0000 ; Transfer buffer (Segment)
dd 0 ; Starting absolute block no.(LBA) (depends upon the location of the kernel on the disk
dd 0 ; Unused (Only for big LBA > 4bytes)
As you can see, some values need to be obtained by parsing the Filesystem(ISO 9660). If you want to play a little, you can hardcode these values and see if it's actually working. I use MagicISO for burning a 'Non-emulation' disk. When I provide a raw
integrated image of my Bootloader & Kernel(CrOS.img) this file doesn't appear on the PATH Table which obliges me to hardcode these values, so that I at least get to know that it's working. I've been using this integrated image and it works fine. You can follow similar procedure or you can separate the Bootloader and Kernel and so that the bootloader can scan the PATH Table to locate the kernel. Eventually, you'll need to write an ATA driver(for protected mode), anyway.
Ah! one common error I recalled is decoding the block no.
Keep in mind that, the CD sector is 2048 bytes long unlike the 512-byte floppy sector.Also don't assume that the bootloader appears at block 1 and the kernel appears at block 2. In my context, the bootloader and kernel(integrated) is located at block 27.
And at last, before worrying about booting from CD, you should focus on creating a better kernel. Once your kernel crosses the 1 Megabyte boundary(in size), you can then start playing with bootable CDs or, let the GRUB do this job for you.
Hope this helped someone.
Cheers.
