How to read some sectors from CD?

[donggas90]

I'm trying to create a booting CD for my OS.
Already made a bootable ISO and launched my booting codes successfully.
but I don't know how to load my kernel and system files.

Code: Select all

org 0x00
bits 16

jmp Main


;=======================================================================
PrintString16:
	lodsb
	or al, al
	jz _PrintString16Return
	mov ah, 0x0E
	int 0x10
	jmp PrintString16
	_PrintString16Return:
		ret


Main:
	cli
	mov ax, 0x07C0
	mov ds, ax
	mov es, ax
	mov fs, ax
	mov gs, ax

	xor ax, ax
	mov ss, ax
	not ax
	mov sp, ax
	sti

	mov si, StartMsg
	call PrintString16
	cli
	hlt

;=======================================================================
StartMsg db "Hello, Bootable CD!", 13, 10, 0


times 0x800 - 2 - ($ - $$ ) db 0
dw 0xAA55

I used INT 13 for previous booting disk that was floppy. but cannot sure it works for CD.
because my ISO NOT using Floppy emulation.

Is it work in my case?
or
Is there any tutorial for this problem?

[Combuster]

Oh my.

Is there any tutorial for this problem?

Welcome to the Beginner Mistakes.

I used INT 13 for previous booting disk that was floppy. but cannot sure it works for CD.

Have you gone looking for the official specification yet like we would have expected from any person asking a question? This particular question is answered on page 16, and I'm sure many of your subsequent questions will be answered somewhere else in that PDF file as well.

[donggas90]

Oh, I did mistakes.
but my mean was just want to see some series of simple examples.
Not lectures what you thinking.

BTW I already searched many docs of this topic, but they do not told exactly so I couldn't understand how to do.
So I wrote this thread.

Yes, I'm a beginner of OS development, but NOT a beginner of programming.
Thus just you can say a keyword or showing some of codes, just link of docs or other answers, that's OK.
I don't want detailed informations like class of professor as well, just want a few clues or examples.

I'll just attempt to use INT 13 what suspected, I was too timided to try that.
If it fail, I will post again.

Thanks.

[max]

BTW I already searched many docs of this topic, but they do not told exactly so I couldn't understand how to do.

Use the specification Combuster linked you to. It will tell you exactly whats necessary. If you have a specific question of what you don't understand, then come back.

[donggas90]

Thanks max.

Finally I got a clue that INT 13 works for CD from here.
In that thread, m99 used 0x9F as disk number. It worked for me.
but quok pointed out that assuming disk number as 0x9F can occur problem. I agree this opinion.
So I checked 0x11 sector is Boot Record Descriptor for each disk number from 0x81 to 0xFF.
because El Torito doc says, disk number of no emulation CD is 0x81-0xFF and Boot Record Descriptor always place in 0x11 sector.
but I think it's ineffecient method. because a sector of CD is 2KB size, so that occur too many overhead just to read several bytes.
I also found some extended INT 13s for CD. but they need disk number, cannot get.
So I want to know about this. Is there a way to get disk number of my CD directly?

[Octocontrabass]

Is there a way to get disk number of my CD directly?

When you boot from a floppy disk, hard disk, USB flash disk, or CD, the disk number is in DL.

[donggas90]

the disk number is in DL.

Wow, thanks.

[donggas90]

I've been returned at ZERO.
INT 13 function 02 wasn't work as predicted.
I expected that loads 0x800bytes per sector but 512bytes.
and Sector 1 is not offset 0 of ISO file. but 0xA000 that is start of Terminate Descriptor + 0x800.

I tried INT 13 function 00 to reset the offset of Sector 1 to zero. but didn't work.
Thus I cannont read Primary Descriptor so cannot read Root Directory Record to search my kernel file.

My ISO is not emulated. and my current testing machine is VMware.

Here is my codes.

Code: Select all

mov byte[DiskNumber], dl

xor ah, ah
int 0x13

mov al, 1
xor ch, ch
mov cl, ISO_PRIMARY_VOLUME_DESCRIPTOR_SECTOR ;0x11
xor dh, dh
push es
push ROOT_DIRECTORY_RECORD_SEGMENT ;0x02E0
pop es
xor bx, bx
call ReadSector
jc Fail

I traced bytes from es:bx and tested some different cases.
Then I recognized that loaded ISO offset is 0xC000 and sector size is 512, Sector 1 offset is 0xA000.

0xC000 = 0xA000 + 512 * 0x10

What can I do for this problem?

[Combuster]

I'm sure many of your subsequent questions will be answered somewhere else in that PDF file as well.

After the boot process has been initiated the INT 13 Extensions (functions 41-48) will access the CD

INT 13 function 02

Being able to read might be a lost art, but it's certainly an important one

[donggas90]

Thanks Combuster.

OMG, that was the point!
I didn't mind that line bacause my manual of BIOS(General Software, Inc.) is not listed that functions.

Finally I got the data of ISO successfully!