Disk Access in Real Mode

[PlayOS]

Hi,

I am having trouble reading the floppy in real mode, I have a routine that I have been using to read the first 17 sectors after the boot sector, however when I put head 1, track 0, sector 1 I get read errors in bochs.

Does anyone have any examples?

thanks.

[beyondsociety]

I do. I used it in my bootsector though, but you shouldn't have any problems with it running in the second stage loader.

Here it is: I have it set up as 17 sectors, sector 2, Head 0

Just change [mov cl,2] to [mov cl,1] and [mov dh,0] to [mov dh,1] and should work for you.

Code: Select all

        mov ah, 02h            ; Read disk sectors function
        mov al, 17             ; Read in 17 sectors
        mov ch, 0               ; Cylinder = 0 
        mov cl, 2               ; Sector = 2 
        mov dh, 0               ; Head = 0 = side one
        mov dl, [bootdrv]       ; disk = what we booted from

I tested it on a real machine, so I haven't tried it on bochs. I've only tested it as I have above in the posted code.

Try it out and see how it works for you. Good luck

[beyondsociety]

By the way:

Code: Select all

mov dl,[bootdrv]  ; The drive we booted from

should be equal to:

Code: Select all

bootdrv db 0       ; Floppy drive A:

[PlayOS]

Hi,

This is what I am doing now and I am not getting what I am supposed to, I know this because if I make my kernel fit within 17 sectors, everything runs fine. But when I exceed this I end up with an error.

It is not the new code because the new code that I add to put it over 17 sectors is a variation of the code that is already working.

Could someone explain exactly how a disk is written to?

I have been told that the disk is written track by track, ie side 0, track 0 then side 1, track 0 then side 0, track 1 then side 1, track 1 etc...

This doesn't sound like a smart way to do it but that is what I was told is done, so does anyone know?

thanks.

[PlayOS]

Well I got it to work, my kernel is being written like this

side 0, track 0
side 0, track 1
etc...

I am using a program called bootcopy so I dont know if it is this program or not but it is now working when I follow this scheme.

Any ideas on the true and correct way to read and write data to disks?

thanks