Extended int 13h

[AeroASM]

Is this code snippet right?

Code Select Expand


;LBA in esi, destination address in di, number of sectors in cx
ReadSectors:
mov [DiskAddressPacket+2h],cx
mov [DiskAddressPacket+4h],di
mov [DiskAddressPacket+8h],esi
mov ah,42h
mov dl,[bBootDrive]
mov si,DiskAddressPacket
int 13h
ret
DiskAddressPacket:
db 10h
db 0h
dw 0h
dd 0h
dq 0h


The reason I have not tested it myself is that my computer takes ages to reboot and I am also in the middle of typing a coursework essay.

[MichaelW]

It looks OK as far as it goes, but it does not check the carry flag on return (CF set on error). And to reliably access a diskette (or other drive that may not be spinning when the call is made) you should make several tries, resetting the disk between tries, before you accept an error return.

[AeroASM]

Is three tries enough for floppy and one try for HDD? Also the bit I really wanted to check was that when writing a far pointer ds:di you first write di, then ds.

Thanks

[MichaelW]

I code for four tries and use the same code for all disks. I differentiate between HDD and floppy only for the code that resets the disk. You need to check the error return for a HDD anyway, so using the same code for both does not slow anything down. The function expects a far pointer to the disk address packet in DS:SI so the proper order of the address components in memory does not matter here. For storage in memory the segment should be in the high-order word, so if you were passing a far pointer on the stack you would push the segment first, followed by the offset.