CODEWITHSUNDEEP
assemblyoperating-systeminterrupt
Brendon Boldt
Brendon Boldt

Reputation: 235

Assembly BIOS Interrupt 13h Disk Timeout

I have been writing a bootloader in NASM and have ran into a problem reading the disk: the disk times-out when I try to read more than one sector:

xor     ax, ax
mov     es, ax
mov     ds, ax
mov     ah, 0
int     0x13      ; Reset disk with int 0x13 ah 0x0

mov     al, 2     ; Read 2 sectors
mov     ah, 0x2   ; int 13 ah 0x2 is read disk
mov     bx, 0x200 ; Load to 0x200
mov     cl, 2     ; Sector 2
mov     ch, 0     ; Cylinder 0
mov     dl, 0     ; Floppy drive 0
mov     dh, 1     ; Head 1
;       LBA is block 19 (root directory of FAT12)
int     0x13

mov     si, bx ; Print first filename to check success

When I use mov cl,1, that is, read one sector instead of two (or more), the program prints the the first filename normally with no errors. Yet when I try to read multiple sector, the filename is not printed normally* and ah contains 0x80 which corresponds to a disk timeout (that is, it is not ready) as well as setting the carry flag. I cannot figure out why this is happening. Thank you.

*string is printed in top right corner of screen--far away from the cursor position

Software: Virtual Floppy Drive + Bochs 2.6.2 (following this tutorial)

Upvotes: 1

Views: 436

Answers (1)

Deleted User
Deleted User

Reputation: 2541

I think you're overwriting the BIOS data area in RAM, by putting your disk buffer too low, on 0000:0200. That could also explain why output goes to a wrong screen postion - the cursor position is stored in there too. All memory at and above address 0000:0500 can be used by the boot loader, so try to set the pointer a bit higher.

I notice that you're ah with 0 on line 4 while it was zeroed already on line 1. Seems unnecessary. Not that it saves a lot, but two bytes are two bytes ...

Upvotes: 2

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