Int 13h question

[thesame]

A friend asked me to post this question:

Can anyone explain me why the 13h interrupt needs a sector-number less than 13?
For example:

Code: Select all

mov ah, 0x02          
mov al, 0x08          
mov ch, 0x00           
mov cl, 0x12            ; Sector 12
mov dh, 0x00             
mov dl, 0x00
int 0x13

That code works but

Code: Select all

mov ah, 0x02          
mov al, 0x08          
mov ch, 0x00           
mov cl, 0x13            ; Sector 13
mov dh, 0x00             
mov dl, 0x00
int 0x13

causes an error (drive not ready)

ts.

[Slasher]

hi,
0x12 is hex for 18 (1*(16^1)+2*(16^0))!!!
0x13 is therefore 19(1*(16^1)+3*(16^0))!!!

1.44 floppys have sectors from 1 to 18!!!