Working With Disk Protection (INT 13h)

INT 13h copy protection schemes are hardly ever used anymore. Nevertheless, it would be good practice for someone to learn how to create and understand the code. Most likely, INT 13h is used with function 2, read sector. This means that:

Register Description

AH will contain the number 2 (function 2)

AL the number of sectors to read in. This is commonly 1 since you just want to check a few sectors for disk validity.

CH will contain the cylinder number

CL will contain the sector number

DH will contain the head number

DL will contain the drive number 00h - 7Fh for floppies 80h - FFh for fixed disks

ES:BX will point to the address into which the data read from the disk will be written to

Also see:
Introduction to disk protection
Advanced discussion

Return

Register	Description
AH	will contain the number 2 (function 2)
AL	the number of sectors to read in. This is commonly 1 since you just want to check a few sectors for disk validity.
CH	will contain the cylinder number
CL	will contain the sector number
DH	will contain the head number
DL	will contain the drive number 00h - 7Fh for floppies 80h - FFh for fixed disks
ES:BX	will point to the address into which the data read from the disk will be written to