INT 0x13 - 0x03 - Write Disk Sector(s)

Request/Call:

reg	value	descr
AH	0x03
AL	var	number of sectors to write (1-128 dec.)
CH	var	track/cylinder number (0-1023 dec., see below)
CL	var	sector number (1-17 dec.)
DH	var	head number (0-15 dec.)
DL	var	drive number (0=A:, 1=2nd floppy, 80h=drive 0, 81h=drive 1)
ES:BX	var	pointer to buffer

reg	descr
CF	0 : succesfull 1 : error
AH	status (see INT 13,01)
AL	number of sectors transferred

errors on a floppy may be due to the motor failing to spin up quickly enough; the write should be retried at least three times, resetting the disk with AH=00h between attempts
AWARD AT BIOS extended to handle more than 1024 cylinders by placing bits 10 and 11 of the cylinder number into bits 6 and 7 of DH
the parameters in CX change depending on the number of cylinders
the track/cylinder number is a 10 bit value taken from the 2 high order bits of CL and the 8 bits in CH (low order 8 bits of track):

bit	descr
0-5	sector number
6-7	high order 2 bits of track/cylinder
8-15	low order 8 bits of track/cyl number