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====== Int 13H, AH=03H ======

===== Version =====

IBM 5150 and higher

===== Brief =====

Write sector

===== Family API =====

[[en:docs:fapi:DosWrite]]

===== Input =====

  * AH = 03h
  * AL = number of sectors to write (must be nonzero)
  * CH = low eight bits of cylinder number
  * CL = sector number 1-63 (bits 0-5) high two bits of cylinder (bits 6-7, hard disk only)
  * DH = head number
  * DL = drive number (bit 7 set for hard disk)
  * ES:BX -> data buffer

===== Return =====

  * CF set on error
  * CF clear if successful
  * AH = status (see [[00234|status]])
  * AL = number of sectors transferred (only valid if CF set for some BIOSes)

===== Macro =====

<code asm>
INCLUDE BIOS.INC

@DskWrite  0, 0, 0, 1, 1, offset buf
</code>

===== Notes =====

  * 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
  * most BIOSes support "multitrack" writes, where the value in AL exceeds the number of sectors remaining on the track, in which case any additional sectors are written beginning at sector 1 on the following head in the same cylinder; the CONFIG.SYS command MULTITRACK can be used to force DOS to split disk accesses which would wrap across a track boundary into two separate calls
  * the IBM AT BIOS and many other BIOSes use only the low four bits of DH (head number) since the WD-1003 controller which is the standard AT controller (and the controller that IDE emulates) only supports 16 heads
  * AWARD AT BIOS and AMI 386sx BIOS have been extended to handle more than 1024 cylinders by placing bits 10 and 11 of the cylinder number into bits 6 and 7 of DH
  * under Windows95, an application must issue a physical volume lock on the drive via INT 21/AX=440Dh before it can successfully write to the disk with this function

===== Note =====

Text based on [[http://www.cs.cmu.edu/~ralf/files.html|Ralf Brown Interrupt List Release 61]]

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