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====== Int 13H, AH=04H ======

===== Version =====

IBM 5150 and higher

===== Brief =====

Verify sector

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

none

===== Input =====

  * AH = 04h
  * AL = number of sectors to verify (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 (PC,XT,AT with BIOS prior to 1985/11/15)

===== Return =====

  * CF set on error
  * CF clear if successful
  * AH = status (see [[00234|status]])
  * AL = number of sectors verified

===== Macro =====

<code asm>
INCLUDE BIOS.INC

@DskVerify  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 (timeout error 80h); the write should be retried at least three times, resetting the disk with AH=00h between attempts
  * on floppys, the operation should also be retried on media change (06h) detection.
  * this function does not compare the disk with memory, it merely checks whether the sector's stored CRC matches the data's actual CRC
  * 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
  * some Epson ROM BIOSes sometimes have problems properly handling this function. The workaround is to reset the disk (INT 13/AH=00h) before the call.

===== Note =====

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

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