Int 13H, AH=04H

IBM 5150 and higher

Verify sector

none

• 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)

• CF set on error
• CF clear if successful
• AH = status (see status)
• AL = number of sectors verified

INCLUDE BIOS.INC
 
@DskVerify  0, 0, 0, 1, 1, offset buf

• 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.

Text based on Ralf Brown Interrupt List Release 61