Automatically called upon detection of unrecoverable
   disk error. Normally prints "Abort, Retry, or Ignore?" message
   and takes the reply, but may be changed if desired.

Provides the following values in registers on entry to interrupt handler:
        AH: bit 7 = 0 disk I/O error
                  = 1 other error -- if block device, bad FAT
                                  -- if char device, code in DI
            bit 6  unused
            bit 5 = 1 if Ignore allowed, 0 if not (DOS 3.2?)
            bit 4 = 1 if Retry allowed, 0 if not (DOS 3.2?)
            bit 3 = 1 if Fail allowed, 0 if not (DOS 3.2+)
            bit 2 \ disk area of error  00 = DOS area  01 = FAT
            bit 1 /                     10 = root dir  11 = data area
            bit 0 = 1 if write, 0 if read
        AL = drive number if AH bit 7 = 1, otherwise undefined
        BP:SI = address of device header for which error occurred
            block device if high bit of BP:SI+4 = 1
        low byte of DI:
           00h write-protect error
           01h unknown unit
           02h drive not ready
           03h unknown command
           04h data error (bad CRC)
           05h bad request structure length
           06h seek error
           07h unknown media type
           08h sector not found
           09h printer out of paper
           0Ah write fault
           0Bh read fault
           0Ch general failure
           0Fh invalid disk change (DOS 3.x)
Handler must return
        AL = 00 ignore error
           = 01 retry operation
           = 02 terminate program through INT 22h
           = 03 fail system call in progress (DOS 3.2+)
-----------------------------------------------------------