INT 13,4 - Verify Disk Sectors

	AH = 04
	AL = number of sectors to verify  (1-128 dec.)
	CH = track/cylinder number  (0-1023 dec., see below)
	CL = sector number  (1-17 dec.)
	DH = head number  (0-15 dec.)
	DL = drive number (0=A:, 1=2nd floppy, 80h=drive 0, 81h=drive 1)
	ES:BX = pointer to buffer

	on return:
	AH = status  (see INT 13,STATUS)
	AL = number of sectors verified
	CF = 0 if successful
	   = 1 if error


	- BIOS disk reads should be retried at least three times and the
	  controller should be reset upon error detection
	- causes controller to calculate the CRC of the disk data and
	  compare it against the CRC stored in the sector header
	- BIOS before 11/15/85 required ES:BX point to a valid buffer
	  that doesn't cross DMA boundaries.   More recent BIOS versions
	  actually ignore the buffer and the DMA boundary requirement
	- use this function to check for valid formatted diskette in a
	  the specified drive and for drive ready for read
	- only the disk number is checked for validity
	- 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):

	  |F|E|D|C|B|A|9|8|7|6|5-0|  CX
	   | | | | | | | | | |	`-----	sector number
	   | | | | | | | | `---------  high order 2 bits of track/cylinder
	   `------------------------  low order 8 bits of track/cyl number

	- see	DETECTING