Category: BIOS


	AH = 08h
	DL = drive (bit 7 set for hard disk)
	ES:DI = 0000h:0000h to guard against BIOS bugs
Return: CF set on error
	    AH = status (07h) (see #00234)
	CF clear if successful
	    AH = 00h
	    AL = 00h on at least some BIOSes
	    BL = drive type (AT/PS2 floppies only) (see #00242)
	    CH = low eight bits of maximum cylinder number
	    CL = maximum sector number (bits 5-0)
		 high two bits of maximum cylinder number (bits 7-6)
	    DH = maximum head number
	    DL = number of drives
	    ES:DI -> drive parameter table (floppies only)
Notes:	may return successful even though specified drive is greater than the
	  number of attached drives of that type (floppy/hard); check DL to
	  ensure validity
	for systems predating the IBM AT, this call is only valid for hard
	  disks, as it is implemented by the hard disk BIOS rather than the
	the IBM ROM-BIOS returns the total number of hard disks attached
	  to the system regardless of whether DL >= 80h on entry.
	Toshiba laptops with HardRAM return DL=02h when called with DL=80h,
	  but fail on DL=81h.  The BIOS data at 40h:75h correctly reports 01h.
	may indicate only two drives present even if more are attached; to
	  ensure a correct count, one can use AH=15h to scan through possible
	Reportedly some Compaq BIOSes with more than one hard disk controller
	  return only the number of drives DL attached to the corresponding
	  controller as specified by the DL value on entry.  However, on
	  Compaq machines with "COMPAQ" signature at F000h:FFEAh,
	  MS-DOS/PC DOS IO.SYS/IBMBIO.COM call INT 15/AX=E400h and 
	  INT 15/AX=E480h to enable Compaq "mode 2" before retrieving the count
	  of hard disks installed in the system (DL) from this function.
	the maximum cylinder number reported in CX is usually two less than
	  the total cylinder count reported in the fixed disk parameter table
	  (see INT 41h,INT 46h) because early hard disks used the last cylinder
	  for testing purposes; however, on some Zenith machines, the maximum
	  cylinder number reportedly is three less than the count in the fixed
	  disk parameter table.
	for BIOSes which reserve the last cylinder for testing purposes, the
	  cylinder count is automatically decremented
	on PS/1s with IBM ROM DOS 4, nonexistent drives return CF clear,
	  BX=CX=0000h, and ES:DI = 0000h:0000h
	machines with lost CMOS memory may return invalid data for floppy
	  drives. In this situation CF is cleared, but AX,BX,CX,DX,DH,DI,
	  and ES contain only 0.  At least under some circumstances, MS-DOS/
	  PC DOS IO.SYS/IBMBIO.COM just assumes a 360 KB floppy if it sees
	  CH to be zero for a floppy.
	the PC-Tools PCFORMAT program requires that AL=00h before it will
	  proceed with the formatting
	if this function fails, an alternative way to retrieve the number
	  of floppy drives installed in the system is to call INT 11h.
	In fact, the MS-DOS/PC-DOS IO.SYS/IBMBIO.COM attempts to get the
	  number of floppy drives installed from INT 13/AH=08h, when INT 11h
	  AX bit 0 indicates there are no floppy drives installed. In addition
	  to testing the CF flag, it only trusts the result when the number of
	  sectors (CL preset to zero) is non-zero after the call.
BUGS:	several different Compaq BIOSes incorrectly report high-numbered
	  drives (such as 90h, B0h, D0h, and F0h) as present, giving them the
	  same geometry as drive 80h; as a workaround, scan through disk
	  numbers, stopping as soon as the number of valid drives encountered
	  equals the value in 0040h:0075h
	a bug in Leading Edge 8088 BIOS 3.10 causes the DI,SI,BP,DS, and ES
	  registers to be destroyed
	some Toshiba BIOSes (at least before 1995, maybe some laptops???
	  with 1.44 MB floppies) have a bug where they do not set the ES:DI
	  vector even for floppy drives. Hence these registers should be
	  preset with zero before the call and checked to be non-zero on
	  return before using them.  Also it seems these BIOSes can return
	  wrong info in BL and CX, as S/DOS 1.0 can be configured to preset
	  these registers as for an 1.44 MB floppy.
	the PS/2 Model 30 fails to reset the bus after INT 13/AH=08h and
	  INT 13/AH=15h. A workaround is to monitor for these functions
	  and perform a transparent INT 13/AH=01h status read afterwards.
	  This will reset the bus. The MS-DOS 6.0 IO.SYS takes care of
	  this by installing a special INT 13h interceptor for this purpose.
	AD-DOS may leave interrupts disabled on return from this function.
	Some Microsoft software explicitly sets STI after return.
SeeAlso: AH=06h"Adaptec",AH=13h"SyQuest",AH=48h,AH=15h,INT 1E
SeeAlso: INT 41"HARD DISK 0"