Interrupt 25h	Absolute Disk Read
                (except DOS 4.0/Compaq DOS 3.31 32M partition (see next page))
entry   AL      drive number (0=A, 1=B, etc) 
	DS:BX   pointer to Disk Transfer Address (buffer) 
	CX      number of sectors to read 
	DX      first relative sector to read 
return  CF      1 if error 
                AL      error code issued to Int\24 in low half of DI
		AH      80h     attachment failed to respond 
			40h     seek operation failed 
			20h     controller failed 
			10h     data error (bad CRC) 
			08h     DMA failure 
			04h     requested sector not found 
			03h     write-protected disk 
			02h     bad address mark 
			01h     bad command 
note    Original flags on stack! 
 
Interrupt 25h   DOS 4.0/Compaq DOS 3.31 - ABSOLUTE DISK READ 
		(32M hard-disk partition) 
entry   AL      drive number (0=A, 1=B, etc) 
	CX      0FFFFh 
	DS:BX   Packet address 
		DWORD  sector number 
		WORD   number of sectors to read 
		DWORD  transfer address 
return  same as above? 
note    Partition is potentially 32Mb (and requires this form of the call) if 
	bit 1 of struct -device attribute code in device\drivers -header is
	set
Interrupt 25h	Absolute Disk Read
		Sample Code	  Ref PC Magazine Vol 9, No 20, Pg 498
   This handy little program returns the status of any floppy disk drive as the
error level in a batch file.
DriveStat:
         MOV AL,[5C]     ;Get first char of command line
	 AND AL,5F	 ;Capitalize it
	 SUB AL,41	 ;Set to 0 for A:, 1 for B:, etc
	 XOR CX,CX
	 INC CX 	 ;Set CX to 0001 (Read one sector)
	 XOR DX,DX	 ;Set DX to 0000 (Start reading at sector zero)
	 MOV BX,BufAddr  ;Set BC to Address of a fake buffer one sector long
	 INT 25 	 ;Call BIOSDiskReadAbs
	 MOV AH,4C	 ;Set AH to 4C (DOSTermProcWErrLvl)
	 INT 21 	 ;Call DOS