Format of DOS device driver header:
Offset	Size	Description	)
 00h	DWORD	pointer to next driver, offset=FFFFh if last driver
 04h	WORD	device attributes (see #01647,#01648)
 06h	WORD	device strategy entry point
		call with ES:BX -> request header
		  (see #02597 at INT 2F/AX=0802h)
 08h	WORD	device interrupt entry point
---character device---
 0Ah  8 BYTEs	blank-padded character device name
---block device---
 0Ah	BYTE	number of subunits (drives) supported by driver
 0Bh  7 BYTEs	normally unused; sometimes contains signature to indicate
		  specific drivers:
		"$PCMATA"   PCMCIA driver PCMATA.SYS (see AX=440Dh"DOS 3.2+")
		"AHADDVR"   Adaptec SCSI disk driver ASPIDISK.SYS
		"DBLSPAC"   MS DoubleSpace or DriveSpace
		"DSKREET"   NortonUtils v5+ Diskreet (see INT 2F/AX=FE00h)
		"GFS	"   LapLink III device driver DD.BIN
		"SIDExxx"   PCMCIA driver ATADRV.EXE (see AX=440Dh"DOS 3.2+")
		"STAC-CD"   Stacker/Stacker Anywhere (see AX=4404h"Stacker")
 12h  2 BYTEs	signature ",." (2Ch 2Eh)
 14h	var	preloading API entry point
 12h	WORD	reserved, must be 0000h
		appears to be another device chain
 14h	BYTE	drive letter, 01h=A:, etc. (must initially be 00h; this byte
		  is set by MSCDEX when it loads)
 15h	BYTE	number of units
 16h  6 BYTEs	signature 'MSCDnn' where 'nn' is version (currently '00')
		(this field does not exist in most drivers)
SeeAlso: #02550 at INT 25/AX=CDCDh,#02845 at INT 2F/AX=5600h