Format of DOS device driver header:
Offset  Size    Description     (Table 01646)
 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")
---DoubleSpace/DriveSpace---
 12h  2 BYTEs   signature ",." (2Ch 2Eh)
 14h    var     preloading API entry point
---CD-ROM---
 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

Bitfields for device attributes (character device):
Bit(s)  Description     (Table 01647)
 15     set (indicates character device)
 14     IOCTL supported (see AH=44h)
 13     (DOS 3.0+) output until busy supported
 12     reserved
 11     (DOS 3.0+) OPEN/CLOSE/RemMedia calls supported
 10-8   reserved
 7      (DOS 5.0+) Generic IOCTL check call supported (driver command 19h)
        (see AX=4410h,AX=4411h)
 6      (DOS 3.2+) Generic IOCTL call supported (driver command 13h)
        (see AX=440Ch,AX=440Dh"DOS 3.2+")
 5      reserved
 4      device is special (use INT 29 "fast console output")
 3      device is CLOCK$ (all reads/writes use transfer record described
          below)
 2      device is NUL
 1      device is standard output
 0      device is standard input
Note:   for European MS-DOS 4.0, bit 11 also indicates that bits 8-6 contain a
          version code (000 = DOS 3.0,3.1; 001 = DOS 3.2;
          010 = European DOS 4.0)
SeeAlso: #01648,#01646

Bitfields for device attributes (block device):
Bit(s)  Description     (Table 01648)
 15     clear (indicates block device)
 14     IOCTL supported
 13     non-IBM format
 12     network device (device is remote)
 11     (DOS 3.0+) OPEN/CLOSE/RemMedia calls supported
 10     reserved
 9      direct I/O not allowed??? (set by DOS 3.3 DRIVER.SYS for "new" drives)
 8      ??? set by DOS 3.3 DRIVER.SYS for "new" drives
 7      (DOS 5.0+) Generic IOCTL check call supported (driver command 19h)
        (see AX=4410h,AX=4411h)
 6      (DOS 3.2+) Generic IOCTL call supported (driver command 13h)
        implies support for commands 17h and 18h
          (see AX=440Ch,AX=440Dh"DOS 3.2+",AX=440Eh,AX=440Fh)
 5-2    reserved
 1      driver supports 32-bit sector addressing (DOS 3.31+)
 0       reserved
Note:   for European MS-DOS 4.0, bit 11 also indicates that bits 8-6 contain a
          version code (000 = DOS 3.0,3.1; 001 = DOS 3.2;
          010 = European DOS 4.0)
SeeAlso: #01647,#01646