-------------------------------------------------------------------------------
INT 21 - CD-ROM device driver - IOCTL INPUT
	AX = 4402h
	BX = file handle referencing character device for CD-ROM driver
	CX = number of bytes to read
	DS:DX -> control block (see #0582)
Return: CF clear if successful
	    AX = number of bytes actually read
	CF set on error
	    AX = error code (01h,05h,06h,0Dh) (see #0789 at AH=59h)
Note:	the data returned depends on the first byte of the control block; the
	  remainder of the control block is filled by the driver
SeeAlso: AX=4403h"CD-ROM",INT 2F/AX=0802h

(Table 0581)
Values for CD-ROM data being requested:
 00h	device driver header address
 01h	drive head location
 02h	reserved
 03h	error statistics
 04h	audio channel info
 05h	raw drive bytes (uninterpreted and device-specific)
 06h	device status
 07h	sector size
 08h	volume size
 09h	media change status
 0Ah	audio disk info
 0Bh	audio track info
 0Ch	audio Q-Channel info
 0Dh	audio sub-channel info
 0Eh	UPC code
 0Fh	audio status info

Format of CD-ROM control block:
Offset	Size	Description	(Table 0582)
 00h	BYTE	data being requested (see #0581)
---function 00h---
 01h	DWORD	device driver header address (see also AH=52h)
---function 01h---
 01h	BYTE	addressing mode
		00h HSG
		01h Red Book
 02h	DWORD	current location of drive's head
		logical sector number in HSG mode
		frame/second/minute/unused in Red Book mode
		(HSG sector = minute * 4500 + second * 75 + frame - 150)
---function 03h---
 01h  N BYTEs	undefined as of 5 Aug 88 specification
---function 04h---
 01h	BYTE	input channel (0-3) for output channel 0
 02h	BYTE	volume for output channel 0
 03h	BYTE	input channel (0-3) for output channel 1
 04h	BYTE	volume for output channel 1
 05h	BYTE	input channel (0-3) for output channel 2
 06h	BYTE	volume for output channel 2
 07h	BYTE	input channel (0-3) for output channel 3
 08h	BYTE	volume for output channel 3
Notes:	output channels 0 and 1 are left and right, 2 and 3 are left prime and
	  right prime; a volume of 00h is off
	the default setting is for each input channel to be assigned to the
	  same-numbered output channel at full (FFh) volume
---function 05h---
 01h	BYTE	number of bytes read
 02h 128 BYTEs	buffer for drive bytes
---function 06h---
 01h	DWORD	device parameters (see #0583)
---function 07h---
 01h	BYTE	read mode
		00h cooked
		01h raw
 02h	WORD	sector size in bytes
---function 08h---
 01h	DWORD	volume size in sectors
---function 09h---
 01h	BYTE	media change status
		00h don't know
		01h media unchanged
		FFh media has been changed
---function 0Ah---
 01h	BYTE	lowest audio track number
 02h	BYTE	highest audio track number
 03h	DWORD	start address of lead-out track (Red Book format)
--function 0Bh---
 01h	BYTE	track number (set by caller)
 02h	DWORD	starting point of track (Red Book format)
 06h	BYTE	track control info
		bits 15,14,12: track type (notice: bits not contiguous!)
			000 two audio channels, no pre-emphasis
			001 two audio channels with pre-emphasis
			010 data track
			100 four audio channels, no pre-emphasis
			101 four audio channels with pre-emphasis
			other reserved
		bit 13: digital copy permitted
---function 0Ch---
 01h	BYTE	CONTROL and ADR byte (as received from drive)
 02h	BYTE	track number
 03h	BYTE	point or index
 04h	BYTE	minute	\
 05h	BYTE	second	 > running time within track
 06h	BYTE	frame	/
 07h	BYTE	zero
 08h	BYTE	"AMIN" or "PMIN"     \
 09h	BYTE	"ASEC" or "PSEC"      > running time on disk
 0Ah	BYTE	"AFRAME" or "PFRAME" /
---function 0Dh---
 01h	DWORD	starting frame address (Red Book format)
 05h	DWORD	transfer address
 09h	DWORD	number of sectors to read
Note:	copies 96 bytes of sub-channel info per sector into buffer
---function 0Eh---
 01h	BYTE	CONTROL and ADR byte
 02h  7 BYTEs	UPC/EAN code (13 BCD digits,low-order nybble of last byte is 0)
 09h	BYTE	zero
 0Ah	BYTE	"AFRAME"
---function 0Fh---
 ??? documentation not yet available
 01h	WORD	pause status (0000h not paused, 0001h paused)
 03h	DWORD	audio play start address
 07h	DWORD	??? audio play length or end address

Bitfields for CD-ROM device parameters:
Bit(s)	Description	(Table 0583)
 0	door open
 1	door unlocked
 2	supports raw reading in addition to cooked
 3	writable
 4	can play audio/video tracks
 5	supports interleaving
 6	reserved
 7	supports prefetch requests
 8	supports audio channel control
 9	supports Red Book addressing in addition to HSG
 10	audio is playing
 11	no disk in drive
 12	supports R-W subchannels
-------------------------------------------------------------------------------
INT 21 - CD-ROM device driver - IOCTL OUTPUT
	AX = 4403h
	BX = file handle referencing character device for CD-ROM driver
	CX = number of bytes to write
	DS:DX -> control block (see #0651)
Return: CF clear if successful
	    AX = number of bytes actually written
	CF set on error
	    AX = error code (01h,05h,06h,0Dh) (see #0789 at AH=59h)
SeeAlso: AX=4402h"CD-ROM",INT 2F/AX=0802h

Format of CR-ROM control block:
Offset	Size	Description	(Table 0651)
 00h	BYTE	function code
		00h eject disk
		01h lock/unlock door
		02h reset drive
		03h control audio channel
		04h write device control string
		05h close tray
---functions 00h,02h,05h---
 no further fields
---function 01h---
 01h	BYTE	lock function
		00h unlock door
		01h lock door
---function 03h---
 01h	BYTE	input channel (0-3) for output channel 0
 02h	BYTE	volume for output channel 0
 03h	BYTE	input channel (0-3) for output channel 1
 04h	BYTE	volume for output channel 1
 05h	BYTE	input channel (0-3) for output channel 2
 06h	BYTE	volume for output channel 2
 07h	BYTE	input channel (0-3) for output channel 3
 08h	BYTE	volume for output channel 3
Note:	output channels 0 and 1 are left and right, 2 and 3 are left prime and
	  right prime; a volume of 00h is off
---function 04h---
 01h  N BYTEs	bytes to send directly to the CD-ROM drive without
		  interpretation
-------------------------------------------------------------------------------
INT 2F - MSCDEX (MS CD-ROM Extensions) - INSTALLATION CHECK
	AX = 1100h subfn DADAh
	STACK: WORD DADAh
Return: AL = status
	    00h not installed, OK to install
		STACK unchanged
	    01h not installed, not OK to install
		STACK unchanged
	    FFh installed
		STACK: WORD	ADADh if MSCDEX installed
				DADBh if Lotus CD/Networker installed
Note:	although MSCDEX sets the stack word to ADADh on return, any value other
	  than DADAh is considered to mean that MSCDEX is already installed;
	  Lotus CD/Networker v4+ uses this feature to fool MSCDEX into
	  thinking it is already installed when it is in fact CD/Networker
	  that is installed
-------------------------------------------------------------------------------
INT 2F - CD-ROM - INSTALLATION CHECK
	AX = 1500h
	BX = 0000h
Return: BX = number of CD-ROM drive letters used
	CX = starting drive letter (0=A:)
Notes:	this installation check DOES NOT follow the format used by other
	  software
	this installation check conflicts with the DOS 4.00 GRAPHICS.COM
	  installation check
SeeAlso: INT 2F/AX=D000h/BX=4D44h"Lotus"
-------------------------------------------------------------------------------
INT 2F - CD-ROM - GET DRIVE DEVICE LIST
	AX = 1501h
	ES:BX -> buffer to hold drive letter list (5 bytes per drive letter)
Return: buffer filled, for each drive letter
	  BYTE	subunit number in driver
	  DWORD address of device driver header
-------------------------------------------------------------------------------
INT 2F - CD-ROM - GET COPYRIGHT FILE NAME
	AX = 1502h
	ES:BX -> 38-byte buffer for name of copyright file
	CX = drive number (0=A:)
Return: CF set if drive is not a CD-ROM drive
	    AX = 000Fh (invalid drive)
	CF clear if successful
SeeAlso: AX=1503h
-------------------------------------------------------------------------------
INT 2F - CD-ROM - GET ABSTRACT FILE NAME
	AX = 1503h
	ES:BX -> 38-byte buffer for name of abstract file
	CX = drive number (0=A:)
Return: CF set if drive is not a CD-ROM drive
	    AX = 000Fh (invalid drive)
	CF clear if successful
SeeAlso: AX=1502h,AX=1504h
-------------------------------------------------------------------------------
INT 2F - CD-ROM - GET BIBLIOGRAPHIC DOC FILE NAME
	AX = 1504h
	ES:BX -> 38-byte buffer for name of bibliographic documentation file
	CX = drive number (0=A:)
Return: CF set if drive is not a CD-ROM drive
	    AX = 000Fh (invalid drive)
	CF clear if successful
SeeAlso: AX=1502h,AX=1503h
-------------------------------------------------------------------------------
INT 2F - CD-ROM - READ VTOC
	AX = 1505h
	ES:BX -> 2048-byte buffer
	CX = drive number (0=A:)
	DX = sector index (0=first volume descriptor,1=second,...)
Return: CF set on error
	    AX = error code (15=invalid drive,21=not ready)
	CF clear if successful
	    AX = volume descriptor type (1=standard,FFh=terminator,0=other)
-------------------------------------------------------------------------------
INT 2F - CD-ROM - TURN DEBUGGING ON
	AX = 1506h
	BX = debugging function to enable
Note:	reserved for development
SeeAlso: AX=1507h
-------------------------------------------------------------------------------
INT 2F - CD-ROM - TURN DEBUGGING OFF
	AX = 1507h
	BX = debugging function to disable
Note:	reserved for development
SeeAlso: AX=1506h
-------------------------------------------------------------------------------
INT 2F - CD-ROM - ABSOLUTE DISK READ
	AX = 1508h
	ES:BX -> buffer
	CX = drive number (0=A:)
	SI:DI = starting sector number
	DX = number of sectors to read
Return: CF set on error
	    AL = error code (15=invalid drive,21=not ready)
	CF clear if successful
SeeAlso: AX=1509h
-------------------------------------------------------------------------------
INT 2F - CD-ROM - ABSOLUTE DISK WRITE
	AX = 1509h
	ES:BX -> buffer
	CX = drive number (0=A:)
	SI:DI = starting sector number
	DX = number of sectors to write
Note:	corresponds to INT 26h and is currently reserved and nonfunctional
SeeAlso: AX=1508h
-------------------------------------------------------------------------------
INT 2F - CD-ROM - RESERVED
	AX = 150Ah
-------------------------------------------------------------------------------
INT 2F - CD-ROM v2.00+ - DRIVE CHECK
	AX = 150Bh
	CX = drive number (0=A:)
Return: BX = ADADh if MSCDEX.EXE installed
	    AX = support status
		0000h if drive not supported
		nonzero if supported
SeeAlso: AX=150Dh
-------------------------------------------------------------------------------
INT 2F - CD-ROM v2.00+ - GET MSCDEX.EXE VERSION
	AX = 150Ch
Return: BH = major version
	BL = minor version
Note:	MSCDEX.EXE versions prior to 2.00 return BX=0
-------------------------------------------------------------------------------
INT 2F - CD-ROM v2.00+ - GET CD-ROM DRIVE LETTERS
	AX = 150Dh
	ES:BX -> buffer for drive letter list (1 byte per drive)
Return: buffer filled with drive numbers (0=A:).  Each byte corresponds
	to the drive in the same position for function 1501h
SeeAlso: AX=150Bh
-------------------------------------------------------------------------------
INT 2F - CD-ROM v2.00+ - GET/SET VOLUME DESCRIPTOR PREFERENCE
	AX = 150Eh
	BX = subfunction
	    00h get preference
		DX = 0000h
		Return: DX = preference settings
	    01h set preference
		DH = volume descriptor preference
		    01h = primary volume descriptor
		    02h = supplementary volume descriptor
		DL = supplementary volume descriptor preference
		    01h = shift-Kanji
	CX = drive number (0=A:)
Return: CF set on error
	    AX = error code (15=invalid drive,1=invalid function)
	CF clear if successful
-------------------------------------------------------------------------------
INT 2F - CD-ROM v2.00+ - GET DIRECTORY ENTRY
	AX = 150Fh
	CL = drive number (0=A:)
	CH bit 0 = copy flag
		clear if direct copy
		set if copy to structure which removes ISO/High Sierra diffs
	ES:BX -> ASCIZ path name
	SI:DI -> buffer for directory entry (see #1289)
		 minimum 255 bytes for direct copy
Return: CF set on error
	    AX = error code
	CF clear if successful
	    AX = disk format (0=High Sierra,1=ISO 9660)

Format of CD-ROM directory entry (direct copy):
Offset	Size	Description	(Table 1289)
 00h	BYTE  length of directory entry
 01h	BYTE  length of XAR in Logical Block Numbers
 02h	DWORD LBN of data, Intel (little-endian) format
 06h	DWORD LBN of data, Motorola (big-endian) format
 0Ah	DWORD length of file, Intel format
 0Eh	DWORD length of file, Motorola format
---High Sierra---
 12h  6 BYTEs date and time
 18h	BYTE  bit flags
 19h	BYTE  reserved
---ISO 9660---
 12h  7 BYTEs date and time
 19h	BYTE  bit flags
---both formats---
 1Ah	BYTE  interleave size
 1Bh	BYTE  interleave skip factor
 1Ch	WORD  volume set sequence number, Intel format
 1Eh	WORD  volume set sequence number, Motorola format
 20h	BYTE  length of file name
 21h  N BYTEs file name
	BYTE (optional) padding if filename is odd length
      N BYTEs system data

Format of CD-ROM directory entry (canonicalized):
Offset	Size	Description	(Table 1290)
 00h	BYTE	length of XAR in Logical Block Numbers
 01h	DWORD	Logical Block Number of file start
 05h	WORD	size of disk in logical blocks
 07h	DWORD	file length in bytes
 0Bh  7 BYTEs	date and time
 12h	BYTE	bit flags
 13h	BYTE	interleave size
 14h	BYTE	interleave skip factor
 15h	WORD	volume set sequence number
 17h	BYTE	length of file name
 18h 38 BYTEs	ASCIZ filename
 3Eh	WORD	file version number
 40h	BYTE	number of bytes of system use data
 41h 220 BYTEs	system use data
-------------------------------------------------------------------------------
INT 2F - CD-ROM v2.10+ - SEND DEVICE DRIVER REQUEST
	AX = 1510h
	CX = CD-ROM drive letter (0 = A, 1 = B, etc)
	ES:BX -> CD-ROM device driver request header (see #1265 at AX=0802h)
-------------------------------------------------------------------------------
INT 2F U - DRIVER.SYS support - EXECUTE DEVICE DRIVER REQUEST
	AX = 0802h
	ES:BX -> device driver request header (see #1265)
Return: request header updated as per requested operation
Notes:	supported by DR-DOS 5.0
	DOS 3.2 executes this function on any AL value from 02h through F7h;
	  DOS 4+ executes this function on AL=02h and AL=04h-F7h
	the command codes and structures described below apply to all drivers
	  which support the appropriate commands; this call is just one of a
	  number of ways in which a device driver request may be invoked
SeeAlso: AX=0800h,AX=0801h,AX=0803h,INT 21/AH=52h,INT 21/AH=99h,INT 21/AH=9Ah

(Table 1263)
Values for device driver command code:
 00h	INIT
 01h	MEDIA CHECK (block devices)
 02h	BUILD BPB (block devices)
 03h	IOCTL INPUT
 04h	INPUT
 05h	NONDESTRUCTIVE INPUT, NO WAIT (character devices)
 06h	INPUT STATUS (character devices)
 07h	INPUT FLUSH (character devices)
 08h	OUTPUT
 09h	OUTPUT WITH VERIFY
 0Ah	OUTPUT STATUS (character devices)
 0Bh	OUTPUT FLUSH (character devices)
 0Ch	IOCTL OUTPUT
 0Dh	(DOS 3+) DEVICE OPEN
 0Eh	(DOS 3+) DEVICE CLOSE
 0Fh	(DOS 3+) REMOVABLE MEDIA (block devices)
 10h	(DOS 3+) OUTPUT UNTIL BUSY (character devices)
 11h	(European MS-DOS 4.0) STOP OUTPUT (console screen drivers only)
 12h	(European MS-DOS 4.0) RESTART OUTPUT (console screen drivers only)
 13h	(DOS 3.2+) GENERIC IOCTL
 14h	unused
 15h	(European MS-DOS 4.0) RESET UNCERTAIN MEDIA FLAG
 16h	unused
 17h	(DOS 3.2+) GET LOGICAL DEVICE
 18h	(DOS 3.2+) SET LOGICAL DEVICE
 19h	(DOS 5+) CHECK GENERIC IOCTL SUPPORT
 80h	(CD-ROM) READ LONG
 81h	(CD-ROM) reserved
 82h	(CD-ROM) READ LONG PREFETCH
 83h	(CD-ROM) SEEK
 84h	(CD-ROM) PLAY AUDIO
 85h	(CD-ROM) STOP AUDIO
 86h	(CD-ROM) WRITE LONG
 87h	(CD-ROM) WRITE LONG VERIFY
 88h	(CD-ROM) RESUME AUDIO

Bitfields for device request status:
Bit(s)	Description	(Table 1264)
 15	error
 14-11	reserved
 10	??? set by DOS kernel on entry to some driver calls
 9	busy
 8	done (may be clear on return under European MS-DOS 4.0)
 7-0	error code if bit 15 set (see #1266)

Format of device driver request header:
Offset	Size	Description	(Table 1265)
 00h	BYTE	length of request header
 01h	BYTE	subunit within device driver
 02h	BYTE	command code (see #1263)
 03h	WORD	status (filled in by device driver) (see #1264)
---DOS---
 05h  4 BYTEs	reserved (unused in DOS 2.x and 3.x)
 09h	DWORD	(European MS-DOS 4.0 only) pointer to next request header in
			  device's request queue
		(other versions) reserved (unused in DOS 2.x and 3.x)
---STARLITE architecture---
 05h	DWORD	pointer to next request header
 09h  4 BYTEs	reserved
---command code 00h---
 0Dh	BYTE	(return) number of units
 0Eh	DWORD	(call) pointer to DOS device helper function (see #1267)
			  (European MS-DOS 4.0 only)
		(call) pointer past end of memory available to driver (DOS 5+)
		(return) address of first free byte following driver
 12h	DWORD	(call) pointer to commandline arguments
		(return) pointer to BPB array (block drivers) or
			  0000h:0000h (character drivers)
 16h	BYTE	(DOS 3+) drive number for first unit of block driver (0=A)
   ---European MS-DOS 4.0---
 17h	DWORD	pointer to function to save registers on stack
   ---DOS 5+ ---
 17h	WORD	(return) error-message flag
		0001h MS-DOS should display error msg on init failure
---command code 01h---
 0Dh	BYTE	media descriptor
 0Eh	BYTE	(return) media status
		00h don't know
		01h media has not changed
		FFh media has been changed
 0Fh	DWORD	(return, DOS 3+) pointer to previous volume ID if OPEN/CLOSE/RM
		  bit in device header is set and disk changed
---command code 02h---
 0Dh	BYTE	media descriptor
 0Eh	DWORD	transfer address
		-> scratch sector if NON-IBM FORMAT bit in device header set
		-> first FAT sector otherwise
 12h	DWORD	pointer to BPB (set by driver) (see INT 21/AH=53h)
---command codes 03h,0Ch--- (see also INT 21/AX=4402h,INT 21/AX=4403h)
 0Dh	BYTE	media descriptor (block devices only)
 0Eh	DWORD	transfer address
 12h	WORD	(call) number of bytes to read/write
		(return) actual number of bytes read or written
---command codes 04h,08h,09h---
 0Dh	BYTE	media descriptor (block devices only)
 0Eh	DWORD	transfer address
 12h	WORD	byte count (character devices) or sector count (block devices)
 14h	WORD	starting sector number (block devices only)
 16h	DWORD	(DOS 3+) pointer to volume ID if error 0Fh returned
 1Ah	DWORD	(DOS 4+) 32-bit starting sector number (block devices with
		  device attribute word bit 1 set only) if starting sector
		  number above is FFFFh (see INT 21/AH=52h)
---command code 05h---
 0Dh	BYTE	byte read from device if BUSY bit clear on return
---command codes 06h,07h,0Ah,0Bh,0Dh,0Eh,0Fh---
 no further fields
---command code 10h---
 0Dh	BYTE	unused
 0Eh	DWORD	transfer address
 12h	WORD	(call) number of bytes to write
		(return) actual number of bytes written
---command codes 11h,12h---
 0Dh	BYTE	reserved
---command code 15h---
 no further fields
---command codes 13h,19h---
 0Dh	BYTE	category code
		00h unknown
		01h COMn:
		03h CON
		05h LPTn:
		07h mouse (European MS-DOS 4.0)
		08h disk
		9Eh (STARLITE) Media Access Control driver
 0Eh	BYTE	function code
		00h (STARLITE) MAC Bind request
 0Fh	WORD	copy of DS at time of IOCTL call (apparently unused in DOS 3.3)
		SI contents (European MS-DOS 4.0)
 11h	WORD	offset of device driver header
		DI contents (European MS-DOS 4.0)
 13h	DWORD	pointer to parameter block from INT 21/AX=440Ch or AX=440Dh
---command codes 80h,82h---
 0Dh	BYTE	addressing mode
		00h HSG (default)
		01h Phillips/Sony Red Book
 0Eh	DWORD	transfer address (ignored for command 82h)
 12h	WORD	number of sectors to read
		(if 0 for command 82h, request is an advisory seek)
 14h	DWORD	starting sector number
		logical sector number in HSG mode
		frame/second/minute/unused in Red Book mode
		(HSG sector = minute * 4500 + second * 75 + frame - 150)
 18h	BYTE	data read mode
		00h cooked (2048 bytes per frame)
		01h raw (2352 bytes per frame, including EDC/ECC)
 19h	BYTE	interleave size (number of sectors stored consecutively)
 1Ah	BYTE	interleave skip factor
		(number of sectors between consecutive portions)
---command code 83h---
 0Dh	BYTE	addressing mode (see above)
 0Eh	DWORD	transfer address (ignored)
 12h	WORD	number of sectors to read (ignored)
 14h	DWORD	starting sector number (see also above)
---command code 84h---
 0Dh	BYTE	addressing mode (see above)
 0Eh	DWORD	starting sector number (see also above)
 12h	DWORD	number of sectors to play
---command codes 85h,88h---
 no further fields
---command codes 86h,87h---
 0Dh	BYTE	addressing mode (see above)
 0Eh	DWORD	transfer address (ignored in write mode 0)
 12h	WORD	number of sectors to write
 14h	DWORD	starting sector number (see also above)
 18h	BYTE	write mode
		00h mode 0 (write all zeros)
		01h mode 1 (default) (2048 bytes per sector)
		02h mode 2 form 1 (2048 bytes per sector)
		03h mode 2 form 2 (2336 bytes per sector)
 19h	BYTE	interleave size (number of sectors stored consecutively)
 1Ah	BYTE	interleave skip factor
		(number of sectors between consecutive portions)

(Table 1266)
Values for device driver error code:
 00h	write-protect violation
 01h	unknown unit
 02h	drive not ready
 03h	unknown command
 04h	CRC error
 05h	bad drive request structure length
 06h	seek error
 07h	unknown media
 08h	sector not found
 09h	printer out of paper
 0Ah	write fault
 0Bh	read fault
 0Ch	general failure
 0Dh	reserved
 0Eh	(CD-ROM) media unavailable
 0Fh	invalid disk change

(Table 1267)
Call European MS-DOS 4.0 device helper function with:
	DL = function
	    00h "SchedClock" called on each timer tick
		AL = tick interval in milliseconds
	    01h "DevDone" device I/O complete
		ES:BX -> request header
		Note:	must update status word first; may be called from
			  an interrupt handler
	    02h "PullRequest" pull next request from queue
		DS:SI -> DWORD pointer to start of device's request queue
		Return: ZF clear if pending request
			    ES:BX -> request header
			ZF set if no more requests
	    03h "PullParticular" remove specific request from queue
		DS:SI -> DWORD pointer to start of device's request queue
		ES:BX -> request header
		Return: ZF set if request header not found
	    04h "PushRequest" push the request onto the queue
		DS:SI -> DWORD pointer to start of device's request queue
		ES:BX -> request header
		interrupts disabled
	    05h "ConsInputFilter" keyboard input check
		AX = character (high byte 00h if PC ASCII character)
		Return: ZF set if character should be discarded
			ZF clear if character should be handled normally
		Note:	called by keyboard interrupt handler so DOS can scan
			  for special input characters
	    06h "SortRequest" push request in sorted order by starting sector
		DS:SI -> DWORD pointer to start of device's request queue
		ES:BX -> request header
		interrupts disabled
	    07h "SigEvent" send signal on keyboard event
		AH = event identifier
		Return: AL,FLAGS destroyed
	    09h "ProcBlock" block on event
		AX:BX = event identifier (typically a pointer)
		CX = timeout in ms or 0000h for never
		DH = interruptable flag (nonzero if pause may be interrupted)
		interrupts disabled
		Return: after corresponding ProcRun call
			CF clear if event wakeup, set if unusual wakeup
			ZF set if timeout wakeup, clear if interrupted
			AL = wakeup code, nonzero if unusual wakeup
			interrupts enabled
			BX,CX,DX destroyed
		Note:	block process and schedules another to run
	    0Ah "ProcRun" unblock process
		AX:BX = event identifier (typically a pointer)
		Return: AX = number of processes awakened
			ZF set if no processes awakened
			BX,CX,DX destroyed
	    0Bh "QueueInit" initialize/clear character queue
		DS:BX -> character queue structure (see #1268)
		Note:	the queue size field must be set before calling
	    0Dh "QueueWrite" put a character in the queue
		DS:BX -> character queue (see #1268)
		AL = character to append to end of queue
		Return: ZF set if queue is full
			ZF clear if character stored
	    0Eh "QueueRead" get a character from the queue
		DS:BX -> character queue (see #1268)
		Return: ZF set if queue is empty
			ZF clear if characters in queue
			    AL = first character in queue
	    10h "GetDOSVar" return pointer to DOS variable
		AL = index of variable
		    03h current process ID
		BX = index into variable if AL specifies an array
		CX = expected length of variable
		Return: CF clear if successful
			    DX:AX -> variable
			CF set on error
			    AX,DX destroyed
			BX,CX destroyed
		Note:	the variables may not be modified
	    14h "Yield" yield CPU if higher-priority task ready to run
		Return: FLAGS destroyed
	    1Bh "CritEnter" begin system critical section
		DS:BX -> semaphore (6 BYTEs, initialized to zero)
		Return: AX,BX,CX,DX destroyed
	    1Ch "CritLeave" end system critical section
		DS:BX -> semaphore (6 BYTEs, initialized to zero)
		Return: AX,BX,CX,DX destroyed
		Note:	must be called in the context of the process which
			  called CritEnter on the semaphore
Note:	the DWORD pointing at the request queue must be allocated by the driver
	  and initialized to 0000h:0000h.  It always points at the next request
	  to be executed

Format of European MS-DOS 4.0 character queue:
Offset	Size	Description	(Table 1268)
 00h	WORD	size of queue in bytes
 02h	WORD	index of next character out
 04h	WORD	count of characters in the queue
 06h  N BYTEs	queue buffer
