--------D-212E--DL00-------------------------
INT 21 - DOS 1+ - SET VERIFY FLAG
	AH = 2Eh
	DL = 00h (DOS 1.x/2.x only)
	AL = new state of verify flag
	    00h off
	    01h on
Notes:	default state at system boot is OFF
	when ON, all disk writes are verified provided the device driver
	  supports read-after-write verification
SeeAlso: AH=54h
--------D-212F-------------------------------
INT 21 - DOS 2+ - GET DISK TRANSFER AREA ADDRESS
	AH = 2Fh
Return: ES:BX -> current DTA
Note:	under the FlashTek X-32 DOS extender, the pointer is in ES:EBX
SeeAlso: AH=1Ah
--------D-2130-------------------------------
INT 21 - DOS 2+ - GET DOS VERSION
	AH = 30h
---DOS 5+ ---
	AL = what to return in BH
	    00h OEM number (as for DOS 2.0-4.0x)
	    01h version flag
Return: AL = major version number (00h if DOS 1.x)
	AH = minor version number
	BL:CX = 24-bit user serial number (most versions do not use this)
---if DOS <5 or AL=00h---
	BH = MS-DOS OEM number (see #1259)
---if DOS 5+ and AL=01h---
	BH = version flag
	    bit 3: DOS is in ROM
	    other: reserved (0)
Notes:	the OS/2 v1.x Compatibility Box returns major version 0Ah (10)
	the OS/2 v2.x Compatibility Box returns major version 14h (20)
	OS/2 Warp 3.0 Virtual DOS Machines report v20.30; Warp 4 VDMs report
	  v20.40.
	the WindowsNT DOS box returns version 5.00, subject to SETVER
	DOS 4.01 and 4.02 identify themselves as version 4.00; use
	  INT 21/AH=87h to distinguish between the original European MS-DOS 4.0
	  and the later PC-DOS 4.0x and MS-DOS 4.0x
	IBM DOS 6.1 reports its version as 6.00; use the OEM number to
	  distinguish between MS-DOS 6.00 and IBM DOS 6.1 (there was never an
	  IBM DOS 6.0)
	MS-DOS 6.21 reports its version as 6.20; version 6.22 returns the
	  correct value
	Windows95 returns version 7.00 (the underlying MS-DOS), as did the
	  "Chicago" beta (reported in _Microsoft_Systems_Journal_,August 1994);
	  Windows95 OSR2 returns version 7.10
	DR DOS 5.0 and 6.0 report version 3.31; Novell DOS 7 reports IBM v6.00,
	  which some software displays as IBM DOS v6.10 (because of the version
	  mismatch in true IBM DOS, as mentioned above).  The Novell DOS 7
	  SETVER.EXE has an undocumented option /G x.y which sets the "global"
	  DOS version returned by this function for all executables not given
	  a specific version number in SETVER to major version x and minor
	  version y.
	generic MS-DOS 3.30, Compaq MS-DOS 3.31, and others identify themselves
	  as PC-DOS by returning OEM number 00h
	the version returned under DOS 4.0x may be modified by entries in
	  the special program list (see #1527 at AH=52h); the version returned
	  under DOS 5+ may be modified by SETVER--use AX=3306h to get the true
	  version number
SeeAlso: AX=3000h/BX=3000h,AX=3306h,AX=4452h,AH=87h,INT 15/AX=4900h
SeeAlso: INT 2F/AX=122Fh,INT 2F/AX=4010h,INT 2F/AX=4A33h,INT 2F/AX=E002h

(Table 1259)
Values for DOS OEM number:
 00h	IBM
 01h	Compaq
 02h	MS Packaged Product
 04h	AT&T
 05h	ZDS (Zenith Electronics)
 06h	Hewlett-Packard
 07h	ZDS (Groupe Bull)
 0Dh	Packard-Bell
 16h	DEC
 23h	Olivetti
 28h	Texas Instruments
 29h	Toshiba
 33h	Novell (Windows/386 device IDs only)
 34h	MS Multimedia Systems (Windows/386 device IDs only)
 35h	MS Multimedia Systems (Windows/386 device IDs only)
 4Dh	Hewlett-Packard
 5Eh	RxDOS
 66h	PhysTechSoft (PTS-DOS)
 99h	General Software's Embedded DOS
 EEh	DR DOS
 EFh	Novell DOS
 FDh	FreeDOS
 FFh	Microsoft, Phoenix
--------D-2131-------------------------------
INT 21 - DOS 2+ - TERMINATE AND STAY RESIDENT
	AH = 31h
	AL = return code
	DX = number of paragraphs to keep resident
Return: never
Notes:	the value in DX only affects the memory block containing the PSP;
	  additional memory allocated via AH=48h is not affected
	the minimum number of paragraphs which will remain resident is 11h
	  for DOS 2.x and 06h for DOS 3.0+
	most TSRs can save some memory by releasing their environment block
	  before terminating (see #1243 at AH=26h,AH=49h)
	any open files remain open, so one should close any files which will
	  not be used before going resident; to access a file which is left
	  open from the TSR, one must switch PSP segments first (see AH=50h)
SeeAlso: AH=00h,AH=4Ch,AH=4Dh,INT 20,INT 22,INT 27
--------D-2132-------------------------------
INT 21 - DOS 2+ - GET DOS DRIVE PARAMETER BLOCK FOR SPECIFIC DRIVE
	AH = 32h
	DL = drive number (00h = default, 01h = A:, etc)
Return: AL = status
	    00h successful
		DS:BX -> Drive Parameter Block (DPB) (see #1260) for specified
			  drive
	    FFh invalid or network drive
Notes:	the OS/2 compatibility box supports the DOS 3.3 version of this call
	  except for the DWORD at offset 12h
	this call updates the DPB by reading the disk; the DPB may be accessed
	  via the DOS list of lists (see #1492 at AH=52h) if disk access is not
	  desirable.
	undocumented prior to the release of DOS 5.0; only the DOS 4.0+
	  version of the DPB has been documented, however
	supported by DR DOS 3.41+; DR DOS 3.41-6.0 return the same data as
	  MS-DOS 3.31
	IBM ROM-DOS v4.0 also reports invalid/network (AL=FFh) on the ROM drive
SeeAlso: AH=1Fh,AH=52h,AX=7302h

Format of DOS Drive Parameter Block:
Offset	Size	Description	(Table 1260)
 00h	BYTE	drive number (00h = A:, 01h = B:, etc)
 01h	BYTE	unit number within device driver
 02h	WORD	bytes per sector
 04h	BYTE	highest sector number within a cluster
 05h	BYTE	shift count to convert clusters into sectors
 06h	WORD	number of reserved sectors at beginning of drive
 08h	BYTE	number of FATs
 09h	WORD	number of root directory entries
 0Bh	WORD	number of first sector containing user data
 0Dh	WORD	highest cluster number (number of data clusters + 1)
		16-bit FAT if greater than 0FF6h, else 12-bit FAT
 0Fh	BYTE	number of sectors per FAT
 10h	WORD	sector number of first directory sector
 12h	DWORD	address of device driver header (see #1511)
 16h	BYTE	media ID byte (see #1221)
 17h	BYTE	00h if disk accessed, FFh if not
 18h	DWORD	pointer to next DPB
---DOS 2.x---
 1Ch	WORD	cluster containing start of current directory, 0000h=root,
		FFFFh = unknown
 1Eh 64 BYTEs	ASCIZ pathname of current directory for drive
---DOS 3.x---
 1Ch	WORD	cluster at which to start search for free space when writing
 1Eh	WORD	number of free clusters on drive, FFFFh = unknown
---DOS 4.0-6.0---
 0Fh	WORD	number of sectors per FAT
 11h	WORD	sector number of first directory sector
 13h	DWORD	address of device driver header (see #1511)
 17h	BYTE	media ID byte (see #1221)
 18h	BYTE	00h if disk accessed, FFh if not
 19h	DWORD	pointer to next DPB
 1Dh	WORD	cluster at which to start search for free space when writing,
		usually the last cluster allocated
 1Fh	WORD	number of free clusters on drive, FFFFh = unknown
SeeAlso: #1222,#1528,#1651 at AX=7302h,#3891 at INT E0/CL=71h
--------D-2133-------------------------------
INT 21 - DOS 2+ - EXTENDED BREAK CHECKING
	AH = 33h
	AL = subfunction
	    00h get current extended break state
		Return: DL = current state, 00h = off, 01h = on
	    01h set state of extended ^C/^Break checking
		DL = new state
		    00h off, check only on character I/O functions
		    01h on, check on all DOS functions
		Return: (Novell DOS 7) DL = old state of extended Break checks
Note:	under DOS 3.1+ and DR DOS, this function does not use any of the
	  DOS-internal stacks and may thus be called at any time
SeeAlso: AX=3302h
--------D-213302-----------------------------
INT 21 - DOS 3.x+ internal - GET AND SET EXTENDED CONTROL-BREAK CHECKING STATE
	AX = 3302h
	DL = new state (00h for OFF, 01h for ON)
Return: DL = old state of extended BREAK checking
Notes:	this function does not use any of the DOS-internal stacks and may thus
	  be called at any time; one possible use is modifying Control-Break
	  checking from within an interrupt handler or TSR
	not supported by DR DOS through version 6.0 (error code 01h);
	  newly-supported by Novell DOS 7
SeeAlso: AH=33h
--------D-213303-----------------------------
INT 21 - DOS 4.0+ - UNUSED
	AX = 3303h
Return: nothing
Note:	this function and AX=3304h were intended to support a proposed
	  code-page switching flag (using two of the ten reserved bytes in
	  the DOS directory entry for codepage information); however, this
	  function has always been a NOP in public releases of DOS and OS/2.
	  See _DOS_Internals_ Chapter 2 for more information
SeeAlso: AX=3304h
--------D-213304-----------------------------
INT 21 - DOS 4.0+ - UNUSED
	AX = 3304h
Return: nothing
Note:	this function and AX=3303h were intended to support a proposed
	  code-page switching flag (using two of the ten reserved bytes in
	  the DOS directory entry for codepage information); however, this
	  function has always been a NOP in public releases of DOS and OS/2.
	  See _DOS_Internals_ Chapter 2 for more information
SeeAlso: AX=3303h
--------D-213305-----------------------------
INT 21 - DOS 4.0+ - GET BOOT DRIVE
	AX = 3305h
Return: DL = boot drive (1=A:,...)
Notes:	fully reentrant
	NEC 9800-series PCs always call the boot drive A: and assign the other
	  drive letters sequentially to the other drives in the system
	this call is supported by OS/2 Warp 3.0, but not earlier versions of
	  OS/2; it is also supported by Novell DOS 7
--------D-213306-----------------------------
INT 21 - DOS 5+ - GET TRUE VERSION NUMBER
	AX = 3306h
Return: BL = major version
	BH = minor version
	DL = revision (bits 2-0, all others 0)
	DH = version flags
	    bit 3: DOS is in ROM
	    bit 4: DOS is in HMA
	AL = FFh if true DOS version < 5.0
Notes:	this function always returns the true version number, unlike AH=30h,
	  whose return value may be changed with SETVER
	because of the conflict from the CBIS redirector (see next
	  entry), programs should check whether BH is less than 100 (64h)
	  and BL is at least 5 before accepting the returned BX as the true
	  version number; however, even this is not entirely reliable when
	  that redirector is loaded
	fully reentrant
	OS/2 v2.1 will return BX=0A14h (version 20.10)
	the Windows NT DOS box returns BX=3205h (version 5.50)
	Novell DOS 7 returns IBM v6.00, which some software displays as
	  IBM DOS v6.10 (because of the version mismatch in true IBM DOS
	  mentioned for INT 21/AH=30h); versions through Update 15 all
	  return revision code 00h
	Windows95 and Windows95 SP1 return version 7.00; Windows95 OSR2 returns
	  version 7.10
BUG:	DR DOS 5.0 and 6.0 return CF set/AX=0001h for INT 21/AH=33h
	  subfunctions other than 00h-02h and 05h, while MS-DOS returns AL=FFh
	  for invalid subfunctions
SeeAlso: AH=30h,INT 2F/AX=122Fh,INT 2F/AX=E000h"SETDRVER"
--------D-213307-----------------------------
INT 21 - Windows95 - SET/CLEAR DOS_FLAG
	AX = 3307h
	DL = subfunction
	    00h clear bit 5 of "DOS_FLAG"
	    01h set bit 5 of "DOS_FLAG"
Return: ???
--------D-2134-------------------------------
INT 21 - DOS 2+ - GET ADDRESS OF INDOS FLAG
	AH = 34h
Return: ES:BX -> one-byte InDOS flag
Notes:	this function executes on the DOS stack, and thus cannot be called
	  while another DOS function is already executing; you should use
	  this function once at the beginning of the program and store the
	  returned pointer rather than calling it when requiring DOS access
	the value of InDOS is incremented whenever an INT 21 function begins
	  and decremented whenever one completes
	during an INT 28 call, it is safe to call some INT 21 functions even
	  though InDOS may be 01h instead of zero
	InDOS alone is not sufficient for determining when it is safe to
	  enter DOS, as the critical error handling decrements InDOS and
	  increments the critical error flag for the duration of the critical
	  error.  Thus, it is possible for InDOS to be zero even if DOS is
	  busy.
	SMARTDRV 4.0 sets the InDOS flag while flushing its buffers to disk,
	  then zeros it on completion
	the critical error flag is the byte immediately following InDOS in
	  DOS 2.x, and the byte BEFORE the InDOS flag in DOS 3.0+ and
	  DR DOS 3.41+ (except COMPAQ DOS 3.0, where the critical error flag
	  is located 1AAh bytes BEFORE the critical section flag)
	for DOS 3.1+, an undocumented call exists to get the address of the
	  critical error flag (see AX=5D06h)
	this function was undocumented prior to the release of DOS 5.0.
SeeAlso: AX=5D06h,AX=5D0Bh,INT 15/AX=DE1Fh,INT 28
--------D-2135-------------------------------
INT 21 - DOS 2+ - GET INTERRUPT VECTOR
	AH = 35h
	AL = interrupt number
Return: ES:BX -> current interrupt handler
Note:	under DR DOS 5.0+, this function does not use any of the DOS-internal
	  stacks and may thus be called at any time
SeeAlso: AH=25h,AX=2503h
--------D-2136-------------------------------
INT 21 - DOS 2+ - GET FREE DISK SPACE
	AH = 36h
	DL = drive number (00h = default, 01h = A:, etc)
Return: AX = FFFFh if invalid drive
	else
	    AX = sectors per cluster
	    BX = number of free clusters
	    CX = bytes per sector
	    DX = total clusters on drive
Notes:	free space on drive in bytes is AX * BX * CX
	total space on drive in bytes is AX * CX * DX
	"lost clusters" are considered to be in use
	according to Dave Williams' MS-DOS reference, the value in DX is
	  incorrect for non-default drives after ASSIGN is run
	this function does not return proper results on CD-ROMs;
	  use AX=4402h"CD-ROM" instead
	(FAT32 drive) the reported total and free space are limited to 2G-32K
	  should they exceed that value
SeeAlso: AH=1Bh,AH=1Ch,AX=4402h"CD-ROM",AX=7303h
--------D-213700-----------------------------
INT 21 - DOS 2+ - "SWITCHAR" - GET SWITCH CHARACTER
	AX = 3700h
Return: AL = status