--------D-20---------------------------------
INT 20 - DOS 1+ - TERMINATE PROGRAM
	CS = PSP segment
Return: never
Notes:	(see INT 21/AH=00h)
	this function sets the program's return code (ERRORLEVEL) to 00h
SeeAlso: INT 21/AH=00h,INT 21/AH=4Ch
--------D-2100-------------------------------
INT 21 - DOS 1+ - TERMINATE PROGRAM
	AH = 00h
	CS = PSP segment
Notes:	Microsoft recommends using INT 21/AH=4Ch for DOS 2+
	this function sets the program's return code (ERRORLEVEL) to 00h
	execution continues at the address stored in INT 22 after DOS performs
	  whatever cleanup it needs to do (restoring the INT 22,INT 23,INT 24
	  vectors from the PSP assumed to be located at offset 0000h in the
	  segment indicated by the stack copy of CS, etc.)
	if the PSP is its own parent, the process's memory is not freed; if
	  INT 22 additionally points into the terminating program, the
	  process is effectively NOT terminated
	not supported by MS Windows 3.0 DOSX.EXE DOS extender
SeeAlso: AH=26h,AH=31h,AH=4Ch,INT 20,INT 22
--------D-2101-------------------------------
INT 21 - DOS 1+ - READ CHARACTER FROM STANDARD INPUT, WITH ECHO
	AH = 01h
Return: AL = character read
Notes:	^C/^Break are checked, and INT 23 executed if read
	^P toggles the DOS-internal echo-to-printer flag
	^Z is not interpreted, thus not causing an EOF if input is redirected
	character is echoed to standard output
	standard input is always the keyboard and standard output the screen
	  under DOS 1.x, but they may be redirected under DOS 2+
SeeAlso: AH=06h,AH=07h,AH=08h,AH=0Ah
--------D-2102-------------------------------
INT 21 - DOS 1+ - WRITE CHARACTER TO STANDARD OUTPUT
	AH = 02h
	DL = character to write
Return: AL = last character output (despite the official docs which state
		nothing is returned) (at least DOS 2.1-7.0)
Notes:	^C/^Break are checked, and INT 23 executed if pressed
	standard output is always the screen under DOS 1.x, but may be
	  redirected under DOS 2+
	the last character output will be the character in DL unless DL=09h
	  on entry, in which case AL=20h as tabs are expanded to blanks
	if standard output is redirected to a file, no error checks (write-
	  protected, full media, etc.) are performed
SeeAlso: AH=06h,AH=09h
--------D-2103-------------------------------
INT 21 - DOS 1+ - READ CHARACTER FROM STDAUX
	AH = 03h
Return: AL = character read
Notes:	keyboard checked for ^C/^Break, and INT 23 executed if detected
	STDAUX is usually the first serial port
SeeAlso: AH=04h,INT 14/AH=02h,INT E0/CL=03h
--------D-2104-------------------------------
INT 21 - DOS 1+ - WRITE CHARACTER TO STDAUX
	AH = 04h
	DL = character to write
Notes:	keyboard checked for ^C/^Break, and INT 23 executed if detected
	STDAUX is usually the first serial port
	if STDAUX is busy, this function will wait until it becomes free
SeeAlso: AH=03h,INT 14/AH=01h,INT E0/CL=04h
--------D-2105-------------------------------
INT 21 - DOS 1+ - WRITE CHARACTER TO PRINTER
	AH = 05h
	DL = character to print
Notes:	keyboard checked for ^C/^Break, and INT 23 executed if detected
	STDPRN is usually the first parallel port, but may be redirected under
	  DOS 2+
	if the printer is busy, this function will wait
SeeAlso: INT 17/AH=00h
--------D-2106-------------------------------
INT 21 - DOS 1+ - DIRECT CONSOLE OUTPUT
	AH = 06h
	DL = character (except FFh)
Return: AL = character output (despite official docs which state nothing is
		returned) (at least DOS 2.1-7.0)
Notes:	does not check ^C/^Break
	writes to standard output, which is always the screen under DOS 1.x,
	  but may be redirected under DOS 2+
SeeAlso: AH=02h,AH=09h
--------D-2106--DLFF-------------------------
INT 21 - DOS 1+ - DIRECT CONSOLE INPUT
	AH = 06h
	DL = FFh
Return: ZF set if no character available
	    AL = 00h
	ZF clear if character available
	    AL = character read
Notes:	^C/^Break are NOT checked
	if the returned character is 00h, the user pressed a key with an
	  extended keycode, which will be returned by the next call of this
	  function
	this function reads from standard input, which is always the keyboard
	  under DOS 1.x, but may be redirected under DOS 2+
	although the return of AL=00h when no characters are available is not
	  documented, some programs rely on this behavior
SeeAlso: AH=0Bh
--------D-2107-------------------------------
INT 21 - DOS 1+ - DIRECT CHARACTER INPUT, WITHOUT ECHO
	AH = 07h
Return: AL = character read from standard input
Notes:	does not check ^C/^Break
	standard input is always the keyboard under DOS 1.x, but may be
	  redirected under DOS 2+
	if the interim console flag is set (see AX=6301h), partially-formed
	  double-byte characters may be returned
SeeAlso: AH=01h,AH=06h,AH=08h,AH=0Ah
--------D-2108-------------------------------
INT 21 - DOS 1+ - CHARACTER INPUT WITHOUT ECHO
	AH = 08h
Return: AL = character read from standard input
Notes:	^C/^Break are checked, and INT 23 executed if detected
	standard input is always the keyboard under DOS 1.x, but may be
	  redirected under DOS 2+
	if the interim console flag is set (see AX=6301h), partially-formed
	  double-byte characters may be returned
SeeAlso: AH=01h,AH=06h,AH=07h,AH=0Ah,AH=64h"DOS 3.2+"
--------D-2109-------------------------------
INT 21 - DOS 1+ - WRITE STRING TO STANDARD OUTPUT
	AH = 09h
	DS:DX -> '$'-terminated string
Return: AL = 24h (the '$' terminating the string, despite official docs which
		state that nothing is returned) (at least DOS 2.1-7.0 and
		NWDOS)
Notes:	^C/^Break are checked, and INT 23 is called if either pressed
	standard output is always the screen under DOS 1.x, but may be
	  redirected under DOS 2+
	under the FlashTek X-32 DOS extender, the pointer is in DS:EDX
SeeAlso: AH=02h,AH=06h"OUTPUT"
--------D-210A-------------------------------
INT 21 - DOS 1+ - BUFFERED INPUT
	AH = 0Ah
	DS:DX -> buffer (see #1209)
Return: buffer filled with user input
Notes:	^C/^Break are checked, and INT 23 is called if either detected
	reads from standard input, which may be redirected under DOS 2+
	if the maximum buffer size (see #1209) is set to 00h, this call returns
	  immediately without reading any input
SeeAlso: AH=0Ch,INT 2F/AX=4810h

Format of DOS input buffer:
Offset	Size	Description	(Table 1209)
 00h	BYTE	maximum characters buffer can hold
 01h	BYTE	(call) number of chars from last input which may be recalled
		(ret) number of characters actually read, excluding CR
 02h  N BYTEs	actual characters read, including the final carriage return
--------D-210B-------------------------------
INT 21 - DOS 1+ - GET STDIN STATUS
	AH = 0Bh
Return: AL = status
	    00h if no character available
	    FFh if character is available
Notes:	^C/^Break are checked, and INT 23 is called if either pressed
	standard input is always the keyboard under DOS 1.x, but may be
	  redirected under DOS 2+
	if the interim console flag is set (see AX=6301h), this function
	  returns AL=FFh if a partially-formed double-byte character is
	  available
SeeAlso: AH=06h"INPUT",AX=4406h
--------D-210C-------------------------------
INT 21 - DOS 1+ - FLUSH BUFFER AND READ STANDARD INPUT
	AH = 0Ch
	AL = STDIN input function to execute after flushing buffer
	other registers as appropriate for the input function
Return: as appropriate for the specified input function
Note:	if AL is not one of 01h,06h,07h,08h, or 0Ah, the buffer is flushed but
	  no input is attempted
SeeAlso: AH=01h,AH=06h"INPUT",AH=07h,AH=08h,AH=0Ah
--------D-210D-------------------------------
INT 21 - DOS 1+ - DISK RESET
	AH = 0Dh
Return: (DOS 6 only) CF clear (earlier versions preserve CF)
Notes:	This function writes all modified disk buffers to disk, but does not
	  update the directory information (that is only done when files are
	  closed or a SYNC call is issued)
SeeAlso: AX=5D01h,AX=710Dh,INT 13/AH=00h,INT 2F/AX=1120h
--------D-210E-------------------------------
INT 21 - DOS 1+ - SELECT DEFAULT DRIVE
	AH = 0Eh
	DL = new default drive (00h = A:, 01h = B:, etc)
Return: AL = number of potentially valid drive letters
Notes:	under Novell NetWare, the return value is always 32, the number of
	  drives that NetWare supports
	under DOS 3.0+, the return value is the greatest of 5, the value of
	  LASTDRIVE= in CONFIG.SYS, and the number of drives actually present
	on a DOS 1.x/2.x single-floppy system, AL returns 2 since the floppy
	  may be accessed as either A: or B:
	otherwise, the return value is the highest drive actually present
	DOS 1.x supports a maximum of 16 drives, 2.x a maximum of 63 drives,
	  and 3+ a maximum of 26 drives
	under Novell DOS 7, this function returns the correct LASTDRIVE value
	  even when the undocumented LASTDRIVE=27..32 directive was used in
	  CONFIG.SYS
SeeAlso: AH=19h,AH=3Bh,AH=DBh
--------D-210F-------------------------------
INT 21 - DOS 1+ - OPEN FILE USING FCB
	AH = 0Fh
	DS:DX -> unopened File Control Block (see #1210,#1211)
Return: AL = status
	    00h successful
	    FFh file not found or access denied
Notes:	(DOS 3.1+) file opened for read/write in compatibility mode
	an unopened FCB has the drive, filename, and extension fields filled
	  in and all other bytes cleared
	not supported by MS Windows 3.0 DOSX.EXE DOS extender
	DR DOS checks password attached with AX=4303h
	(FAT32 drive) this function will only succeed for creating a volume
	  label; FAT32 does not support FCBs for file I/O
BUG:	APPEND for DOS 3.3+ corrupts DX if the file is not found
SeeAlso: AH=10h,AH=16h,AH=3Dh,AX=4303h

Format of File Control Block:
Offset	Size	Description	(Table 1210)
 00h	BYTE	drive number (0 = default, 1 = A, etc)
		FFh is not allowed (signals extended FCB, see #1211)
 01h  8 BYTEs	blank-padded file name
 09h  3 BYTEs	blank-padded file extension
 0Ch	WORD	current block number
 0Eh	WORD	logical record size
 10h	DWORD	file size
 14h	WORD	date of last write (see #1531 at AX=5700h)
 16h	WORD	time of last write (see #1530 at AX=5700h) (DOS 1.1+)
 18h  8 BYTEs	reserved (see #1212,#1213,#1214,#1215,#1216)
 20h	BYTE	record within current block
 21h	DWORD	random access record number (if record size is > 64 bytes, high
		  byte is omitted)
SeeAlso: #1211

Format of Extended File Control Block (XFCB):
Offset	Size	Description	(Table 1211)
 00h	BYTE	FFh signature for extended FCB
 01h  5 BYTEs	reserved
 06h	BYTE	file attribute if extended FCB
 07h 36 BYTEs	standard FCB (all offsets are shifted by seven bytes)
SeeAlso: #1111

Format of FCB reserved field for DOS 1.0:
Offset	Size	Description	(Table 1212)
 16h	WORD	location in directory (if high byte = FFh, low byte is device
		  ID)
 18h	WORD	number of first cluster in file
 1Ah	WORD	current absolute cluster number on disk
 1Ch	WORD	current relative cluster number within file
		(0 = first cluster of file, 1 = second cluster, etc.)
 1Eh	BYTE	dirty flag (00h = not dirty)
 1Fh	BYTE	unused

Format of FCB reserved field for DOS 1.10-1.25:
Offset	Size	Description	(Table 1213)
 18h	BYTE	bit 7: set if logical device
		bit 6: not dirty
		bits 5-0: disk number or logical device ID
 19h	WORD	starting cluster number on disk
 1Bh	WORD	current absolute cluster number on disk
 1Dh	WORD	current relative cluster number within file
 1Fh	BYTE	unused

Format of FCB reserved field for DOS 2.x:
Offset	Size	Description	(Table 1214)
 18h	BYTE	bit 7: set if logical device
		bit 6: set if open???
		bits 5-0: ???
 19h	WORD	starting cluster number on disk
 1Bh	WORD	???
 1Dh	BYTE	???
 1Eh	BYTE	???
 1Fh	BYTE	???

Format of FCB reserved field for DOS 3.x:
Offset	Size	Description	(Table 1215)
 18h	BYTE	number of system file table entry for file
 19h	BYTE	attributes
		bits 7,6: 00 = SHARE.EXE not loaded, disk file
			  01 = SHARE.EXE not loaded, character device
			  10 = SHARE.EXE loaded, remote file
			  11 = SHARE.EXE loaded, local file or device
		bits 5-0: low six bits of device attribute word
---SHARE.EXE loaded, local file---
 1Ah	WORD	starting cluster of file on disk
 1Ch	WORD	(DOS 3.x) offset within SHARE of sharing record
		  (see #1502 at AH=52h)
 1Eh	BYTE	file attribute
 1Fh	BYTE	???
---SHARE.EXE loaded, remote file---
 1Ah	WORD	number of sector containing directory entry (see #1217)
 1Ch	WORD	relative cluster within file of last cluster accessed
 1Eh	BYTE	absolute cluster number of last cluster accessed
 1Fh	BYTE	???
---SHARE.EXE not loaded---
 1Ah	BYTE	(low byte of device attribute word AND 0Ch) OR open mode
 1Bh	WORD	starting cluster of file
 1Dh	WORD	number of sector containing directory entry (see #1217)
 1Fh	BYTE	number of directory entry within sector
Note:	if FCB opened on character device, DWORD at 1Ah is set to the address
	  of the device driver header, then the BYTE at 1Ah is overwritten.
SeeAlso: #1511

Format of FCB reserved field for DOS 5.0:
Offset	Size	Description	(Table 1216)
 18h	BYTE	number of system file table entry for file
 19h	BYTE	attributes
		bits 7,6: 00 = SHARE.EXE not loaded, disk file
			  01 = SHARE.EXE not loaded, character device
			  10 = SHARE.EXE loaded, remote file
			  11 = SHARE.EXE loaded, local file or device
		bits 5-0: low six bits of device attribute word
---SHARE.EXE loaded, local file---
 1Ah	WORD	starting cluster of file on disk
 1Ch	WORD	unique sequence number of sharing record
 1Eh	BYTE	file attributes
 1Fh	BYTE	unused???
---SHARE.EXE loaded, remote file---
 1Ah	WORD	network handle