For more information, see the following topics:

~INT 21,0~ Program terminate

~INT 21,1~ Keyboard input with echo

~INT 21,2~ Display output

~INT 21,3~ Wait for auxiliary device input

~INT 21,4~ Auxiliary output

~INT 21,5~ Printer output

~INT 21,6~ Direct console I/O

~INT 21,7~ Wait for direct console input without echo

~INT 21,8~ Wait for console input without echo

~INT 21,9~ Print string

~INT 21,A~ Buffered keyboard input

~INT 21,B~ Check standard input status

~INT 21,C~ Clear keyboard buffer, invoke keyboard function

~INT 21,D~ Disk reset

~INT 21,E~ Select disk

~INT 21,F~ Open file using ~FCB~

~INT 21,10~ Close file using FCB

~INT 21,11~ Search for first entry using FCB

~INT 21,12~ Search for next entry using FCB

~INT 21,13~ Delete file using FCB

~INT 21,14~ Sequential read using FCB

~INT 21,15~ Sequential write using FCB

~INT 21,16~ Create a file using FCB

~INT 21,17~ Rename file using FCB

~INT 21,18~ DOS dummy function (CP/M) (not used/listed)

~INT 21,19~ Get current default drive

~INT 21,1A~ Set disk transfer address

~INT 21,1B~ Get allocation table information

~INT 21,1C~ Get allocation table info for specific device

~INT 21,1D~ DOS dummy function (CP/M) (not used/listed)

~INT 21,1E~ DOS dummy function (CP/M) (not used/listed)

~INT 21,1F~ Get pointer to default drive parameter table (undocumented)

~INT 21,20~ DOS dummy function (CP/M) (not used/listed)

~INT 21,21~ Random read using ~FCB~

~INT 21,22~ Random write using FCB

~INT 21,23~ Get file size using FCB

~INT 21,24~ Set relative record field for FCB

~INT 21,25~ Set interrupt vector

~INT 21,26~ Create new program segment

~INT 21,27~ Random block read using FCB

~INT 21,28~ Random block write using FCB

~INT 21,29~ Parse filename for FCB

~INT 21,2A~ Get date

~INT 21,2B~ Set date

~INT 21,2C~ Get time

~INT 21,2D~ Set time

~INT 21,2E~ Set/reset verify switch

~INT 21,2F~ Get disk transfer address

~INT 21,30~ Get DOS version number

~INT 21,31~ Terminate process and remain resident

~INT 21,32~ Get pointer to drive parameter table (undocumented)

~INT 21,33~ Get/set ~Ctrl-Break~ check state & get boot drive

~INT 21,34~ Get address to DOS critical flag (undocumented)

~INT 21,35~ Get vector

~INT 21,36~ Get disk free space

~INT 21,37~ Get/set switch character (undocumented)

~INT 21,38~ Get/set country dependent information

~INT 21,39~ Create subdirectory (mkdir)

~INT 21,3A~ Remove subdirectory (rmdir)

~INT 21,3B~ Change current subdirectory (chdir)

~INT 21,3C~ Create file using handle

~INT 21,3D~ Open file using handle

~INT 21,3E~ Close file using handle

~INT 21,3F~ Read file or device using handle

~INT 21,40~ Write file or device using handle

~INT 21,41~ Delete file

~INT 21,42~ Move file pointer using handle

~INT 21,43~ Change file mode

~INT 21,44~ I/O control for devices (~IOCTL~)

~INT 21,45~ Duplicate file handle

~INT 21,46~ Force duplicate file handle

~INT 21,47~ Get current directory

~INT 21,48~ Allocate memory blocks

~INT 21,49~ Free allocated memory blocks

~INT 21,4A~ Modify allocated memory blocks

~INT 21,4B~ EXEC load and execute program (func 1 undocumented)

~INT 21,4C~ Terminate process with return code

~INT 21,4D~ Get return code of a sub-process

~INT 21,4E~ Find first matching file

~INT 21,4F~ Find next matching file

~INT 21,50~ Set current process id (undocumented)

~INT 21,51~ Get current process id (undocumented)

~INT 21,52~ Get pointer to DOS "INVARS" (undocumented)

~INT 21,53~ Generate drive parameter table (undocumented)

~INT 21,54~ Get verify setting

~INT 21,55~ Create ~PSP~ (undocumented)

~INT 21,56~ Rename file

~INT 21,57~ Get/set file date and time using handle

~INT 21,58~ Get/set memory allocation strategy (3.x+, undocumented)

~INT 21,59~ Get extended error information (3.x+)

~INT 21,5A~ Create temporary file (3.x+)

~INT 21,5B~ Create new file (3.x+)

~INT 21,5C~ Lock/unlock file access (3.x+)

~INT 21,5D~ Critical error information (undocumented 3.x+)

~INT 21,5E~ Network services (3.1+)

~INT 21,5F~ Network redirection (3.1+)

~INT 21,60~ Get fully qualified file name (undocumented 3.x+)

~INT 21,62~ Get address of program segment prefix (3.x+)

~INT 21,63~ Get system lead byte table (MSDOS 2.25 only)

~INT 21,64~ Set device driver look ahead (undocumented 3.3+)

~INT 21,65~ Get extended country information (3.3+)

~INT 21,66~ Get/set global code page (3.3+)

~INT 21,67~ Set handle count (3.3+)

~INT 21,68~ Flush buffer (3.3+)

~INT 21,69~ Get/set disk serial number (undocumented DOS 4.0+)

~INT 21,6A~ DOS reserved (DOS 4.0+)

~INT 21,6B~ DOS reserved

~INT 21,6C~ Extended open/create (4.x+)

~INT 21,F8~ Set OEM INT 21 handler (functions F9-FF) (undocumented)

- int 21 functions are called with the function number in AH

- register AX may be altered, its contents are not guaranteed

- if an error occurs, CF is set to 1 and AX contains a simple

error code; ~INT 21,59~ can be used to determine cause.

- most INT 21 functions do not restore the flags to pre-interrupt

state to allow returning of information via the flags register