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 DOS int 21 function table
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 ah   Description
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 00   Program terminate
 01   Keyboard input with echo
 02   Display output
 03   Wait for auxiliary device input
 04   Auxiliary output
 05   Printer output
 06   Direct console I/O
 07   Direct console input without echo
 08   Console input without echo
 09   Print string
 0a   Buffered keyboard input
 0b   Check standard input status
 0c   Clear keyboard buffer and invoke keyboard function
 0d   Disk reset
 0e   Select disk
 0f   Open a file using FCB
 10   Close a file using FCB
 11   Search for first entry using FCB
 12   Search for next entry using FCB
 13   Delete file using FCB
 14   Sequential read using FCB
 15   Sequential write using FCB
 16   Create a file using FCB
 17   Rename a file using FCB
 19   Get current default drive
 1a   Set Disk Transfer Address
 1b   Get allocation table information
 1c   Get allocation table info for specified drive
 1f   Get pointer to current drive parameter table
 21   Random read using FCB
 22   Random write using FCB
 23   Get file size using FCB
 24   Set relative record field in FCB
 25   Set interrupt vector
 26   Create new Program Segment Prefix
 27   Random block read using FCB
 28   Random block write using FCB
 29   Parse a filename for FCB
 2a   Get date
 2b   Set date
 2c   Get time
 2d   Set time
 2e   Set/reset verify switch
 2f   Get Disk Transfer Address
 30   Get DOS version number
 31   Terminate and Stay Resident
 32   Get pointer to drive parameter table
 33   Get/set system values
 34   Get address to DOS critical flag
 35   Get interrupt vector
 36   Get disk free space
 37   Get/set switch character
 38   Get/set country dependent information
 39   Create subdirectory
 3a   Remove subdirectory
 3b   Change current directory
 3c   Create file using handle
 3d   Open file using handle
 3e   Close file using handle
 3f   Read from file or device using handle
 40   Write to file or device using handle
 41   Delete file
 42   Move file pointer using handle
 43   Get/set file attributes
 44   I/O control for devices
 45   Duplicate file handle
 46   Force duplicate file handle
 47   Get current directory
 48   Allocate memory
 49   Free allocated memory
 4a   Modify allocated memory block
 4b   Execute program
 4c   Terminate process with return code
 4d   Get return code of subprocess
 4e   Find first matching file
 4f   Find next matching file
 50   Set current process ID
 51   Get current process ID
 52   Get pointer to DOS INVARS
 53   Generate drive parameter table
 54   Get verify setting
 55   Create new Program Segment Prefix
 56   Rename file
 57   Get/set file date and time using handle
 58   Get/set memory allocation strategy
 59   Get extended error information
 5a   Create temporary file
 5b   Create file
 5c   Lock/unlock file access
 5d   Critical error information
 5e   Get machine name
 5f   Get redirection mode
 60   Get fully qualified file name
 62   Get PSP address
 63   Get lead byte table
 64   Set device driver look ahead
 65   Get extended country information
 66   Get/set global code page
 67   Set handle count
 68   Flush buffer using handle
 69   Get/set disk serial number
 6c   Extended open/create
 f8   Set OEM int 21 handler
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