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           DOS Undocumented Interrupts, Functions and Structures

        INT 15,20   PRINT.COM Critical Region Flag (DOS 3.x+)
        INT 21,1F   Get pointer to current drive parameter table
        INT 21,32   Get pointer to drive parameter table
        INT 21,34   Get address to DOS critical flag (INDOS flag)
        INT 21,37   Get/set switch char
        INT 21,4B   EXEC/Load and execute (Function 1 undocumented)
        INT 21,50   Set current process id (DOS 2.x)
        INT 21,51   Get current process id (DOS 2.x)
        INT 21,52   Get pointer to DOS "INVARS"
        INT 21,53   Generate drive parameter table
        INT 21,55   Create new PSP
        INT 21,58   Get/set memory allocation strategy (DOS 3.x+)
        INT 21,5D   Critical error information (DOS 3.x+)
        INT 21,60   Get fully qualified file name (DOS 3.x+)
        INT 21,64   Set device driver look ahead  (DOS 3.3+)
        INT 21,69   Get/set disk serial number (DOS 4.0+)
        INT 21,F8   Set INT 21 OEM handler
        INT 28      DOS idle loop/scheduler
        INT 29      Fast character output
        INT 2E      Execute command using base level COMMAND.COM

        BCB       Batch Control Block
        DTA       Disk Transfer Area  (partially undocumented)
        MCB       Memory Control Block
        PSP       Program Segment Prefix Layout (partially undocumented)
        SFT       System File Table


        - the above items are undocumented with respect to IBM/Microsoft
          and should be not be used unless one fully understands the
          repercussions
        - several functions are version dependant and OEM specific