Important Notice

The pages on this site contain documentation for very old MS-DOS software, purely for historical purposes. If you're looking for up-to-date documentation, particularly for programming, you should not rely on the information found here, as it will be woefully out of date.

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       0   1   2   3   4   5   6   7   8   9   A   B   C   D   E   F
     ┌───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┐
  00 │ int20 │  mem  │rsv│      calldisp     │     adr22     │adr23∙∙│
     ├───────┼───────┴───┴───┬───────────────┴───────────────┴───────┤
  10 │ ∙∙∙   │     adr24     │                                       │
     ├───────┴───────────────┘                       ┌───────┐       ┤
  20 │                                               │  env  │       │
     ├                                               └───────┘       ┤
  30 │                             rsv                               │
     ├                                                               ┤
  40 │                                                               │
     ├───────┬───┐                                   ┌───────────────┤
  50 │ int21 │far│                                   │      fcb1∙∙∙  │
     ├───────┴───┴───────────────────────────────────┼───────────────┤
  60 │ ∙∙∙                                           │      fcb2∙∙∙  │
     ├───────────────────────────────────────────────┼───────────────┤
  70 │ ∙∙∙                                           │      rsv      │
     ├───┬───────────────────────────────────────────┴───────────────┤
  80 │num│                                                           │
     ├───┘                                                           ┤
  90 │                                                               │
     ∙                                                               ∙
     ∙                            command                            ∙
     ∙                                                               ∙
  F0 │                                                               │
     └───┴───┴───┴───┴───┴───┴───┴───┴───┴───┴───┴───┴───┴───┴───┴───┘
 
  Field      Description
 
  int20      Opcode for Int 20h (CDh 20h).
  mem        Segment address of next free memory block.
  rsv        Reserved.
  calldisp   Opcode for far call to DOS function dispatcher.
  adr22      Far address of terminate routine (interrupt 22h).*
  adr23      Far address of Ctrl+C Handler routine (interrupt 23h).*
  adr24      Far address of Critical Error Handler routine (interrupt 24h).*
  env        Segment address of program's environment table.
  int21      Opcode for Int 21h (CDh 21h).
  far        Opcode for RETF (CBh).
  fcb1       Initial 16 bytes of first file control block (FCB #1).£
  fcb2       Initial 20 bytes of second file control block (FCB #2).£
  num        Number of characters in command line. Also begins default
             128-byte disk transfer area (DTA).
  command    Command-line arguments as entered from keyboard.
 
  *  DOS copies these addresses from the interrupt vector table to the PSP
     when the program is loaded. When the program terminates, DOS restores
     the addresses (or "vectors") from the program's PSP back to the
     interrupt vector table.
 
  £  Each FCB is 37 bytes wide, but can be preceded by a 7-byte extension.
     The FCBs cannot be open at the same time, since each overwrites the
     other.
 
  See also: Interrupt 20h (Program Terminate),
            Interrupt 22h (Terminate Address),
            Interrupt 23h (Ctrl+C Handler),
            Interrupt 24h (Critical Error Handler), FCB Diagram
                                    -♦-