About DOS Functions

 The DOS functions are a haphazard set of file, device, memory, and process
 control services that are available to any program which is capable of
 setting registers and invoking software interrupts.

 To access DOS functions:
  ■ Place a DOS Function Number in the AH register (if one or more
    sub-functions exist, its value usually goes in AL)
  ■ Load up the other registers as described in the function summary
  ■ Prepare any buffers, ASCIIZ strings, and control blocks needed
  ■ Be aware of (or set) the location of the DTA if necessary
  ■ Invoke INT 21H
  ■ Look for error indicators returned by DOS (examine the Carry Flag)

 DOS preserves the entry values of all registers except when information is
 returned in a register.

 As DOS evolved, later version have remained mostly compatible with earlier
 versions.  See DOS Versions for a discussion of incompatibilities.

See Also: DOS Functions
          DOS Interrupts
          DOS Fns by Category