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
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About DOS Functions