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 Fns by Category