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.
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
Look for error indicators returned by DOS (examine the Carry Flag)
DOS preserves the entry values of all registers except when information is
As DOS evolved, later version have remained mostly compatible with earlier
versions. See DOS Versions for a discussion of incompatibilities.