Compatibility: 1.0+
These low-level I/O functions are given special treatment.  DOS switches
to a special stack when calling these fns.  The idea is to allow higher-
level fns to call these without reentrancy problems.  In particular, it is
OK for device drivers to use fn 01H-0cH during initialization.  And popup
TSR programs must pay special attention to when these fns are available
and when not.

See TSR and InDOS Flag and ErrorMode Flag and INT 24H for related details.

See: 01H Keyboard Input
02H Display Output
03H Auxiliary Input
04H Auxiliary Output
05H Printer Output
06H Console I/O
07H No Echo Unfiltered Console Input
08H No Echo Console Input
09H Display String
0aH Buffered String Input
0bH Check Input Status
0cH Clear & Input

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DOS Fns 01H-0cH: Low-level I/O