The
fourth
part
specifies how various signals relate
to
the available ports of the 8520.
This
information enables the programmer
to
relate the
port
addresses
to
the outside-
world items (or internal control signals)
that
are
to
be affected.
The
third and fourth
parts
are primarily for the use of the systems programmer and should generally not be
utilized by applications programmers. Systems software normally is configured
to
handle
the
setting of particular signals, no
matter
how the physical connections may change. In
other
words, if you have a version of the system software
that
matches the revision level
of the machine (normally a true condition), when you ask
that
a particular bit be set,
you
don't
care which
port
that
bit
is
connected to. Thus, applications programmers
should rely on system documentation instead of going directly
to
the ports. Note also
that
in a multitasking operating system, many different tasks may
be
competing for the
use
of
the system resources. Applications programmers should follow the established
rules for resource access in order
to
assure compatibility of their software with the sys-
tem.
See the figures
at
the end of this appendix for more information about the fire buttons,
light pen, mouse, and the
"pot"
counters.
E-2
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