Radio Shack TRS-80 Model II Reference Manual page 261

Technical Information
C. The most common high-memory module used is probably the communications
drivers, as these are used by the serial printer software in the operating system.
If you use a serial printer, you will not want to overlay X'FOOO-F3FF' (64K
machine) orX'7000-73FF' (32K machine). The code in that area must be left intact
at all times that the comm drivers are active. Overlaying anything here will
cause problems, some of which could be very serious.
D. SPOOLER is another high-memory module that can be used by almost any
application that has printed output. It has been designed to be used without any
changes in the application software. It may be used at any time, and the
decision to use it may rest in the operator of the Model II. To provide, in your
application software, the most flexibility for the operator, you should not use
the RAM area of 'F640-F800' (64K machine) or '7640-7C80' (32K machine). If you
need this RAM, you must make it clear to the operator of the system that spooler
may not be used. Serious problems will occur if any of the spooler code space is
overlayed after the system has loaded the spooler code in that area.
E.
HOST is another high-RAM routine that is designed to be "transparent" to the
application software. Host allows for a remote terminal to execute your
applications and programs as if the remote were local (the local keyboard and
video). If this high-RAM space is overlayed with a user-subroutine, serious
problems will occur. Operation flexibility suffers if an application is written
that depends upon the use of this space for a subroutine or program.
As you can see, the use of any high-RAM space will prevent the use of some or all of
the system functions that use this space. Careful planning of what will be needed,
when it will be needed, and what you want to instruct the operator of the system to
do (or not to do) when the operator runs your application, may provide you with the
extra RAM space you need.
An example might be that your application program will not create printed output.
In this case, even if you have a serial printer, you will not need the spooler RAM area
or the comm drivers RAM area (if comm drivers were needed just for the serial
printer support). If you make sure that the spooler and the comm drivers are not
active before or during the run of the application, then you may use this RAM space.
One way to be sure of this is to start execution of the application with a DO file
(which resides in the top of RAM). In this DO file, you may shut off spooler and
comm before the application itself is executed. That way, you can be sure that these
high-RAM modules will not be resident and will not be overlayed. Remember that
you must shut down both comm channels (A
& B)
before the high-RAM comm
drivers are considered inactive by the system.
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