Radio Shack TRS-80 Model II Reference Manual page 260

Modell! TRSDOS
High-Memory Modules and Their Addresses
and Use
As mentioned earlier, there are five high-memory modules that are conditionally
loaded. There are DO, SPOOLER, HOST, DEBUG and communications drivers.
In
a
64K machine, these reside above X'FOOO'; in a 32K machine, these reside at X'7000'
and above. The approximate memory map is as follows:
Note: Addresses may change in future releases.
Module
Comm drivers
DEBUG
HOST*
SPOOLER*
DO*
32K
X'7000'
X'7400'
X'7400'
X'7640'
X'7C80'
64K
X'FOOO'
X'F400'
X'F400'
X'F640'
X'FC80'
*Mutually exclusive with DEBUG
Some applications require a large amount of RAM to be able to execute (e. g. , a large
BASIC program or a machine-language subroutine that must link to BASIC and you
need full RAM for the BASIC program itself). If this is the case, you may use the
upper portion of RAM for your application if you consider the following:
A. The use of RAM above the X'7000' (32K machine) or X'FOOO' (64K machine)
boundary will preclude the use of certain high-memory modules. If you invoke
a system function that loads this high-RAM with a module, and then your
application loads something on top of this, you will certainly experience
problems. Refer to the memory map above for details. The system cannot
prevent you from overlaying an already loaded high-memory module. You
must take the appropriate care to prevent any problems if you choose to use this
RAM space.
B. The operating system now, upon program-load, will either give X'EFFF' (64K
machine) or X'6FFF' (32K machine) in register DE upon entry to the program as
the high-RAM address. To the user of BASIC , this will give you slightly less RAM
for program and variable storage than was available in earlier versions of
TRSDOS. This may be over-ridden, if necessary, by specifying the command
line: BASIC - M:XXXXX where XXXXX is the decimal high-RAM address to use
as the upper limit.
This must never exceed decimal 63487fora
64K
machine!
The same rule applies to an applications program. The intent of this RAM
restriction is that if you never use the RAM above this high-memory mark, you
will be able to use all ofthe high-memory routines during the execution of your
programs.
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