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28
The Processor and the Operating System
General Memory Organization
In order to find your way around the machine effectively, you should be aware of where things
are stored in memory. Occasionally these areas can become considerations in your
programming.
First in the memory come the internal registers. They were discussed above.
Next in the memory comes the ICOM area. The starting location is dependent upon system
needs, but is always at least 418. The size of the ICOM region depends upon the size designated
by the ICOM statement. Its maximum ending address is 777568. This is the reason for the
limitation on the size in the ICOM statement.
Next in the memory comes the area reserved for the system to store programs and the like. This
area extends from the end of the ICOM region to 1776178.
This area is followed by the registers in the read/write memory (see the list in the previous
section) with a number of interspersed system-reserved areas.
Figure 6 is a graphical presentation of this organization.
The immediately addressable memory consists of 65 536 words, which is all that can be ad-
dressed by a 16-bit word (the basic unit of memory in the system). Note that the memory is
divided into two blocks - an "upper" block and a "lower" one. This distinction between
blocks becomes significant when addressing individual bytes in memory.
Protected Memory
All of the reserved areas mentioned above are known as "protected memory". To give some
measure of security to the operating system, it is advised that no attempt should be made to
write or branch into these areas.
Access to certain portions of protected memory (e.g., BASIC variables) is provided by utilities
within the assembly system. The user should access those areas only through the utilities.
Some measure of protection against access into these areas is provided during debugging. See
Chapter 8 for a discussion of how this is done and the extent of the protection provided.
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