Xerox 550 Reference Manual page 161

Table 26. Functions of Memory Unit Configuration Control Panel Row (cont.)
Label
Switch/Indicator
STARTING ADDRESS 7 switches
154 Configuration Control Panel
Function
Used to set the starting addresses of the memory units. From left to
right the switches are labeled S12, S13, S14, S15, S16, S17, and
S 18. Using the switches as a 7-position binary field, this allows
memory to address up to 1 mi Ilion words.
When the memory system comprises memory units of different sizes,
some precautions are necessary to prevent false address recognition
as well as to prevent gaps in the memory range.
1. If all memory units have sizes that are powers of two, they can
a
II
be di fferent; they must , however, be assi gned in order of
decreasing size in the address continuum.
For instance, three memory units can be used in this manner:
Memory Unit No. Size Address Range
- -
1 64K words
o
to 64K
word~
2 32K words 64 to 96K words
3 16K words
96 to 112K words
2.
If a memory unit has a size that is not a power of two, it must
be situated in a memory system that satisfies the following
rules:
a. All other memory units must have sizes that are a power
of two.
b. The starting address of the non-power-of-two unit must be
a multiple of the'next power of two above the size of that
unit.
c. The memory unit whose size is not a power of two must be
at the upper end of the address range.