Cache Memory; Ka49 Cache/Memory Hierarchy - Digital Equipment VAXstation 4000 90 Service Information

Cache Memory

Overview
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The NVAX memory subsystem follows a hierarchical structure.
The VIC, Pcache, Bcache, and finally the main memory form the
hierarchical memory subsystem of the KA49. The hierarchical
ordering of the various levels of KA49 memory is shown in
Figure 1–3. For I-stream references, the memory hierarchy
starts with the VIC, whereas for D-stream references the memory
hierarchy starts with the Pcache.
Figure 1–3 KA49 Cache/Memory Hierarchy
References generated by the NVAX CPU are issued to the memory
subsystem at the first hierarchical level, as determined by the
reference type (I-stream or D-stream). The reference then passes
up through the hierarchy until it is serviced by one of the layers.
References that are serviced at lower layers take less time than
references that must pass to higher layers. For this reason, it is
the intent of the memory subsystem to service most references
within the lower layers, thus maximizing system performance.
By creating successively faster layers of memory hierarchy below
the main memory, the KA49 decreases the average amount
of time required to access information. Because each layer in
the hierarchy tends to be smaller in size than the next higher
(slower) layer, there is the problem of allocating space at each
M a s s S t o r a g e
M a i n M e m o r y
B a c k u p C a c h e 2 5 6 K B
P r i m a r y C a c h e 8 K B
V I C 2 K B
L J - 0 1 8 1 7 - T I 0
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