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Since there are 8 sectors of 128 words each, in the internal program RAM, the 32 bit ad-
dress is divided into the following two fields:
•
7 LSBs for the word displacement or offset in the sector
•
25 MSBs for the tag
The sector placement algorithm is fully associative so that each external program memory
sector could be placed in any of the 8 internal program RAM sectors, essentially making
it an eight-way fully associative cache.
A 25-bit tag is associated with every one of the eight internal program memory RAM sec-
tors. When the cache controller searches for a tag equal to the tag field of the current ad-
dress, it compares it to the eight tags in parallel using the eight comparators.
Each word in each cache sector is associated with a cache-word-valid-bit (or valid-bit),
that specifies whether the data in that word has already been fetched from external mem-
ory and is therefore valid. There are a total of 1024 valid-bits, arranged as eight banks of
128 valid-bits each, one bank for every sector. Note that the valid-bits are not available to
the user for direct use. They are cleared by the processor RESET to indicate that the
PRAM context has not been initialized.
MOTOROLA
32 bit address
comparator 0
tag 0
Tag Values
Figure 2 - Cache Controller Block Diagram
25 bit Tag Field
7 bits
hit/miss
128 valid
bits for
sector 0
5
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