Service manual

Cache Coherency B-19
B.9.2 IOP Tag Store
Each IOP in the system implements two fully associative data caches; one
“write” cache and one “read” cache. As such, each IOP also implements a fully
associative tag store. Table B–7 describes information stored in the IOP tag
store.
Table B– 7 IOP Tag Cache Coherency Storage
IOP Tag Field Field Description
Valid
When set, indicates that a memory block is cached at the
associated cache location.
Dirty Indicates that the cached block is writeable.
Tag
Identifies the cached memory block; equivalent to the high
order address bits of the cached block.
Both the IOP and the QBB Switch QSA chip access the IOP tag store. The main
functions of the IOP tag store are:
• To provide coherency status for requests to local addresses.
• To respond to any coherency updates required as a result of requests to both
local and remote addresses.