AMD AMD5K86 - Inquire Cycles

AMD~
AMD5f1J6 Processor Technical Reference Manual 18524BjO-Mar1996
6.2.4
6-12
• Repetitive writes to the same location are slower than in
writeback mode.
• No updates to the data cache are hidden from the sys-
tem.
• When returning from S:MlVI with S:MlVI memory cache-
able, there is no need to write back modified lines in the
data cache, so the mode transition may be faster. (Both
caches, however, must be invalidated.)
• Writeback Protocol:
• Repetitive writes to the same location are faster than in
writethrough mode.
• Updates that hit exclusive or modified lines in the data
cache are hidden from the system.
• When returning from S:MlVI, in which S:MlVI memory is
cache able, modified lines in the data cache must be writ-
ten back before invalidating both caches, so the mode
transition may be slower.
In single-processor systems with no other caching master, WBI
WT is typically tied High. This allows the processor to cache all
cacheable reads in the exclusive state, and all cacheable writes
update only the cache. In systems with multiple caching mas-
ters, WBIWT can be generated after inquire cycles to all other
caching masters by the logical OR of HIT from all of the mas-
ters. This allows the processor to cache reads in the exclusive or
modified state only if no other master has a copy.
The write-once protocol, as described in Section 6.2.6 on page
6-19, combines the system visibility features of pure
writethrough and write back protocols. While the write back
function can support higher performance in systems with a sin-
gle
caching master, the writethrough function is required for
certain transitions in the write-once protocol in systems with
multiple caching masters.
Inquire Cycles
System logic maintains coherency between external caching
devices and the processor's internal caches by driving inquire
cycles to the processor during shared-memory accesses by
other caching masters. Inquire cycles are often called snoops or
invalidations, but these terms are too general to clearly differ-
System Design