AMD AMD-K6-2/500AFX Data Sheet - Page 214
Cache Coherency, Inquire Cycles, Internal Snooping
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AMD-K6®-2 Processor Data Sheet Preliminary Information 21850J/0-February 2000 7.10 Cache Coherency Inquire Cycles Internal Snooping Different ways exist to maintain coherency between the system memory and cache memories. Inquire cycles, internal snoops, FLUSH#, WBINVD, INVD, and line replacements all prevent inconsistencies between memories. Inquire cycles are bus cycles initiated by system logic which ensure coherency between the caches and main memory. In systems with multiple bus masters, system logic maintains cache coherency by driving inquire cycles to the processor. System logic initiates inquire cycles by asserting AHOLD, BOFF#, or HOLD to obtain control of the address bus and then driving EADS#, INV (optional), and an inquire address (A[31:5]). This type of bus cycle causes the processor to compare the tags for both its instruction and data caches with the inquire address. If there is a hit to a shared or exclusive line in the data cache or a valid line in the instruction cache, the processor asserts HIT#. If the compare hits a modified line in the data cache, the processor asserts HIT# and HITM#. If HITM# is asserted, the processor writes the modified line back to memory. If INV was sampled asserted with EADS#, a hit invalidates the line. If INV was sampled negated with EADS#, a hit leaves the line in the shared state or transitions it from the exclusive or modified state to the shared state. Table 37 on page 197 shows the effects of inquire cycles- performed with INV equal to 0 (non-invalidating) and INV equal to 1 (invalidating)-snoops, and invalidations. Internal snooping is initiated by the processor (rather than system logic) during certain cache accesses. It is used to maintain coherency between the instruction cache and the data cache. The processor automatically snoops its instruction cache during read or write misses to its data cache, and it snoops its data cache during read misses to its instruction cache. Table 37 summarizes the actions taken during this internal snooping. 194 Cache Organization Chapter 7