Embedded Systems Design, 2nd Edition

Memory systems 111

Bus snooping

With bus snooping, a memory cache monitors the external

bus for any access to data within the main memory that it already

has. If the cache data is more recent, the cache can either supply it

direct or force the other master off the bus, update main memory

and start a retry, thus allowing the original master access to valid

data. As an alternative to forcing a retry, the cache containing the

valid data can act as memory and supply the data directly. As

previously discussed, bus snooping is essential for any multimaster

system to ensure cache coherency.

MC88100s

MC88200s

5

Main

memory

BA

Shared

unmodified

Invalid

SNOOPED

5

5

Processor ...