IBM PowerPC 604 User Manual page 135

and a retry indicator (that is, another processor either has a modified copy of the addressed
block that it needs
to
push out of the chip, or another processor had a queuing problem that
prevented appropriate snooping from occurring).
To maximize performance, the 604 provides a second path into the data cache directory for
snooping. This allows the mainstteam instruction processing to operate concurrently with
the snooping operation. The instruction processing is affected only when the snoop conttol
logic detects a situation where a snoop push of modified data is required
to
maintain
memory coherency.
M
-
E
-
3·12
Modified in Cache A
Cache A
Cache B
Valid Data
System Memory
Data invalid\
not congruent
Exclusive in Cache A
Cache A Cache B
Valid Data
System Memory
Valid Data
s
-
---!
Shared in Cache A
Cache A CacheB
Valid Data
s
-
Valid Data
System Memory
Valid Data
Invalid in Cache A
Cache A Cache B
Invalid Date
--i
Don't Care
System Memory
Don't Care
Figure 3-5. MESI States
PowerPC 604 RISC Microproceasor User's Manual