Intel IA-32 - Manual (Page 445)

Vol. 3A 10-5
MEMORY CACHE CONTROL
When the processor attempts to write an operand to a cacheable area of memory, it first checks
if a cache line for that memory location exists in the cache. If a valid cache line does exist, the
processor (depending on the write policy currently in force) can write the operand into the cache
instead of writing it out to system memory. This operation is called a write hit. If a write misses
the cache (that is, a valid cache line is not present for area of memory being written to), the
processor performs a cache line fill, write allocation. Then it writes the operand into the cache
line and (depending on the write policy currently in force) can also write it out to memory. If the
operand is to be written out to memory, it is written first into the store buffer, and then written
from the store buffer to memory when the system bus is available. (Note that for the Pentium
processor, write misses do not result in a cache line fill; they always result in a write to memory.
For this processor, only read misses result in cache line fills.)
When operating in an MP system, IA-32 processors (beginning with the Intel486 processor)
have the ability to snoop other processor’s accesses to system memory and to their internal
caches. They use this snooping ability to keep their internal caches consistent both with system
memory and with the caches in other processors on the bus. For example, in the Pentium and P6
family processors, if through snooping one processor detects that another processor intends to
write to a memory location that it currently has cached in shared state, the snooping processor
will invalidate its cache line forcing it to perform a cache line fill the next time it accesses the
same memory location.
Beginning with the P6 family processors, if a processor detects (through snooping) that another
processor is trying to access a memory location that it has modified in its cache, but has not yet
written back to system memory, the snooping processor will signal the other processor (by
means of the HITM# signal) that the cache line is held in modified state and will preform an
implicit write-back of the modified data. The implicit write-back is transferred directly to the
initial requesting processor and snooped by the memory controller to assure that system memory
has been updated. Here, the processor with the valid data may pass the data to the other proces-
sors without actually writing it to system memory; however, it is the responsibility of the
memory controller to snoop this operation and update memory.
10.3 METHODS OF CACHING AVAILABLE
The processor allows any area of system memory to be cached in the L1, L2, and L3 caches. In
individual pages or regions of system memory, it allows the type of caching (also called
memory type) to be specified (see Section 10.5). Memory types currently defined for the IA-32
architecture are as follows (see Table 10-2):
•
Strong Uncacheable (UC) —System memory locations are not cached. All reads and
writes appear on the system bus and are executed in program order without reordering. No
speculative memory accesses, page-table walks, or prefetches of speculated branch targets
are made. This type of cache-control is useful for memory-mapped I/O devices. When
used with normal RAM, it greatly reduces processor performance.