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Unformatted text preview: , or both). This operation is called a cache line fill. If the memory location containing that operand is still cached the next time the processor attempts to access the operand, the processor can read the operand from the cache instead of going back to memory. This operation is called a cache hit. 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 the 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 write buffer, and then written from the write buffer to memory when the system bus is available. (Note that for the Intel486™ and Pentium® processors, write misses do not result in a cache line fill; they always result in a write to memory. For these processors, only read misses result in cache line fills.) When operating in a multiple-processor system, Intel Architecture 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 processors without actually writing it to system memory; however, it is the responsibility of the memory controller to snoop this operation and update memory. 9.3. METHODS OF CACHING AVAILABLE The processor allows any area of system memory to be cached in the L1 and L2 caches. Within individual pages or regions of system memory, it also allows the type of caching (also called memory type) to be specified, using a variety of system flags and registers. For more information, see Section 9.5., “Cache Control”. The caching methods currently defined for the Intel Architecture are as follows. (Table 9-2 lists which types of caching are available on specific Intel Architecture processors.) • 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 9-5 MEMORY CACHE CONTROL 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. Table 9-2. Methods of Caching Available in P6 Family, Pentium®, and Intel486™ Processors Caching Method Uncacheable (UC) Write Combining (WC) Write Through (WT) Write Back (WB) Write Protected (WP) NOTES: 1. Requires programming of MTRRs to implement. 2. Speculative reads not supported. P6 Family Processors Yes Yes1 Yes Yes Yes 1 Pentium® Processor Yes No Yes 2 Intel486™ Processor Yes No Yes2 No No Yes2 No • Write Combining (WC)—System memory locations are not cached (as with uncacheable memory) and coherency is not enforced by the processor’s bus coherency protocol. Speculative reads are allowed. Writes may be delayed and combined in the write buffer to reduce memory accesses. The writes may be delayed until the next occurrence of a buffer or processor serialization event, e.g., CPUID execution, a read or write to uncached memory, interrupt occurrence, LOCKed instruction execution, etc. if the WC buffer is partially filled. This type of cache-control is appropriate for video frame buffers, where the order of writes is unimportant as long as the writes update memory so they can be seen on the graphics display....
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This note was uploaded on 06/07/2013 for the course ECE 1234 taught by Professor Kwhon during the Spring '10 term at University of California, Berkeley.

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