29 Facts About Data cache

CPU cache is a hardware cache used by the central processing unit of a computer to reduce the average cost to access data from the main memory.

The cache memory is typically implemented with static random-access memory, in modern CPUs by far the largest part of them by chip area, but SRAM is not always used for all levels, or even any level, sometimes some latter or all levels are implemented with eDRAM.

The first CPUs that used a cache had only one level of cache; unlike later level 1 cache, it was not split into L1d and L1i.

Split L1 Data cache started in 1976 with the IBM 801 CPU, became mainstream in the late 1980s, and in 1997 entered the embedded CPU market with the ARMv5TE.

The L2 Data cache is usually not split and acts as a common repository for the already split L1 Data cache.

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L4 Data cache is currently uncommon, and is generally on dynamic random-access memory, rather than on static random-access memory, on a separate die or chip.

Alternatively, in a write-back or copy-back Data cache, writes are not immediately mirrored to the main memory, and the Data cache instead tracks which locations have been written over, marking them as dirty.

The Data cache hit rate and the Data cache miss rate play an important role in determining this performance.

One benefit of this scheme is that the tags stored in the Data cache do not have to include that part of the main memory address which is implied by the Data cache memory's index.

Since the Data cache tags have fewer bits, they require fewer transistors, take less space on the processor circuit board or on the microprocessor chip, and can be read and compared faster.

An effective memory address which goes along with the Data cache line is split into the tag, the index and the block offset.

An instruction Data cache requires only one flag bit per Data cache row entry: a valid bit.

The first hardware cache used in a computer system was not actually a data or instruction cache, but rather a TLB.

Alternatively, if Data cache entries are allowed on pages not mapped by the TLB, then those entries will have to be flushed when the access rights on those pages are changed in the page table.

Also, during miss processing, the alternate ways of the Data cache line indexed have to be probed for virtual aliases and any matches evicted.

Since virtual hints have fewer bits than virtual tags distinguishing them from one another, a virtually hinted Data cache suffers more conflict misses than a virtually tagged Data cache.

Cache entry replacement policy is determined by a Data cache algorithm selected to be implemented by the processor designers.

The victim Data cache is usually fully associative, and is intended to reduce the number of conflict misses.

The main disadvantage of the trace Data cache, leading to its power inefficiency, is the hardware complexity required for its heuristic deciding on caching and reusing dynamically created instruction traces.

Smart Data cache is a level 2 or level 3 caching method for multiple execution cores, developed by Intel.

Furthermore, the shared Data cache makes it faster to share memory among different execution cores.

Typically, sharing the L1 Data cache is undesirable because the resulting increase in latency would make each core run considerably slower than a single-core chip.

However, for the highest-level cache, the last one called before accessing memory, having a global cache is desirable for several reasons, such as allowing a single core to use the whole cache, reducing data redundancy by making it possible for different processes or threads to share cached data, and reducing the complexity of utilized cache coherency protocols.

An associative Data cache is more complicated, because some form of tag must be read to determine which entry of the Data cache to select.

An N-way set-associative level-1 cache usually reads all N possible tags and N data in parallel, and then chooses the data associated with the matching tag.

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Early history of Data cache technology is closely tied to the invention and use of virtual memory.

The popularity of on-motherboard Data cache continued through the Pentium MMX era but was made obsolete by the introduction of SDRAM and the growing disparity between bus clock rates and CPU clock rates, which caused on-motherboard Data cache to be only slightly faster than main memory.

Early Data cache designs focused entirely on the direct cost of Data cache and RAM and average execution speed.

Multi-ported Data cache is a Data cache which can serve more than one request at a time.