Top Tip: What's the difference between RAM and Cache? - ExtremeTech
Cache special high-speed storage mechanism. It can be either a reserved section of main memory or an independent high-speed storage device. This definition explains the meaning of cache memory, also known as CPU memory, from the cache rather than having to get it from computer's main memory. .. What is the relationship between virtual memory, cache memory and RAM?. In computing, a cache is a hardware or software component that stores data so that future Hardware implements cache as a block of memory for temporary storage of data . While the disk buffer, which is an integrated part of the hard disk drive, is sometimes misleadingly referred to as "disk cache", its main functions are.
The victim cache exploits this property by providing high associativity to only these accesses. Generally, instructions are added to trace caches in groups representing either individual basic blocks or dynamic instruction traces. Having this, the next time an instruction is needed, it does not have to be decoded into micro-ops again.
The WCC's task is reducing number of writes to the L2 cache. The main disadvantage of the trace cache, leading to its power inefficiency, is the hardware complexity required for its heuristic deciding on caching and reusing dynamically created instruction traces. This is used by low-powered processors which do not need a normal instruction cache because the memory system is capable of delivering instructions fast enough to satisfy the CPU without one. However, this only applies to consecutive instructions in sequence; it still takes several cycles of latency to restart instruction fetch at a new address, causing a few cycles of pipeline bubble after a control transfer.
A branch target cache provides instructions for those few cycles avoiding a delay after most taken branches. This allows full-speed operation with a much smaller cache than a traditional full-time instruction cache. Cache hierarchy Another issue is the fundamental tradeoff between cache latency and hit rate.
Larger caches have better hit rates but longer latency. To address this tradeoff, many computers use multiple levels of cache, with small fast caches backed up by larger, slower caches. Multi-level caches generally operate by checking the fastest, level 1 L1 cache first; if it hits, the processor proceeds at high speed.
If that smaller cache misses, the next fastest cache level 2, L2 is checked, and so on, before accessing external memory. As the latency difference between main memory and the fastest cache has become larger, some processors have begun to utilize as many as three levels of on-chip cache.
Price-sensitive designs used this to pull the entire cache hierarchy on-chip, but by the s some of the highest-performance designs returned to having large off-chip caches, which is often implemented in eDRAM and mounted on a multi-chip moduleas a fourth cache level.
The benefits of L3 and L4 caches depend on the application's access patterns. Examples of products incorporating L3 and L4 caches include the following: However, with register renaming most compiler register assignments are reallocated dynamically by hardware at runtime into a register bank, allowing the CPU to break false data dependencies and thus easing pipeline hazards.
Register files sometimes also have hierarchy: The Cray-1 circa had eight address "A" and eight scalar data "S" registers that were generally usable. There was also a set of 64 address "B" and 64 scalar data "T" registers that took longer to access, but were faster than main memory. The "B" and "T" registers were provided because the Cray-1 did not have a data cache. The Cray-1 did, however, have an instruction cache. Multi-core chips[ edit ] When considering a chip with multiple coresthere is a question of whether the caches should be shared or local to each core.
Implementing shared cache inevitably introduces more wiring and complexity. But then, having one cache per chip, rather than core, greatly reduces the amount of space needed, and thus one can include a larger cache. Typically, sharing the L1 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.
Top Tip: What’s the difference between RAM and Cache?
Shared highest-level cache, which is called before accessing memory, is usually referred to as the last level cache LLC. Additional techniques are used for increasing the level of parallelism when LLC is shared between multiple cores, including slicing it into multiple pieces which are addressing certain ranges of memory addresses, and can be accessed independently.
Exclusive versus inclusive[ edit ] Multi-level caches introduce new design decisions. For instance, in some processors, all data in the L1 cache must also be somewhere in the L2 cache. These caches are called strictly inclusive. Other processors like the AMD Athlon have exclusive caches: There is no universally accepted name for this intermediate policy. This advantage is larger when the exclusive L1 cache is comparable to the L2 cache, and diminishes if the L2 cache is many times larger than the L1 cache.
When the L1 misses and the L2 hits on an access, the hitting cache line in the L2 is exchanged with a line in the L1. This exchange is quite a bit more work than just copying a line from L2 to L1, which is what an inclusive cache does. In cache hierarchies which do not enforce inclusion, the L1 cache must be checked as well. As a drawback, there is a correlation between the associativities of L1 and L2 caches: Another disadvantage of inclusive cache is that whenever there is an eviction in L2 cache, the possibly corresponding lines in L1 also have to get evicted in order to maintain inclusiveness.
This is quite a bit of work, and would result in a higher L1 miss rate. Exclusive caches require both caches to have the same size cache lines, so that cache lines can be swapped on a L1 miss, L2 hit.
If the secondary cache is an order of magnitude larger than the primary, and the cache data is an order of magnitude larger than the cache tags, this tag area saved can be comparable to the incremental area needed to store the L1 cache data in the L2.
The K8 has four specialized caches: Each of these caches is specialized: The instruction cache keeps copies of byte lines of memory, and fetches 16 bytes each cycle. Each byte in this cache is stored in ten bits rather than eight, with the extra bits marking the boundaries of instructions this is an example of predecoding. The cache has only parity protection rather than ECCbecause parity is smaller and any damaged data can be replaced by fresh data fetched from memory which always has an up-to-date copy of instructions.
Each cycle's instruction fetch has its virtual address translated through this TLB into a physical address. Each entry is either four or eight bytes in memory. The split allows the fully associative match circuitry in each section to be simpler.
The operating system maps different sections of the virtual address space with different size PTEs. The data TLB has two copies which keep identical entries. The two copies allow two data accesses per cycle to translate virtual addresses to physical addresses.
The data cache keeps copies of byte lines of memory.
Difference between Cache Memory and Main Memory | Cache Memory vs Main Memory
There are two copies of the tags, because each byte line is spread among all eight banks. Each tag copy handles one of the two accesses per cycle. The K8 also has multiple-level caches. Both instruction and data caches, and the various TLBs, can fill from the large unified L2 cache.
This cache is exclusive to both the L1 instruction and data caches, which means that any 8-byte line can only be in one of the L1 instruction cache, the L1 data cache, or the L2 cache. It is, however, possible for a line in the data cache to have a PTE which is also in one of the TLBs—the operating system is responsible for keeping the TLBs coherent by flushing portions of them when the page tables in memory are updated.
The K8 also caches information that is never stored in memory—prediction information. These caches are not shown in the above diagram. As is usual for this class of CPU, the K8 has fairly complex branch predictionwith tables that help predict whether branches are taken and other tables which predict the targets of branches and jumps. Some of this information is associated with instructions, in both the level 1 instruction cache and the unified secondary cache.
The K8 uses an interesting trick to store prediction information with instructions in the secondary cache. Lines in the secondary cache are protected from accidental data corruption e.
Since the parity code takes fewer bits than the ECC code, lines from the instruction cache have a few spare bits. These bits are used to cache branch prediction information associated with those instructions.
- Cache (computing)
- Computer - Memory
- Difference between Cache Memory and Main Memory
The net result is that the branch predictor has a larger effective history table, and so has better accuracy. More hierarchies[ edit ] Other processors have other kinds of predictors e. These predictors are caches in that they store information that is costly to compute.
Some of the terminology used when discussing predictors is the same as that for caches one speaks of a hit in a branch predictorbut predictors are not generally thought of as part of the cache hierarchy. The K8 keeps the instruction and data caches coherent in hardware, which means that a store into an instruction closely following the store instruction will change that following instruction.
Other processors, like those in the Alpha and MIPS family, have relied on software to keep the instruction cache coherent.
What is Cache Memory? - Definition from Techopedia
Stores are not guaranteed to show up in the instruction stream until a program calls an operating system facility to ensure coherency. Higher associative caches usually employ content-addressable memory. Cache algorithms Cache reads are the most common CPU operation that takes more than a single cycle.cache memory in computer architecture
Program execution time tends to be very sensitive to the latency of a level-1 data cache hit. A great deal of design effort, and often power and silicon area are expended making the caches as fast as possible. Secondary Memory This type of memory is also known as external memory or non-volatile.
It is slower than the main memory. CPU directly does not access these memories, instead they are accessed via input-output routines.
The contents of secondary memories are first transferred to the main memory, and then the CPU can access it. Characteristics of Secondary Memory These are magnetic and optical memories. It is known as the backup memory. It is a non-volatile memory. Data is permanently stored even if power is switched off.
It is used for storage of data in a computer. Computer may run without the secondary memory. Slower than primary memories. The data in these locations are written back to the backing store only when they are evicted from the cache, an effect referred to as a lazy write. For this reason, a read miss in a write-back cache which requires a block to be replaced by another will often require two memory accesses to service: Other policies may also trigger data write-back.
The client may make many changes to data in the cache, and then explicitly notify the cache to write back the data. Since no data is returned to the requester on write operations, a decision needs to be made on write misses, whether or not data would be loaded into the cache.
This is defined by these two approaches: Write allocate also called fetch on write: In this approach, write misses are similar to read misses. No-write allocate also called write-no-allocate or write around: In this approach, data is loaded into the cache on read misses only. Both write-through and write-back policies can use either of these write-miss policies, but usually they are paired in this way: A write-through cache uses no-write allocate.
Here, subsequent writes have no advantage, since they still need to be written directly to the backing store. Entities other than the cache may change the data in the backing store, in which case the copy in the cache may become out-of-date or stale. Alternatively, when the client updates the data in the cache, copies of those data in other caches will become stale.
Communication protocols between the cache managers which keep the data consistent are known as coherency protocols. Examples of hardware caches[ edit ] Main article: Most CPUs since the s have used one or more caches, sometimes in cascaded levels ; modern high-end embeddeddesktop and server microprocessors may have as many as six types of cache between levels and functions .