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Text of the page (random words):
of cpu the k8 has fairly complex branch prediction with 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 g by an alpha particle strike by either ecc or parity depending on whether those lines were evicted from the data or instruction primary caches 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 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 g the store to load bypass predictor in the dec alpha 21264 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 predictor but 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 stores are not guaranteed to show up in the instruction stream until a program calls an operating system facility to ensure coherency tag ram edit tag ram on board of a intel pentium iii in computer engineering a tag ram is used to specify which of the possible memory locations is currently stored in a cpu cache 62 63 for a simple direct mapped design fast sram can be used higher associative caches usually employ content addressable memory implementation edit main article cache algorithms cache reads are the most common cpu operation that takes more than a single cycle 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 the simplest cache is a virtually indexed direct mapped cache the virtual address is calculated with an adder the relevant portion of the address extracted and used to index an sram which returns the loaded data the data are byte aligned in a byte shifter and from there are bypassed to the next operation there is no need for any tag checking in the inner loop in fact the tags need not even be read later in the pipeline but before the load instruction is retired the tag for the loaded data must be read and checked against the virtual address to make sure there was a cache hit on a miss the cache is updated with the requested cache line and the pipeline is restarted an associative cache is more complicated because some form of tag must be read to determine which entry of the 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 level 2 caches sometimes save power by reading the tags first so that only one data element is read from the data sram read path for a 2 way associative cache the adjacent diagram is intended to clarify the manner in which the various fields of the address are used address bit 31 is most significant bit 0 is least significant the diagram shows the srams indexing and multiplexing for a 4 kib 2 way set associative virtually indexed and virtually tagged cache with 64 byte b lines a 32 bit read width and 32 bit virtual address because the cache is 4 kib and has 64 b lines there are just 64 lines in the cache and we read two at a time from a tag sram which has 32 rows each with a pair of 21 bit tags although any function of virtual address bits 31 through 6 could be used to index the tag and data srams it is simplest to use the least significant bits similarly because the cache is 4 kib and has a 4 b read path and reads two ways for each access the data sram is 512 rows by 8 bytes wide a more modern cache might be 16 kib 4 way set associative virtually indexed virtually hinted and physically tagged with 32 b lines 32 bit read width and 36 bit physical addresses the read path recurrence for such a cache looks very similar to the path above instead of tags virtual hints are read and matched against a subset of the virtual address later on in the pipeline the virtual address is translated into a physical address by the tlb and the physical tag is read just one as the virtual hint supplies which way of the cache to read finally the physical address is compared to the physical tag to determine if a hit has occurred some sparc designs have improved the speed of their l1 caches by a few gate delays by collapsing the virtual address adder into the sram decoders see sum addressed decoder history edit the early history of cache technology is closely tied to the invention and use of virtual memory citation needed because of scarcity and cost of semi conductor memories early mainframe computers in the 1960s used a complex hierarchy of physical memory mapped onto a flat virtual memory space used by programs the memory technologies would span semi conductor magnetic core drum and disc virtual memory seen and used by programs would be flat and caching would be used to fetch data and instructions into the fastest memory ahead of processor access extensive studies were done to optimize the cache sizes optimal values were found to depend greatly on the programming language used with algol needing the smallest and fortran and cobol needing the largest cache sizes disputed discuss in the early days of microcomputer technology memory access was only slightly slower than register access but since the 1980s 64 the performance gap between processor and memory has been growing microprocessors have advanced much faster than memory especially in terms of their operating frequency so memory became a performance bottleneck while it was technically possible to have all the main memory as fast as the cpu a more economically viable path has been taken use plenty of low speed memory but also introduce a small high speed cache memory to alleviate the performance gap this provided an order of magnitude more capacity for the same price with only a slightly reduced combined performance first tlb implementations edit the first documented uses of a tlb were on the ge 645 65 and the ibm 360 67 66 both of which used an associative memory as a tlb first instruction cache edit the first documented use of an instruction cache was on the cdc 6600 67 first data cache edit the first documented use of a data cache was on the ibm system 360 model 85 68 in 68k microprocessors edit the 68010 released in 1982 has a loop mode which can be considered a tiny and special case instruction cache that accelerates loops that consist of only two instructions the 68020 released in 1984 replaced that with a typical instruction cache of 256 bytes being the first 68k series processor to feature true on chip cache memory the 68030 released in 1987 is basically a 68020 core with an additional 256 byte data cache an on chip memory management unit mmu a process shrink and added burst mode for the caches the 68040 released in 1990 has split instruction and data caches of four kilobytes each the 68060 released in 1994 has the following 8 kib data cache four way associative 8 kib instruction cache four way associative 96 byte fifo instruction buffer 256 entry branch cache and 64 entry address translation cache mmu buffer four way associative in x86 microprocessors edit example of a motherboard with an i386 microprocessor 33 mhz 64 kib cache 25 ns 8 chips in the bottom left corner 2 mib dram 70 ns 8 simms to the right of the cache and a cache controller austek a38202 to the right of the processor as the x86 microprocessors reached clock rates of 20 mhz and above in the 386 small amounts of fast cache memory began to be featured in systems to improve performance this was because the dram used for main memory had significant latency up to 120 ns as well as refresh cycles the cache was constructed from more expensive but significantly faster sram memory cells which at the time had latencies around 10 25 ns the early caches were external to the processor and typically located on the motherboard in the form of eight or nine dip devices placed in sockets to enable the cache as an optional extra or upgrade feature some versions of the intel 386 processor could support 16 to 256 kib of external cache with the 486 processor an 8 kib cache was integrated directly into the cpu die this cache was termed level 1 or l1 cache to differentiate it from the slower on motherboard or level 2 l2 cache these on motherboard caches were much larger with the most common size being 256 kib there were some system boards that contained sockets for the intel 485turbocache daughtercard which had either 64 or 128 kbyte of cache memory 69 70 the popularity of on motherboard 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 cache to be only slightly faster than main memory the next development in cache implementation in the x86 microprocessors began with the pentium pro which brought the secondary cache onto the same package as the microprocessor clocked at the same frequency as the microprocessor on motherboard caches enjoyed prolonged popularity thanks to the amd k6 2 and amd k6 iii processors that still used socket 7 which was previously used by intel with on motherboard caches k6 iii included 256 kib on die l2 cache and took advantage of the on board cache as a third level cache named l3 motherboards with up to 2 mib of on board cache were produced after the socket 7 became obsolete on motherboard cache disappeared from the x86 systems the three level caches were used again first with the introduction of the intel xeon mp foster core 71 where the l3 cache was added to the cpu die it became common for the total cache sizes to be increasingly larger in newer processor generations and recently as of 2011 it is not uncommon to find level 3 cache sizes of tens of megabytes 72 intel introduced a level 4 on package cache with the haswell microarchitecture crystalwell 38 haswell cpus equipped with the gt3e variant of intel s integrated iris pro graphics effectively feature 128 mib of embedded dram edram on the same package this l4 cache is shared dynamically between the on die gpu and cpu and serves as a victim cache to the cpu s l3 cache 39 in arm microprocessors edit the apple m1 cpu has 128 or 192 kib of l1 instruction cache for each core important for latency single thread performance depending on core type this is an unusually large l1 cache for any cpu type not just for a laptop the total cache memory size is not unusually large the total is more important for throughput for a laptop and much larger total e g l3 or l4 sizes are available in ibm s mainframes current research edit early cache designs focused entirely on the direct cost of cache and ram and average execution speed more recent cache designs also consider energy efficiency fault tolerance and other goals 73 74 there are several tools available to computer architects to help explore tradeoffs between the cache cycle time energy and area the cacti cache simulator 75 and the simplescalar instruction set simulator are two open source options multi ported cache edit a multi ported cache is a cache which can serve more than one request at a time when accessing a traditional cache we normally use a single memory address whereas in a multi ported cache we may request n addresses at a time where n is the number of ports that connected through the processor and the cache the benefit of this is that a pipelined processor may access memory from different phases in its pipeline another benefit is that it allows the concept of super scalar processors through different cache levels see also edit branch predictor cache computing cache algorithms cache coherence cache control instructions cache hierarchy cache placement policies cache prefetching dinero cache simulator instruction unit locality of reference memoization memory hierarchy micro operation no write allocation scratchpad ram sum addressed decoder write buffer notes edit the very first paging machine the ferranti atlas 29 30 had no page tables in main memory there was an associative memory with one entry for every 512 word page frame of core references edit torres gabriel september 12 2007 how the cache memory works su chao zeng qingkai 2021 06 10 nicopolitidis petros ed survey of cpu cache based side channel attacks systematic analysis security models and countermeasures security and communication networks 2021 1 15 doi 10 1155 2021 5559552 issn 1939 0122 landy barry november 2012 atlas 2 at cambridge mathematical laboratory and aldermaston and cad centre two tunnel diode stores were developed at cambridge one which worked very well speeded up the fetching of operands the other was intended to speed up the fetching of instructions the idea was that most instructions are obeyed in sequence so when an instruction was fetched that word was placed in the slave store in the location given by the fetch address modulo 32 the remaining bits of the fetch address were also stored if the wanted word was in the slave it was read from there instead of main memory this would give a major speedup to instruction loops up to 32 instructions long and reduced effect for loops up to 64 words ibm system 360 model 85 functional characteristics pdf ibm june 1968 a22 6916 1 liptay john s march 1968 structural aspects of the system 360 model 85 part ii the cache pdf ibm systems journal 7 1 15 21 doi 10 1147 sj 71 0015 smith alan jay september 1982 cache memories pdf computing surveys 14 3 473 530 doi 10 1145 356887 356892 s2cid 6023466 altering computer architecture is way to raise throughput suggest ibm researchers electronics 49 25 30 31 december 1976 1 2 3 4 hennessy john l patterson david a 2011 computer architecture a quantitative approach elsevier p b 9 isbn 978 0 12 383872 8 white bill de leon cecilia a et al march 2016 ibm z13 and ibm z13s technical introduction pdf ibm p 20 product fact sheet accelerating 5g network infrastructure from the core to the edge intel newsroom press release intel corporation 25 february 2020 retrieved 2024 04 18 l1 cache of 32kb core l2 cache of 4 5mb per 4 core cluster and shared llc cache up to 15mb smith ryan int...
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