Memoire Cache Definition Simple

To reduce the time it takes to access these caches, some researchers have invented path prediction, which involves placing bets on the next path. Instead of waiting for tag comparisons to work, the processor automatically selects a channel and configures multiplexers in advance. If the processor makes no mistake, it accesses data one to two cycles earlier than expected. If it makes a mistake, the processor cancels the pre-read and starts again retrieving the data in the right direction. This technique hibernates channels that the processor has not adjusted, which can reduce the processor`s power consumption. This is more efficient than reading multiple data in different channels and keeping only one. The L1 instruction cache is often „read-only”: you cannot edit its contents, you can only read or load instructions. This makes it difficult to manage self-modifying code, i.e. programs whose instructions modify others, which is used to optimize, compress or hide a program (computer viruses use many types of processes). When the processor executes this type of code, it cannot write to this L1 instruction cache, but must write to the L2 cache or RAM, and then load the changed instructions into the L1 cache, which takes time! Even worse, errors can sometimes occur if the L1 cache is not updated.

Cache memory is a chip-based computer component that makes retrieving data from your computer`s memory more efficient. It acts as a temporary memory area from which the computer`s processor can easily recover data. This temporary area of memory, called the cache, is more easily accessible to the processor than the computer`s main memory source, usually a form of DRAM. The cache data is (pre)loaded from memory, so all cache data is a copy of the data in RAM. The cache must match a data item in the cache to the corresponding memory address. From an operational point of view, the cache can be thought of as a kind of lookup table that stores data, each associated with its memory address. Therefore, the cache contains pairs of address-to-cache lines that allow it to associate the cache line with the address. This is true from a processor perspective, as the inner workings of the cache are slightly different depending on the cache. There are caches whose internal function is that of a hardware lookup table, others that are much more optimized. In addition to hardware cache, cache memory can also be a disk cache where a reserved part on a hard drive stores data/applications and provides access to frequently used data/applications from the hard drive. When the processor accesses the data for the first time, a copy is created in the cache.

The cache memory, which is faster and closer to the computer hardware requesting the data, is smaller than the memory for which it mediates because of its performance and therefore its cost. Commercially, the concept of caching appeared in 1968 on the IBM 360/85 mainframe. You might think that a single cache is more than enough to compensate for slow memory. Unfortunately, the processors have become so fast that the caches themselves are very slow! As a reminder, the more data a storage can hold, the slower it is. And caches are not spared. If we were to use a single cache, it would be very large and therefore too slow. The situation we try to avoid with RAM comes back. This makes merging two cache errors quite easy. If two cache errors at the same address read different memory words, simply configure the entries accordingly. The first error configures its entries and the second has its own. For this organization, the number of MSHRs indicates how many cache lines can be read from memory at the same time. The number of records per MSHR determines how much non-overlapping memory access can occur simultaneously.

Still other caches are technically not memory caches at all. For example, disk caches can use DRAM or flash memory to provide data caching, similar to what memory caches do with processor instructions. When data is frequently accessed from the hard drive, it is cached in DRAM or silicon flash memory technology to speed up access time and responsiveness. Same problem, same solution: If we decide to divide the main memory into several memories of different size and speed, we can do the same with the cache memory. For the past twenty years, a processor has contained several caches with very different capacities: L1, L2, and sometimes an L3 cache. Some of these caches are small but very fast: they are the ones we will access first. Then come other caches, of variable size, but slower. Processors therefore have a hierarchy of caches that becomes more and more complex over time. This hierarchy consists of several levels of cache, ranging from lower levels near RAM to higher levels near the processor. The higher you go, the smaller and faster the caches. Each memory access is intercepted by the cache, which checks whether the requested data exists in the cache or not. If the desired data is present in the cache, we have access to the cache and access the cache data.

Otherwise, it is a missed cache and we are forced to access the RAM. The number of cache successes per number of memory hits, called the success rate, is crucial for performance: the higher it is, the more efficient the cache. The three types of mapping used for cache memory are: direct mapping, associative mapping, and defined associative mapping. The details are as follows: If the program modifies the instructions, the separate caches cause an issue with the consistency of the instruction cache: the program must then invalidate the corresponding entries in the command cache itself in order to update them before executing the modified instructions, otherwise an earlier version of these instructions could be taken over and executed by the processor (or even an unpredictable mixing of instructions). new and old instructions). In these caches, each address is divided into three parts: a tag, an index, and an offset, as with directly addressed caches. As you can see, the organization is identical to that of a fully associative cache, except that each set of cache slogans is replaced by RAM that contains several. In addition to command and data caches, other caches are designed to provide specialized system functionality.