![]() "dirty" block: a cache block that has been written to but which has not been copied back to main memory is "dirty." It must be copied back to main memory when that block is discarded. Thrashing can reduce a cache to the same performance (or worse) than a system with no cache at all. Thrash: load and unload a cache block repeatedly - cache loads, then some other data is accessed and it's flushed and loads the new data, and then flushed and the original is loaded, and so on. Tag: some set of bits attached to a block that define it's characteristics (i.e., the address it is currently mapped to, whether it is "dirty" or not, etc.) Source block: the block in main memory to be copied into the cache or the block in the cache being written back to main memoryĭiscard / discarded : a discard is a block that has been flushed or removed from the cache and replaced with a block newly read from memory. For small moves (accessing one byte in a block, or writing one byte in a block) copying the whole block wastes some memory bandwidth but this is necessary concession to design practicality.ĭestination block: the block in the cache or in memory to be written to (e.g., when loading, the destination block is in the cache, when writing a changed cache block to memory, the destination block is main memory). A block is the smallest unit that may be copied to or from memory. (5) Caches are loaded in terms of blocks. Blocks contain some number of words, depending on their size. Caches usually contain a power of 2 # of blocks but this isn't a requirement. ![]() Except as noted, the size of all caches is an integer multiple of the block size. (4) All caches are broken into blocks of fixed size. (This is relevant to comparisons in H&P of the 200MHz 21064 Alpha AXP to the IBM 70MHz POWER2. It isn't just a matter of cranking the clockspeed up. ![]() (3) Measures such as "work per cycle" or "instructions per clock" are meaningless metrics for comparing different architectures. There's a pretty damn good reason, since modern CPUs almost exclusively use set-associative and direct mapped caches. For instance, one might ask why direct and set-associative caches (see below) would even need to exist when fully associative caches are so much more flexible. If you could, somebody would already have done it and you wouldn't be debating it. (2) You usually can't improve anything in computers without giving up a little of something else. This is accomplished by copying the contents into the faster cache. Caches work by mapping blocks of slow main RAM into fast cache RAM. Square bitmaps are stored one scanline after another, rectangular arrays of numbers are stored in memory as a series of linearly appended rows, and so on. All memory is addressed, stored, accessed, written to, copied, moved, and so on in a linear way. What's different about a fully associative cache? What are some key points I need to understand this section?ĥ. So tags of the all four resident cache lines are checked against the tag of the generated address for a match.1. When an address is generated by the processor, 8 middle bits of the 30 bit address is used to select the cache set. ![]() As different memory blocks can be mapped to same cache line, this tag value helps in differentiating among them. So that leaves us with (30 - 5 - 8) = 17 bits for tag. Middle bits represent to which cache set this byte(or cache line) will be mappedīits needed to represent 1GB of memory = 30 (1GB = (2^30)B)īits needed to represent offset in cache line = 5 (32B = (2^5)B)īits needed to represent 256 cache sets = 8 (2^8 = 256).Rightmost bits represent byte offset within a cache line or block.Number of blocks in each page = (4KB / 32B) = 128Įach byte address in the system can be divided into 3 parts: Number of blocks in main memory = (1GB / 32B) = 2^25 In your example, each memory block can be stored in any of the 4 cache lines of a set. But it can be stored in any of the cache lines of the set. In set associative cache, each memory block will be mapped to a fixed set in the cache. So each block of main memory will be mapped to a cache line (but not always to a particular cache line, as it is set associative cache). If we think of the main memory as consisting of cache lines, then each memory region of one cache line size is called a block. Your cache size is 32KB, it is 4 way and cache line size is 32B. So the relationship stands like this :Ĭache size = number of sets in cache * number of cache lines in each set * cache line size The whole cache is divided into sets and each set contains 4 cache lines(hence 4 way cache). The cache you are referring to is known as set associative cache.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |