1 CMPE 421 Advanced Computer Architecture Accessing a Cache PART1.

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Presentation transcript:

1 CMPE 421 Advanced Computer Architecture Accessing a Cache PART1

2 Direct Mapped Caching: A Simple First Example Cache Main Memory Q1: How do we find it? Use next 2 low order memory address bits – the index – to determine which cache block (i.e., modulo the number of blocks in the cache) TagData Q2: Is it there? Compare the cache tag to the high order 2 memory address bits to tell if the memory block is in the cache Valid 0000xx 0001xx 0010xx 0011xx 0100xx 0101xx 0110xx 0111xx 1000xx 1001xx 1010xx 1011xx 1100xx 1101xx 1110xx 1111xx Two low order bits define the byte in the word (32b words) (block address) modulo (# of blocks in the cache) Index Valid bit indicates whether an entry contains valid information – if the bit is not set, there cannot be a match for this block

3 Direct Mapped Cache Example 1  8-blocks, 1 word/block, direct mapped  Initial state IndexVTagData 000N 001N 010N 011N 100N 101N 110N 111N

4 Direct Mapped Cache Example 1 IndexVTagData 000N 001N 010N 011N 100N 101N 110Y10Mem[10110] 111N Word addrBinary addrHit/missCache block Miss110

5 Direct Mapped Cache Example 1 IndexVTagData 000N 001N 010Y11Mem[11010] 011N 100N 101N 110Y10Mem[10110] 111N Word addrBinary addrHit/missCache block Miss010

6 Direct Mapped Cache Example 1 IndexVTagData 000N 001N 010Y11Mem[11010] 011N 100N 101N 110Y10Mem[10110] 111N Word addrBinary addrHit/missCache block Hit Hit010

7 Direct Mapped Cache Example 1 IndexVTagData 000Y10Mem[10000] 001N 010Y11Mem[11010] 011Y00Mem[00011] 100N 101N 110Y10Mem[10110] 111N Word addrBinary addrHit/missCache block Miss Miss Hit000

8 Direct Mapped Cache Example 1 IndexVTagData 000Y10Mem[10000] 001N 010Y10Mem[10010] 011Y00Mem[00011] 100N 101N 110Y10Mem[10110] 111N Word addrBinary addrHit/missCache block Miss010

9 Direct Mapped Cache Example 2 ( 4, 1-word blocks )  Consider the main memory word reference string Start with an empty cache - all blocks initially marked as not valid

10 Direct Mapped Cache Example 2 ( 4, 1-word blocks )  Consider the main memory word reference string Mem(0) 00 Mem(1) 00 Mem(0) 00 Mem(1) 00 Mem(2) miss hit 00 Mem(0) 00 Mem(1) 00 Mem(2) 00 Mem(3) 01 Mem(4) 00 Mem(1) 00 Mem(2) 00 Mem(3) 01 Mem(4) 00 Mem(1) 00 Mem(2) 00 Mem(3) 01 Mem(4) 00 Mem(1) 00 Mem(2) 00 Mem(3) Mem(1) 00 Mem(2) 00 Mem(3) Start with an empty cache - all blocks initially marked as not valid l 8 requests, 6 misses, 2 hits

11  One word/block, cache size = 1K words 20 Tag 10 Index Data IndexTagValid Byte offset What kind of locality are we taking advantage of? 20 Data 32 Hit Address Subdivision : Direct Mapped Cache FIGURE 7.7 For this cache, the lower portion of the address is used to select a cache entry consisting of a data word and a tag.

12 Another Example for Direct Mapping  Consider the main memory word reference string Start with an empty cache - all blocks initially marked as not valid

13 Another Example for Direct Mapping  Consider the main memory word reference string miss 00 Mem(0) Mem(4) Mem(0) Mem(0) Mem(0) Mem(4) Mem(4) 0 00 Start with an empty cache - all blocks initially marked as not valid  Ping pong effect due to conflict misses - two memory locations that map into the same cache block l 8 requests, 8 misses

14 Multiword (4 word) Block Direct Mapped Cache 12 Index Data IndexTagValid Byte offset 16 Tag HitData 32 Block offset  Four words/block, cache size = 16K words What kind of locality are we taking advantage of? 32 2

15 Taking Advantage of Spatial Locality 0  Let cache block hold more than one word Start with an empty cache - all blocks initially marked as not valid

16 Taking Advantage of Spatial Locality 0  Let cache block hold more than one word Mem(1) Mem(0) miss 00 Mem(1) Mem(0) hit 00 Mem(3) Mem(2) 00 Mem(1) Mem(0) miss hit 00 Mem(3) Mem(2) 00 Mem(1) Mem(0) miss 00 Mem(3) Mem(2) 00 Mem(1) Mem(0) hit 00 Mem(3) Mem(2) 01 Mem(5) Mem(4) hit 00 Mem(3) Mem(2) 01 Mem(5) Mem(4) 00 Mem(3) Mem(2) 01 Mem(5) Mem(4) miss Start with an empty cache - all blocks initially marked as not valid l 8 requests, 4 misses

17 Miss Rate vs Block Size vs Cache Size  Miss rate goes up if the block size is too large relative to the cache size.  because the number of blocks that can be held in the same size cache is smaller (increasing capacity misses)

18