Managing cache: issues

Cache misses: types

compulsory: first reference to a data item

capacity: not enough space in cache

too few slots

cache line too small

conflict: space is available, but data block already stored at that location

2 cache lines map to same cache slot

Instruction and data cache

instructions and data have different access patterns, use different memory areas

use separate instruction and data caches

one reason to have separate instruction and data memories in datapath

Modifying data in the cache

write-back: update main memory only when block is removed from cache

saves time required to write main memory for each store

write-through

update main memory at the same time as cache

save time by continuing execution while main memory write completes

Replacement policy

how to choose cache line to replace

LRU: least recently used: slot which has not been used in the longest time

LFU: least frequently used

FIFO: first in, first out: slot which has been in the cache the longest

Random: may be only 10% worse than LRU

may require additional hardware to keep track