Lecture Notes for Computer Architecture II - St. Cloud State University

Direct Mapping
Each main memory block has one specific location in the cache
Block j of the main memory maps onto block (J modulo cache size)
Example
Cache size 128
Block 0, 128, 256, … mapped one at a time to cache block 0.
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Direct Mapped Cache
Memory address divided into 3 fields
Tag,
block
word
Tag represents the higher order bits of the memory address
Match the tags in the memory address and cache address to determine whether
the block is in cache
Block field determines the cache position
Word field determines the number of words in a block
Example
Word field 4 16 words in a block
Word field 2 4 words in a block
Direct Mapped Cache
• Taking advantage of spatial locality:
Hits vs. Misses
• Read hits
– this is what we want!
• Read misses
– stall the CPU, fetch block from memory, deliver to cache, restart
• Write hits:
– can replace data in cache and memory (write-through)
– write the data only into the cache (write-back the cache later)
• Write misses:
– read the entire block into the cache, then write the word