Address Subdivision
·
32-bit
byte address
·
A
direct-mapped cache
·
The
cache size is 2^n blocks, so n bits are used for the index
·
The
block size is 2^m words (2^m+2 bytes), so m bits are used for the word within
the block, and two bits are used for the byte part of the address
Accessing Cache
▪ Total number of bits needed for a cache
1) size of tag field is
o 32-(n+m+2)
2) The total number of bits
in a direct-mapped cache
o 2^n x (block size + tag size +valid field size)
▪ Total number of bits in cache is
- 2^n x (2^m x 32 +(32-n-m-2)+1) = 2^n x (2^m x 32+31-n-m)
Example: Large Block Size
▪ 64 blocks ,16 bytes/block
o To what block number does address 1200 map?
▪ Block Address= 1200/16 = 75
▪ Block number= 75 modulo 64=11
Block Size Considerations
▪ Larger blocks should reduce miss rate
o
Due
to spatial locality
▪ But in a
fixed-sized cache
o
Larger blocks------->fewer of them
§ More competition-------->increased miss rate
o
Larger blocks------->pollution
▪ Larger miss penalty
o
Can
override benefit of reduced miss rate
o
Early
restart and critical-word-first can help
Cache
Misses
▪ On cache hit, CPU
proceed normally
▪ On cache miss
o
Stall
the CPU pipeline
o
Fetch
block from next level of hierachy
o
Instruction
cache miss
-
Restart
Instruction fetch
o
Data
cache miss
-
Complete
data access
Write-Through
▪ On data-write hit, could just the block in cache
- But then cache and memory would be inconsistent
▪ Write through: also update
memory
▪ But makes writes take longer
- e.g., if base CPI=1, 10% of instruction are stores,
writes to memory takes 100 cycles
§ Effective
CPI=1+0.1x100=11
▪ Solution: write
buffer
- Holds data waiting to be written to memory
- CPU continues immediately
§ Only stalls on write if write buffer is already full
Write-Back
▪ Alternative: On
data-write hit, just update the block in cache
-
keep
track of whether each block is dirty
▪When a dirty block
is replaced
-
Write
it back to memory
-
Can
use a write buffer to allow replacing block to read first
Write
Allocation
▪ What should happen
on a write miss?
▪ Alternatives for
write-through
-
Allocate
on miss: fetch the block
-
Write
around: don’t fetch the block
o
Since
programs often write a whole block before reading it(e.g., Initialization)
▪ For write-back
-
Usually
fetch the block
