What is a Sub-block?

• Sub-block:

  • Share one cache tag between all sub-blocks in a block
  • A unit within a block that has its own valid bit
  • Example: 1 KB Direct Mapped Cache, 32-B Block, 8-B Sub-block
    • Each cache entry will have: 32/8 = 4 valid bits

• Write miss: only the bytes in that sub-block is brought in.

  • reduce cache fill bandwidth (penalty).

0

1

2

3

:

Cache Data

:

SB0’s V Bit

:

31

Cache Tag

SB1’s V Bit

:

SB2’s V Bit

:

SB3’s V Bit

:

Sub-block0

Sub-block1

Sub-block2

Sub-block3

:

B0

B7

:

B24

B31

Byte 992

Byte 1023

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