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FUJITSU MB91460 SERIES FR60 User Manual

Page 196

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Chapter 12 Instruction Cache

2.Main body structure

Figure 2-2 Instruction Cache Tag

[Bits 31 to 9] Address tag

This area stores the upper 23 bits of the memory address of the instruction cached in the
corresponding block. For example, memory address IA of the instruction data stored in sub-
block k in block i is obtained from the following equation:

IA = address tag x 2

11

+ i x 2

4

+ k x 2

2

The address tag is used to check for a match with the instruction address requested for
access by the CPU. The CPU and cache behave as follows depending on the result of the
cache check:

1) When the requested instruction data exists in the cache (hit), the cache transfers the data
to the CPU within the cycle.

2) When the requested instruction data does not exist in the cache (miss), the CPU and
cache obtain the data loaded by external access at the same time.

[Bits 7 to 4] SBV3 to SBV0: Sub-block valid bits

When SBV* contains "1", the corresponding sub-block holds the current instruction data at
the address located by the tag. Each sub-block usually holds two instructions (including
immediate-value transfer instructions).

[Bit 3] TAGV: TAG valid bit

This bit indicates whether the address tag value is valid. When the bit contains "0", the
corresponding block is invalid regardless of the settings of the sub-block validation bits. (The

Address tag

Reserved

ETLK

LRU

Reserved

TAGV

SBV0

SBV1

SBV2

SBV3

Way 1

08

09

07

05

06

03

04

01

00

02

31

Sub-block valid

LRU

Entry lock

TAG valid

Address tag

Reserved

ETLK

Reserved

TAGV

SBV0

SBV1

SBV2

SBV3

Way 2

08

09

07

05

06

03

04

01

00

02

31

Sub-block valid

Entry lock

TAG valid