CACHE AND ON-CHIP DATA RAM
4-7
4
4.5.2
Multi-Word Data Accesses that Partially Hit the Data Cache
The following applies only when data caching is enabled for an access.
For a multi-word load access (
ldl
,
ldt
,
ldq
) in which none of the requested words hit the data cache,
an external bus transaction is started to acquire all the words of the access.
For a multi-word load access that partially hits the data cache, the processor may either:
•
Load or reload all words of the access (even those that hit) from the external bus
•
Load only missing words from the external bus and interleave them with words found in the data cache
The multi-word alignment determines which of the above methods is used:
•
Naturally aligned multi-word accesses cause all words to be reloaded
•
An unaligned multi-word access causes only missing words to be loaded
When any words accessed by a
ldl
,
ldt
, or
ldq
instruction miss the data cache, every word accessed
by that load instruction is updated in the cache.
In each case, the external bus accesses used to acquire the data may consist of none, one, or several
burst accesses based on the alignment of the data and the bus-width of the memory region that
contains the data. See
CHAPTER 14, EXTERNAL BUS
for more details.
A multi-word load access that completely hits in the data cache does not cause external bus accesses.
For a multi-word store access (
stl
,
stt
,
stq
) an external bus transaction is started to write all words
of the access regardless when any or all words of the access hit the data cache. External bus
accesses used to write the data may consist of either one or several burst accesses based on data
alignment and the bus-width of the memory region that receives the data. (See
CHAPTER 14,
EXTERNAL BUS
for more details.) The cache is also updated accordingly as described earlier in
this chapter.
Table 4.1.
Load Instruction
Number of Updated Words
ldq
4 words
ldt
3 words
ldl
2 words
Содержание i960 Jx
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Страница 25: ...1 INTRODUCTION ...
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Страница 47: ...3 PROGRAMMING ENVIRONMENT ...
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Страница 503: ...B OPCODES AND EXECUTION TIMES ...
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