March 2002 Release
Virtex-II Pro™ Platform FPGA Documentation
1-800-255-7778
Memory-System Organization
R
•
A
tag
used to uniquely identify the line within the congruence class.
•
32 bytes of
data
that are a copy of a contiguous, 32-byte block of system memory,
aligned on a 32-byte address boundary. The data can represent either instructions (in
the instruction cache) or operands (in the data cache).
•
An
LRU
bit that specifies which cacheline within the congruence class is least-recently
used. Each time a cacheline is accessed, the cache controller marks the
other
line
within that congruence class as least-recently used. When a new cacheline is read
from memory during a cacheline fill, the line in the congruence class marked least-
recently used is replaced.
•
A
dirty
bit that indicates whether the cacheline contains modified information. A
modified cacheline contains data that is more recent than the copy in system memory.
The instruction cache does not have a dirty bit.
The 512 total lines of 32 bytes each yields a 16 KB cache size.
Data is selected from the data cache using fields within the data address. Likewise, an
instruction is selected from the instruction cache using fields within the instruction
address. The data cache is
physically tagged
and
physically indexed
. This means that the
physical address alone is used to access the data-cache array. The instruction cache is
physically tagged
and
virtually indexed
. Here, the effective address is used to specify a
congruence class (set of lines) within the cache, and the physical address is used to specify
a specific tag. The instruction cache is accessed in this manner for performance reasons, but
care is required to avoid cache synonyms (see
).
shows the address fields used in accessing the two caches.
Figure 5-2:
Logical Structure of the PPC405
Cache Arrays
UG011_34_033101
Way A
Tag
Tag 0
Tag 1
Data 254
Tag 254
Data 255
Tag 255
. . .
Line 0
Line 1
Line 254
Line 255
. . .
Way B
LRU &
Dirty
Data
Tag
Data 0
Tag 0
Data 1
Tag 1
Data 254
Tag 254
Data 255
Tag 255
. . .
. . .
Line 254
Line 255
LRU &
Dirty
Data
Data 0
Data 1
. . .
Line 0
Line 1
. . .
Cache
Содержание Virtex-II Pro PPC405
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