Reconfiguring The Data Cache As Data Ram - Intel IXP45X Developer's Manual

Intel
The time it takes to execute a global clean operation depends on the number of dirty
lines in cache.
3.4.3

Reconfiguring the Data Cache as Data RAM

Software has the ability to lock tags associated with 32-byte lines in the data cache,
thus creating the appearance of data RAM. Any subsequent access to this line will
always hit the cache unless it is invalidated. Once a line is locked into the data cache it
is no longer available for cache allocation on a line fill. Up to 28 lines in each set can be
reconfigured as data RAM, such that the maximum data RAM size is 28 Kbytes for the
32-Kbyte.
Hardware does not support locking lines into the mini-data cache; any attempt to do
this will produce unpredictable results.
There are two methods for locking tags into the data cache; the method of choice
depends on the application. One method is used to lock data that resides in external
memory into the data cache and the other method is used to re-configure lines in the
data cache as data RAM. Locking data from external memory into the data cache is
useful for lookup tables, constants, and any other data that is frequently accessed. Re-
configuring a portion of the data cache as data RAM is useful when an application needs
scratch memory (bigger than the register file can provide) for frequently used
variables. These variables may be strewn across memory, making it advantageous for
software to pack them into data RAM memory.
Code examples for these two applications are shown in
Example 11 on page
on page 93
Example 11 on page 94
No external memory request is made, which means software can map any unallocated
area of memory as data RAM. However, the line-allocate operation does validate the
target address with the MMU, so system software must ensure that the memory has a
valid descriptor in the page table.
Another item to note in
newly allocated line in the cache must be initialized by software before it can be read.
The line allocate operation does not initialize the 32 bytes and therefore reading from
that line will produce unpredictable results.
In both examples, the code drains the pending loads before and after locking data. This
step ensures that outstanding loads do not end up in the wrong place -- either
unintentionally locked into the cache or mistakenly left out in the proverbial cold. Note
also that a drain operation has been placed after the operation that locks the tag into
the cache. This drains ensures predictable results if a programmer tries to lock more
than 28 lines in a set; the tag will get allocated in this case but not locked into the
cache.
® ®
Intel IXP45X and Intel IXP46X Product Line of Network Processors
Developer's Manual
92
® ®
IXP45X and Intel IXP46X Product Line of Network Processors—Intel XScale
94. The difference between these two routines is that
actually requests the entire line of data from external memory and
uses the line-allocate operation to lock the tag into the cache.
Example 11 on page 94
Example 10 on page 93
is that the 32 bytes of data located in a
Order Number: 306262-004US
®
Processor
and
Example 10
August 2006