![Intel IXP45X Developer'S Manual Download Page 175](http://html1.mh-extra.com/html/intel/ixp45x/ixp45x_developers-manual_2073092175.webp)
Intel
®
IXP45X and Intel
®
IXP46X Product Line of Network Processors
August 2006
Developer’s Manual
Order Number: 306262-004US
175
Intel XScale
®
Processor—Intel
®
IXP45X and Intel
®
IXP46X Product Line of Network Processors
3.8.3.2
New Page Attributes
The Intel XScale processor extends the page attributes defined by the C and B bits in
the page descriptors with an additional X bit. This bit allows four more attributes to be
encoded when X=1. These new encodings include allocating data for the mini-data
cache and write-allocate caching. A full description of the encodings can be found in
“Memory Attributes” on page 70
The Intel XScale processor retains Intel StrongARM definitions of the C and B encoding
when X = 0, which is different than the Intel StrongARM products. The memory
attribute for the mini-data cache has been moved and replaced with the write-through
caching attribute.
When write-allocate is enabled, a store operation that misses the data cache
(cacheable data only) will generate a line fill. If disabled, a line fill only occurs when a
load operation misses the data cache (cacheable data only).
Write-through caching causes all store operations to be written to memory, whether
they are cacheable or not cacheable. This feature is useful for maintaining data cache
coherency.
Bit 1 in the Control Register (coprocessor 15, register 1, opcode=1) is used reserved
for the P bit memory attribute for memory accesses made during page table walks. The
P bit is not implemented on the IXP45X/IXP46X network processors.
These attributes are programmed in the translation table descriptors, which are
highlighted in:
•
Table 72, “First-Level Descriptors” on page 175
•
Table 73, “Second-Level Descriptors for Coarse Page Table” on page 175
•
Table 74, “Second-Level Descriptors for Fine Page Table” on page 176
Two second-level descriptor formats have been defined for the IXP45X/IXP46X network
processors, one is used for the coarse page table and the other is used for the fine page
table.
Table 72.
First-Level Descriptors
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10
9
8
7
6
5
4
3
2
1
0
SBZ
0 0
Coarse page table base address
P
Domain
SBZ
0 1
Section base address
SBZ
TEX
AP
P
Domain
0 C B 1 0
Fine page table base address
SBZ
P
Domain
SBZ
1 1
Table 73.
Second-Level Descriptors for Coarse Page Table
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10
9
8
7
6
5
4
3
2
1
0
SBZ
0 0
Large page base address
TEX
AP3
AP2
AP1
AP0
C B 0 1
Small page base address
AP3
AP2
AP1
AP0
C B 1 0
Extended small page base address
SBZ
TEX
AP
C B 1 1