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Intel
®
IXP45X and Intel
®
IXP46X Product Line of Network Processors—Intel XScale
®
Processor
Intel
®
IXP45X and Intel
®
IXP46X Product Line of Network Processors
Developer’s Manual
August 2006
176
Order Number: 306262-004US
The TEX (Type Extension) field is present in several of the descriptor types. In the Intel
XScale processor, only the LSB of this field is defined; this is called the X bit. The
remaining bits are reserved for future use and should be programmed as zero (SBZ) on
the IXP45X/IXP46X network processors.
A Small Page descriptor does not have a TEX field. For these descriptors, TEX is
implicitly zero; that is, they operate as if the X bit had a ‘0’ value.
The X bit, when set, modifies the meaning of the C and B bits. Description of page
attributes and their encoding can be found in
“Memory Management Unit” on page 69
3.8.3.3
Additions to CP15 Functionality
To accommodate the functionality in the Intel XScale processor, registers in CP15 and
CP14 have been added or augmented. See
for details.
At times it is necessary to be able to guarantee exactly when a CP15 update takes
effect. For example, when enabling memory address translation (turning on the MMU),
it is vital to know when the MMU is actually guaranteed to be in operation. To address
this need, a processor-specific code sequence is defined for the Intel XScale processor.
The sequence — called CPWAIT — is shown in
.
Table 74.
Second-Level Descriptors for Fine 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
Tiny Page Base Address
TEX
AP
C B 1 1
Example 17. CPWAIT: Canonical Method to Wait for CP15 Update
;; The following macro should be used when software needs to be
;; assured that a CP15 update has taken effect.
;; It may only be used while in a privileged mode, because it
;; accesses CP15.
MACRO CPWAIT
MRC P15, 0, R0, C2, C0, 0
; arbitrary read of CP15
MOV R0, R0
; wait for it
SUB PC, PC, #4
; branch to next instruction
; At this point, any previous CP15 writes are
; guaranteed to have taken effect.
ENDM