![Intel IXP45X Скачать руководство пользователя страница 72](http://html1.mh-extra.com/html/intel/ixp45x/ixp45x_developers-manual_2073092072.webp)
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
72
Order Number: 306262-004US
3.1.1.2.4
Memory Operation Ordering
A fence memory operation (memop) is one that guarantees all memops issued prior to
the fence will execute before any memop issued after the fence. Thus software may
issue a fence to impose a partial ordering on memory accesses.
shows the circumstances in which memops act as fences.
Any swap (SWP or SWPB) to a page that would create a fence on a load or store is a
fence.
3.1.1.2.5
Exceptions
The MMU may generate prefetch aborts for instruction accesses and data aborts for
data memory accesses. The types and priorities of these exceptions are described in
“Event Architecture” on page 177
.
Data address alignment checking is enabled by setting bit 1 of the Control Register
(CP15, register 1). Alignment faults are still reported even if the MMU is disabled. All
other MMU exceptions are disabled when the MMU is disabled.
3.1.2
Interaction of the MMU, Instruction Cache, and Data Cache
The MMU, instruction cache, and data/mini-data cache may be enabled/disabled
independently. The instruction cache can be enabled with the MMU enabled or disabled.
However, the data cache can only be enabled when the MMU is enabled. Therefore only
three of the four combinations of the MMU and data/mini-data cache enables are valid.
The invalid combination will cause undefined results.
Table 6.
Data Cache and Buffer Behavior When X = 1
C B
Cacheable
Bufferable
Write Policy
Line
Allocation
Policy
Notes
0 0
-
-
-
-
Unpredictable -- do not use
0 1
N
Y
-
-
Writes will not coalesce into buffers
1 0
(Mini Data
Cache)
-
-
-
Cache policy is determined by MD field
of Auxiliary Control register
1 1
Y
Y
Write Back
Read/Write
Allocate
Notes:
1.
Normally, bufferable writes can coalesce with previously buffered data in the same address range.
2.
See
“Register 1: Control and Auxiliary Control Registers” on page 100
for a description of this
register.
Table 7.
Memory Operations that Impose a Fence
Operation
X
C
B
load
-
0
-
store
1
0
1
load or store
0
0
0