Intel IXP45X Developer's Manual page 179

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Intel XScale Processor—Intel
Table 78. Intel XScale
Priorit
y
Highest
Lowest
† All other encodings not listed in the table are reserved.
Imprecise Data Aborts
• A data cache parity error is imprecise; the extended Status field of the Fault Status
Register is set to 0xb11000.
• All external data aborts except for those generated on a data MMU translation are
imprecise.
The Fault Address Register for all imprecise data aborts is undefined and R14_ABORT is
the address of the next instruction to execute + 4, which is the same for both Intel
StrongARM
Although the Intel XScale processor guarantees the Base Restored Abort Model for
precise aborts, it cannot do so in the case of imprecise aborts. A Data Abort handler
may encounter an updated base register if it is invoked because of an imprecise abort.
Imprecise data aborts may create scenarios that are difficult for an abort handler to
recover. Both external data aborts and data cache parity errors may result in corrupted
data in the targeted registers. Because these faults are imprecise, it is possible that the
corrupted data will have been used before the Data Abort fault handler is invoked.
Because of this, software should treat imprecise data aborts as unrecoverable.
Note that even memory accesses marked as "stall until complete" (see
Cache and Write Buffer Behavior" on page
these types of accesses, the fault is somewhat less imprecise than the general case: it
is guaranteed to be raised within three instructions of the instruction that caused it. In
other words, if a "stall until complete" LD or ST instruction triggers an imprecise fault,
then that fault will be seen by the program within three instructions.
With this knowledge, it is possible to write code that accesses "stall until complete"
memory with impunity. Simply place several NOP instructions after such an access. If
an imprecise fault occurs, it will do so during the NOPs; the data abort handler will see
identical register and memory state as it would with a precise exception, and so should
be able to recover. An example of this is shown in
August 2006
Order Number: 306262-004US
® ®
IXP45X and Intel IXP46X Product Line of Network Processors
®
Processor Encoding of Fault Status for Data Aborts
Sources
Alignment
External Abort on Translation
Translation
Domain
Permission
Lock Abort
This data abort occurs on an MMU lock operation (data or
instruction TLB) or on an Instruction Cache lock operation.
Imprecise External Data Abort
Data Cache Parity Error Exception
*
and Thumb mode.
First level
Second level
Section
Page
Section
Page
Section
Page
71) can result in imprecise data aborts. For
Example 18 on page
® ®
Intel IXP45X and Intel IXP46X Product Line of Network Processors
†
FS[10,3:0] Domain
0b000x1
invalid
0b01100 invalid
0b01110 valid
0b00101 invalid
0b00111 valid
0b01001 valid
0b01011 valid
0b01101 valid
0b01111 valid
0b10100 invalid
0b10110 invalid
0b11000 invalid
"Details on Data
180.
Developer's Manual
FAR
valid
valid
valid
valid
valid
valid
valid
valid
valid
invalid
invalid
invalid
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