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IBM PowerPC 750GX and 750GL RISC Microprocessor
gx_05.fm.(1.2)
March 27, 2006
Memory Management
Page 193 of 377
The state saved by the processor for each of these exceptions contains information that identifies the address
of the failing instruction. See Chapter 4, Exceptions, on page 151 for a more detailed description of exception
processing.
In addition to the translation exceptions, there are other MMU-related conditions (some of them defined as
implementation-specific, and therefore not required by the architecture) that can cause an exception to occur.
These exception conditions map to processor exceptions as shown in Table 5-4. The only MMU exception
conditions that occur when MSR[DR] = 0 are those that cause an alignment exception for data accesses. For
more detailed information about the conditions that cause an alignment exception (in particular for
string/multiple instructions), see Section 4.5.6, Alignment Exception (0x00600), on page 170.
Notes:
• Some exception conditions depend upon whether the memory area is set up as write-though (W = 1) or
caching-inhibited (I = 1).
• These bits are described fully in “Memory/Cache Access Attributes,” in Chapter 5, “Cache Model and
Memory Coherency,” of the PowerPC Microprocessor Family: The Programming Environments Manual.
• Also see Chapter 4, Exceptions, on page 151 and Chapter 6, “Exceptions,” in the PowerPC Microproces-
sor Family: The Programming Environments Manual for a complete description of the SRR1 and DSISR
bit settings for these exceptions.
Instruction fetch from direct-store seg-
ment
Attempt to fetch instruction when SR[T] = 1
ISI exception
SRR1[3] =1
Data access to direct-store segment
(including floating-point accesses)
Attempt to perform load or store (including floating-point
(FP) load or store) when SR[T] = 1
DSI exception
DSISR[5] =1
Instruction fetch from guarded memory
Attempt to fetch instruction when MSR[IR] = 1 and either
matching xBAT[G] = 1, or no matching BAT entry and
PTE[G] = 1
ISI exception
SRR1[3] =1
Table 5-4. Other MMU Exception Conditions for the 750GX Processor (Page 1 of 2)
Condition Description Exception
Data Cache Block Set to Zero (dcbz) with
W = 1 or I = 1
dcbz instruction to write-through or cache-
inhibited segment or block
Alignment exception (not required by archi-
tecture for this condition)
Load Word and Reserve Indexed (lwarx),
Store Word Conditional Indexed (stwcx).,
External Control In Word Indexed (eciwx),
or External Control Out Word Indexed
(ecowx) instruction to direct-store segment
Reservation instruction or external control
instruction when SR[T] = 1
DSI exception
DSISR[5] = 1
Floating-point load or store to direct-store
segment
Floating-point memory access when SR[T]
=1
See data access to direct-store segment in
Table 5-3 on page 192.
Load or store that results in a direct-store
error
A DSI exception is taken when a load or
store is attempted to a direct-store segment
(SR[T] = 1)
DSI exception
For additional information, see
Section 4.5.3, DSI Exception (0x00300), on
page 169.
Table 5-3. Translation Exception Conditions (Page 2 of 2)
Condition Description Exception