
4-28
MPC603e & EC603e RISC Microprocessors User's Manual
MOTOROLA
The 603e can initiate an alignment exception for the access shown in Table 4-14. In this
case, the appropriate range check is performed before the instruction begins execution. As
a result, if an alignment exception is taken, it is guaranteed that no portion of the instruction
has been executed.
4.5.6.1.1 Page Address Translation Access
A page-address translation access occurs when MSR[DR] is set, SR[T] is cleared and there
is not a match in the BAT. Note the following points:
The following is true for all loads and stores except strings/multiples:
— Byte operands never cause an alignment exception.
— Half-word operands can cause an alignment exception if the EA ends in 0xFFF.
— Word operands can cause an alignment exception if the EA ends in 0xFFD–FFF.
— Double-word operands cause an alignment exception if the EA ends in
0xFF9–FFF.
The
dcbz
instruction causes an alignment exception if the access is to a page or
block with the W (write-through) or I (cache-inhibit) bit set in the TLB or BAT,
respectively.
A misaligned memory access that does not cause an alignment exception will not perform
as well as an aligned access of the same type. The resulting performance degradation due
to misaligned accesses depends on how well each individual access behaves with respect to
the memory hierarchy. At a minimum, additional cache access cycles are required that can
delay other processor resources from using the cache. More dramatically, for an access to
a noncacheable page, each discrete access involves individual processor bus operations that
reduce the effective bandwidth of that bus.
Finally, note that when the 603e is in page address translation mode, there is no special
handling for accesses that fall into BAT regions.
4.5.6.2 Floating-Point Alignment Exceptions
The 603e implements the alignment exception as it is defined in the PowerPC architecture.
For information on bit settings and how exception conditions are detected, refer to
The
Programming Environments Manual
.
Table 4-14. Access Types
MSR[DR]
SR[T]
Access Type
1
0
Page-address translation access