IBM POWERPC 750GL User Manual
Page 297
User’s Manual
IBM PowerPC 750GX and 750GL RISC Microprocessor
gx_08.fm.(1.2)
March 27, 2006
Bus Interface Operation
Page 297 of 377
The 750GX supports misaligned memory operations, although their use can substantially degrade perfor-
mance. Misaligned memory transfers address memory that is not aligned to the size of the data being trans-
ferred (such as, a word read of an odd byte address). Although most of these operations hit in the primary
cache (or generate burst memory operations if they miss), the 750GX interface supports misaligned transfers
within a word (32-bit aligned) boundary, as shown in Table 8-5 on page 298.
Note: The 4-byte transfer in Table 8-5 is only one example of misalignment. As long as the attempted trans-
fer does not cross a word boundary, the 750GX can transfer the data on the misaligned address (for example,
a half-word read from an odd byte-aligned address). An attempt to address data that crosses a word bound-
ary requires two bus transfers to access the data.
Due to the performance degradations associated with misaligned memory operations, they are best avoided.
In addition to the double-word straddle boundary condition, the address-translation logic can generate
substantial exception overhead when the load/store multiple and load/store string instructions access
misaligned data. It is strongly recommended that software attempt to align data where possible.
Word
1
0
0
000
x
x
x
x
—
—
—
—
1
0
0
100
—
—
—
—
x
x
x
x
Double word
0
0
0
000
x
x
x
x
x
x
x
x
Table 8-4. Aligned Data Transfers
(Page 2 of 2)
Transfer Size
TSIZ0
TSIZ1
TSIZ2
A[29–31]
Data-Bus Byte Lane(s)
0
1
2
3
4
5
6
7
Note: The entries with an “x” indicate the byte portions of the requested operand that are read or written during a bus transaction.
The entries with a “–” are not required and are ignored during read transactions, and they are driven with undefined data during all write
transactions.