
MOTOROLA
Chapter 9. Core Interface Operation
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9-15
Address Bus Tenure
size of the data. For example, Table 9-5 shows that 1-byte data is always aligned; however,
for a 4-byte word to be aligned, it must be at on an address that is a multiple of 4.
The G2 core supports misaligned memory operations, although their use may substantially
degrade performance. Misaligned memory transfers address memory that is not aligned to
the size of the data being transferred (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 core interface supports misaligned transfers within a word
(32-bit aligned) boundary, as shown in Table 9-6. Note that the 4-byte transfer in Table 9-6
is only one example of misalignment. As long as the attempted transfer does not cross a
word boundary, the core 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 boundary requires two bus transfers to access the data. Note that an attempt to load
or store a floating-point operand that is not word-aligned results in a floating-point
alignment exception. For more information, refer to Section 5.5.6, “Alignment Exception
(0x00600).”
Table 9-5. Aligned Data Transfers (64-Bit Bus)
Transfer Size
tsiz0
tsiz1
tsiz2
core_a_out
[29:31]
Data Bus Byte Lanes
1
1
A: These entries indicate the byte portions of the requested operand that are read or written during that bus
transaction.
—: These entries are not required and are ignored during read transactions and are driven with undefined data during
all write transactions.
0
1
2
3
4
5
6
7
Byte
0
0
1
0 0 0
A
—
—
—
—
—
—
—
0
0
1
0 0 1
—
A
—
—
—
—
—
—
0
0
1
0 1 0
—
—
A
—
—
—
—
—
0
0
1
0 1 1
—
—
—
A
—
—
—
—
0
0
1
1 0 0
—
—
—
—
A
—
—
—
0
0
1
1 0 1
—
—
—
—
—
A
—
—
0
0
1
1 1 0
—
—
—
—
—
—
A
—
0
0
1
1 1 1
—
—
—
—
—
—
—
A
Half word
0
1
0
0 0 0
A
A
—
—
—
—
—
—
0
1
0
0 1 0
—
—
A
A
—
—
—
—
0
1
0
1 0 0
—
—
—
—
A
A
—
—
0
1
0
1 1 0
—
—
—
—
—
—
A
A
Word
1
0
0
0 0 0
A
A
A
A
—
—
—
—
1
0
0
1 0 0
—
—
—
—
A
A
A
A
Double word
0
0
0
0 0 0
A
A
A
A
A
A
A
A
F
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