
Chapter 6. Instruction Timing
6-31
Execution Unit Timings
Note that the MPC7400 differs from the MPC750 in some aspects of little-endian
operation; in little-endian mode, MPC7400 does not work with the MPC106.
6.4.5.2 Integer Store Gathering
The MPC7400 performs store gathering for write-through operations to nonguarded space.
It performs cache-inhibited stores to nonguarded space for 4-byte, word-aligned stores.
These stores are combined in the LSU to form a double word sent out on the 60x bus as a
single-beat operation. However, stores are gathered only if the successive stores meet the
criteria and are queued and pending. Store gathering occurs regardless of the address order
of the stores. Store gathering is enabled by setting HID0[SGE]. Stores can be gathered in
big-endian modes.
Store gathering is
not
done for the following:
¥
¥
¥
¥
¥
Stores to guarded cache-inhibited or write-through space
Byte-reverse store operations
stwcx.
instructions
ecowx
instructions
A store that occurs during a table search operation
Table 6-1. Performance Effects of Memory Operand Placement
Operand
Boundary Crossing
1
1
Vector operands are not shown because they are always aligned.
optimal: One EA calculation occurs.
good: Multiple EA calculations occur which may cause additional bus activities with multiple bus transfers.
poor: Alignment exception occurs.
2
These operations are not supported in little-endian mode, and would cause an alignment exception.
Size
Byte Alignment
None
8 Byte
Cache Line
Protection Boundary
Integer
4 Byte
4
<4
Optimal
Optimal
Good
Good
Good
2 Byte
2
<2
Optimal
Optimal
Good
Good
Good
1 Byte
1
Optimal
lmw
,
stmw
2
4
<4
Good
Poor
Good
Poor
Good
Poor
Good
Poor
String
2
Good
Good
Good
Good
Floating-Point
8 Byte
8
4
<4
Optimal
Good
Poor
Good
Poor
Good
Poor
4 Byte
4
<4
Optimal
Poor
Poor
Poor
Poor