Chapter 3. L1 and L2 Cache Operation
3-73
MPC7400 Caches and System Bus Transactions
Figure 3-38. Double-Word Address OrderingCritical Double Word First
3.9.1 Bus Operations Caused by Cache Control Instructions
The cache control, TLB management, and synchronization instructions supported by the
MPC7400 may affect or be affected by the operation of the system bus. The operation of
the instructions may also indirectly cause bus transactions to be performed, or their
completion may be linked to the bus.
When memory coherency is required (WIMG = xx1x), the
dcbst
,
dcbf
, and
dcbi
instructions cause a broadcast on the system bus to maintain coherency. The
icbi
instruction
is always broadcast, regardless of the state of the memory-coherency-required attribute. For
detailed information on the cache control instructions, refer to Chapter 2, òProgramming
Model,ó in this book and Chapter 8, òInstruction Set,ó in
The Programming Environments
Manual
.
Table 3-13 provides an overview of the bus operations initiated by cache control
instructions. Note that Table 3-13 assumes that the WIM bits are set to 001; that is, the
cache is operating in write-back mode, caching is allowed, and memory coherency is
enforced.
Table 3-13. Bus Operations Caused by Cache Control
Instructions (WIM = 001)
Instruction
Current Cache
State
Next Cache
State
Bus Operation
Comment
sync
Dont care
No change
sync
Waits for memory queues to
complete bus activity
tlbie
Dont care
No change
tlbie
If the address requested is in double word A, the address placed on the bus is that of double word A, and
the four data beats are ordered in the following manner:
If the address requested is in double word C, the address placed on the bus will be that of double word C,
and the four data beats are ordered in the following manner:
A
B
C
D
11
10
00
01
A
B
C
D
3
2
0
1
Beat
MPC7400 Cache Address
Bits (27... 28)
C
D
A
B
3
2
0
1
Beat