
MOTOROLA
Chapter 8. Internal Control
8-9
8.2 Internal Arbitration
The arbitration for the PCI bus is performed externally. All processor to PCI transactions
are performed strictly in-order. Also, all snoops for PCI accesses to system memory are
performed in order (if snooping is enabled). However, the MPC106 performs arbitration
internally for the shared processor/memory data bus. The arbitration for the processor/
memory data bus employs the priority scheme shown in Table 8-2.
Table 8-2. Internal Arbitration Priorities
Priority
Operation
1
A high-priority copy-back buffer flush due to one of the following:
A PCI access to system memory hits in the copy-back buffer.
An L2 cast-out occurs simultaneously with a PCI access to system memory and both the cast-out
and the PCI access are to the same address in system memory.
An L2 cast-out hits a nonsnooped address in the PCMWB.
A 60x processor burst write to system memory with ECC enabled hits a nonsnooped address in
the PCMWB.
A 60x processor burst write to system memory with ECC enabled hits a nonsnooped address in
the PCMRB.
2
A PCI read from system memory (with snoop complete)
3
Priority 60x processor or L2 cache transfers including the following:
An L2 cache copy-back (or PCMRB data transfer) due to a PCI read snoop hit
A 60x processor read from system memory
A 60x processor to L2 cache transfer
A fast L2 cache cast-out
4
A high priority PCMWB flush due to one of the following:
A PCI read hits in the PCMWB
The PCMWB is full and another PCI write to system memory starts
A 60x processor to system memory read hits in the PCMWB
A 60x processor to system memory single-beat write hits in the PCMWB
5
A medium priority copy-back buffer flush due to one of the following:
A 60x processor read hits in the copy-back buffer.
A 60x processor single-beat write hits in the copy-back buffer.
The copy-back buffer is full and new data needs to be written to it.
6
Normal 60x processor or L2 cache transfers including the following:
A 60x processor write to system memory
A snoop copy-back due to a PCI write snoop
A 60x processor read from PCI
A 60x processor write to PCI
A copy-back buffer fill
7
A PCI read from system memory (with snoop not complete)
8
A low-priority copy-back buffer flush
9
A low-priority PCMWB flush
10
A PCMRB prefetch from system memory due to a speculative PCI read operation