MSC8113 Reference Manual, Rev. 0
10-2
Freescale Semiconductor
MQBus and M2 Memory
10.1 MQBus Arbitration Model
The arbitration algorithm between the three SC140 cores controls the bus accesses so that the bus
is efficiently used and there are zero gaps between accesses. The zero gaps are kept between
accesses from the same SC140 core and also between accesses from different SC140 cores. The
arbitration model is based on a round-robin algorithm. Each client requests a shared resource
from an arbiter. One of the requesting clients—the one with the highest priority—is granted
access and uses the shared resource. Through the parked grant mechanism, the arbitration winner
holds the MQBus grant until another request is initiated. Therefore, the arbitration winner access
path to the MQBus is shorter starting from the first consecutive access. After using the shared
resource, this client is assigned the lowest priority. Then its priority increases each time another
client is granted access to the shared resource.
The incoming request signals define three priority levels: high, middle, and low:
High priority
— Read accesses that are not prefetched
— Immediate write accesses
Middle priority
— Non-immediate write accesses from the EQBS write buffer
Low priority
— Prefetch read accesses
During each clock cycle the access requests are handled as follows:
If there are high-priority requests, the MQBus arbiter performs the round-robin algorithm
between the high-priority requesting SC140 cores.
If there are no high-priority requests but there are mid-priority requests, the MQBus
arbiter performs the round-robin algorithm between the mid-priority requesting SC140
cores.
If there are no high-priority or mid-priority requests but there are low-priority requests, the
MQBus arbiter performs the round-robin algorithm between the low requesting SC140
cores.
Each SC140 core participating in the arbitration process may have a few queued requests in
addition to the current request. These requests may have a higher priority than the current one. In
these cases, the current priority is upgraded to the highest priority. For example, if the current
request is for a prefetch read (low priority) and in addition the same SC140 core has another
prefetch and a program miss (high priority) waiting for execution, the current request is upgraded
to high priority so that the arbitration latency for the program miss access is reduced. In addition,
Содержание MSC8113
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