AMD-762 Datasheet, PDF (33/122 Pages) Advanced Micro Devices – System Controller

24416C—December 2001
Preliminary Information
AMD-762™ System Controller Data Sheet
Chapter 2
Requests from the SBA bus are multiplexed with PIPE#
requests and written to the same queues. High-priority
requests are inserted in front of low-priority requests so that
the request to be serviced is at the top of the queue. This
reordering is done dynamically as a new request is written into
the queue.
Requests from each of the queues can be read out of both the
queues at the same time. The reads start fetching data from
memory and the write data is sent across the AGP bus at the
same time.
The AGP ordering rules specify that writes are ordered ahead
of reads. Reads are serviced only when all the preceding writes
have been written to memory, which is only required for
low-priority requests and does not affect high-priority read
requests. When a low-priority request is the next one to be
serviced from the read queue, the tag of that request is
compared with all valid entries in the write queue. If any entry
matches, then the read request is blocked. Only after the write
requests are serviced are the read requests allowed to proceed.
AGP Request Queue. In general, the AGP request queue services
AGP requests in the order received, subject to their priority
(write High, read High, write, read).
Ordering Rules. The request queue is subject to the following
AGP ordering rules:
n High-priority write requests are processed in the order they
are received.
n High-priority read requests are processed in the order they
are received.
n Low-priority write requests are processed in the order they
are received.
n Low-priority read requests are processed in the order they
are received.
n Low-priority reads push low-priority writes, meaning that a
write request is serviced before a subsequently received
read request is serviced.
n Low-priority writes can pass low-priority reads, meaning
that a write request can be serviced before a previously
received read request.
Functional Operation
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