Accelerated Graphics Port Interface Specification

AGP3.0 Interface Specification
Rev. 1.0
106
An AGP3.0 Master’s request queue depth must be sufficient to cover the AGP3.0 core-logic’s
isochronous pipeline latency (L). Plus, any additional isochronous latency contributed by an optional
Fan-out Bridge, according to the following expression:
Y
LBW
Q
isoch
*
=
Where BW/Y = N which is the number of transactions (read and write) per time period T (1
microsecond) of payload size Y.
Request queue size for an AGP3.0 Master device is the sum of its asynchronous and isochronous
queue allocations.
isochasyncdevice
QQQ +=
The core-logic request queue depth (specified in register AGPSTAT field RQ) must be greater than or
equal to the sum of the AGP3.0 device request allocations. Note that the normal case is that of a single
device on the AGP Interface.
device_2
QQQ
devicechipset
+≥
1_
Data (N) and request (Q) resources are shared between AGP3.0 Master devices connected to a Fan-
out Bridge. This specification requires AGP3.0 setup software to establish these parameters in a
compatible way between core-logic and AGP3.0 Master device(s) following the sequence of steps
outlined above.
If a Fan-out Bridge is present, the transaction capability and request queue resources will be first
allocated to the device with the smallest requirement, with any remaining resource allocated to the other
device. Driver software for the AGP3.0 devices are free to change this allocation later, if desired.
An exception to the above rule is in the case of isochronous writes with payload size greater than 64
bytes. In this case, the setup software that establishes the contract parameters must ensure that
Q
isoch
includes the increase in write transactions to satisfy the isochronous payload. The programmed
value of the request queue size for the Master device must not exceed the Core-
Logic’s request queue
depth as stated in AGPSTAT.RQ.
Take the case of Y=256 bytes.
writereaddevice
NNN += in terms of payload size Y = 256 bytes.
64
***
Y
LNLNQ
writereadisoch
+=
isochasyncdevice
QQQ +=
And the software must program the parameters such that
devicechipset
NN ≥
chipsetdevice
QQ ≤