Accelerated Graphics Port Interface Specification

AGP3.0 Interface Specification
Rev. 1.0
135
5.8.7 NICMD: AGP ISOCHRONOUS COMMAND REGISTER
This register is accessible in the Target only if the Target’s AGPSTAT.isoch_support bit is set.
Similarly, the register is accessible in the Master only if the Master’s AGPSTAT.isoch_support bit is set.
When the isoch_support bit is not set, reads to this register will return undefined data and writes will
have no effect.
Offset: CAPPTR + 20h
Size: 2 bytes
Table 67: Command Register
Bits Access Field Description
15:8 MST: R-W-
D’0
TGT: RZ-IW
PISOCH_N MASTER: Software programs this field with the maximum
number of isochronous transactions that the master is
allowed to request from the target in a single isochronous
period. It includes both Read and Write requests.
Target: IGNORED
7:6 R-W PISOCH_Y
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Software programs this field with the Isochronous payload
size to be used by all AGP3.0 devices; software ensures it is
the same value for all AGP3.0 devices. It should be set to the
smallest size (NISTAT.ISOCH_Y) that is supported by all
devices AND satisfies the bandwidth requirements for all
ports. It is encoded in the same way as NISTAT.ISOCH_Y.
Master: Default value is master’s NISTAT.ISOCH_Y.
Software can modify value based on the smallest payload
size acceptable to all devices. Changing PISOCH_Y from
default would require software to re-calculate ISOCH_N for
the master in order to ensure that the isochronous
throughput requirements are met. This is because the
master’s throughput requirement is determined by
(NISTAT.ISOCH_Y) * (NISTAT.ISOCH_N). Target: Default
value is NISTAT.ISOCH_Y
In the target, each ISOCH “Y” has a corresponding ISOCH
“N”. Since PISOCH_Y overrides the value in
NICMD.ISOCH_Y, the target must re-compute (in hardware)
the value of NISTAT.ISOCH_N based on the new PISOCH_Y.
The algorithm is implementation specific.
Software must first write the final value in PISOCH_Y in target
prior to looking at the target’s NISTAT.ISOCH_N. It must also
re-calculate the master’s ISOCH_N based on the final value
of PISOCH_Y.
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PISOCH_Y applies to Isoch Read and Writes. For Isochronous Writes, the AGP Master must queue 2 and 4 consecutive
64 byte requests to handle PISOCH_Y of 128 and 256 bytes respectively.