Accelerated Graphics Port Interface Specification
AGP3.0 Interface Specification
Rev. 1.0
133
5.8.6 NISTAT: AGP ISOCHRONOUS STATUS 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. How the core-logic responds to Isochronous transactions when the Isoch_support bit is
not set is undefined. Refer to the AGP Status Register, AGPSTAT, for register set definition.
Offset:
CAPPTR + 0Ch
Size: 4 bytes
Table 66: Status Register
Bits Access Field Description
31-
24
RZ-IW Reserved Reserved. Reads return zeros; writes are ignored
23:16
R-IW-Dx MAXBW
28
Maximum Bandwidth (both asynchronous and isochronous) of the
device per us in units of 32 bytes. If this field is all “0s,” it should be
ignored.
TARGET: Maximum bandwidth that the target can support.
MASTER: Maximum bandwidth required by the Master.
15:8 R-IW-Dx ISOCH_N Maximum number of isochronous transactions in a single
isochronous period.
Target: This is the maximum that the target can accept at its
corresponding isochronous payload size
(NICMD.PISOCH_Y). Must be at least 1.
In the target, each ISOCH “Y” has a corresponding ISOCH
“N”. The value reported in NISTAT.ISOCH_N is controlled by
the value in NICMD.PISOCH_Y (which defaults to
NISTAT.ISOCH_Y).
After software configures the target’s NICMD.PISOCH_Y, it
can then read the target’s corresponding ISOCH “N” here.
Master: This is the maximum number required by the master
at its corresponding isochronous payload size (ISOCH_Y).
7:6 R-IW-Dx ISOCH_Y Target and Master: Isochronous payload sizes supported.
Encoded as:
‘00’ 32bytes, 64bytes, 128bytes, 256bytes
‘01’ 64bytes, 128bytes, 256bytes
‘10’ 128bytes, 256bytesand greater
‘11’ 256bytes
28
MAXBW should be used by configuration software for the allocation of PISOCH_N and PRQ parameters when dividing
the available bandwidth among several devices.