Accelerated Graphics Port Interface Specification

AGP3.0 Interface Specification
Rev. 1.0
134
Bits Access Field Description
31-
24
RZ-IW Reserved Reserved. Reads return zeros; writes are ignored
5:3 MST: RW-
D101’b
TGT: R-IW-
Dx
ISOCH_L Target: Maximum isochronous data transfer latency in units
of isochronous periods (1 us). Allowable values are 1 to 5.
Master: Software should write the value of Target’s ISOCH_L
into this field. Master may use this value if needed although it
can always use a default max value of 5 for ISOCH_L.
2 RZ-IW Reserved Reserved
1:0 R-W1C/D’0 Isoch-ErrorCode TARGET: 00 = No Error/ 01 = Isoch Req Overflow
29
/ 10 =
Reserved/ 11 = Reserved
MASTER: 00 = No Error/ 01 = Read Buffer under-run
30
/ 10 =
Write Buffer Overflow
31
/ 11 = Reserved
The calculation of NICMD.PISOCH_Y, NICMD.PISOCH_N, and NICMD.PRQ are very dependent upon
fields in AGP_STAT and NISTAT. Refer to Section 4.1.10 for possible methods in calculating these
fields.
29
Indicates that the number of isochronous requests from master has exceeded isoch request queue in target.
30
Indicates that the target has failed to return isochronous read data in a timely manner resulting in a read buffer under-run.
31
Indicates that the target has failed to fetch isochronous write data from the master in a timely manner resulting in this
error.