Accelerated Graphics Port Interface Specification

AGP3.0 Interface Specification
Rev. 1.0
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Table 48: Transaction Size Vs Payload Size
Type Payload Size
(ISOCH_Y)
Transaction Size # of Consecutive
Requests/Payload
Read 32Bytes 32Bytes 1
Read 64Bytes 64Bytes 1
Read 128Bytes 128Bytes 1
Read 256Bytes 256Bytes 1
Write 32Bytes 32Bytes 1
Write 64Bytes 64Bytes 1
Write 128Bytes 64Bytes 2
Write 256Bytes 64Bytes 4
4.1.2.3 Number of Isochronous Transactions Per Period
For each payload size the core-logic supports, it must report the maximum number of transactions it will
accept in an isochronous period. This parameter, Isoch_N, is specified in the configuration register
NISTAT. An AGP3.0 device (or Fan-out Bridge) may initiate up to the maximum number of isochronous
transactions supported by the AGP3.0 core-logic. At a minimum, the core-logic must support enough
transactions to provide at least 128MB/sec of isochronous bandwidth.
Table 49: Minimum Isochronous Bandwidth
Payload Size Transactions Per Period
(Isoch_N)
32 bytes 4
64 bytes 2
128 bytes 1
256 bytes 1
When the payload size is set at 128 or 256 bytes, the core-logic must implicitly allow for 2x or 4x the
value of Isoch_N for isochronous write transactions. This requires the total number of pending
transactions (isochronous and asynchronous) not exceeding the maximum request queue specified by
the field AGPCMD.PRQ. It is the responsibility of the Master to ensure that the maximum request queue
never overflows. The isochronous contract established by the Master must take this into account when
programming the isochronous parameters PISOCH_Y and PISOCH_N in the configuration register
NICMD.
4.1.2.4 Transaction Latency
Isochronous latency (L) is defined as the maximum time from an isochronous request to the beginning
of a corresponding data transfer (to or from AGP3.0 Target) and is specified in integer multiples of the
isochronous period, T
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. To illustrate, assume that the isochronous latency is 5, and a transaction
request begins crossing the AGP3.0 Port 11 common clocks after the beginning of an isochronous
period (T). The AGP3.0 core-logic must initiate the corresponding data transfer no later than the 11
common clocks after the beginning of the fifth period following the one containing the request.
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AGP3.0 does not require the core-logic to indicate its minimum latency.