Accelerated Graphics Port Interface Specification

AGP3.0 Interface Specification
Rev. 1.0
97
4.1.1 4x Speed and Isochronous Support
Isochronous operation is not supported when the 4x speed of operation is selected even when operating
in AGP3.0 signaling mode. The core-logic hardware must ensure that the isochronous operation is
disabled when this speed is selected and the Isoch_Support bit in AGPSTAT [17] is cleared.
4.1.2 Contract Parameters
Isochronous parameters are defined in this part of the AGP3.0 specification. The values for other
parameters are contained in an AGP3.0 core-logic’s PCI configuration registers.
4.1.2.1 Isochronous Time Period
The isochronous time period, T, on all AGP3.0 systems is 1.0 microsecond (66.667 common clock
periods). All isochronous components (i.e., master, bridge, core-logic…) in the system use this same
“isochronous period.”
4.1.2.2 Isochronous Data Payload Size
AGP3.0 core-logics may support up to four isochronous data payload sizes: 32, 64, 128 or 256 bytes.
By default the core-logic specifies a default size based on the internal design of the core-logic. The
default size is indicated in an AGP3.0 core-logic PCI configuration register NISTAT.ISOCH_Y. The core-
logic must also support all larger sizes. Core-logics may be able to support higher isochronous
bandwidth when large payload sizes are used.
AGP3.0 Master devices may use the default size or they may request a larger size. Once selected, all
isochronous devices use the same payload size.
4.1.2.2.1 PAYLOAD SIZE VS TRANSACTION SIZE
Isochronous data payload size (ISOCH_Y) specifies the minimum number of bytes of isochronous data
that will be transferred on the AGP Interface without other intervening transactions. The data within the
payload has consecutively increasing addresses (naturally aligned to 8-byte boundaries) in AGP
memory.
Isochronous transaction size specifies the number of data bytes transferred in one isochronous read or
write transaction. Isochronous reads, transaction size, and payload size are one and the same. This is
because the transaction size supported for isochronous reads are: 32, 64, 128, or 256 bytes.
However, for isochronous writes, only two transaction sizes are allowed: 32 or 64 bytes. Therefore, for
payload sizes of 128Bytes and 256Bytes, the Master must generate, without interruption by a different
transaction, multiple consecutive isochronous write requests. The rate at which these consecutive
requests are generated is one per common clock cycle, which is the fastest allowed by the AGP
interface.