Accelerated Graphics Port Interface Specification

AGP3.0 Interface Specification

Rev. 1.0

4 Appendix A: Workstation Enhancements

The following workstation specific changes and enhancements are described in this Appendix. The

remaining workstation specific changes are described in Appendix B.

1. Isochronous Operation

2. Fast Write Flow Control Change

3. Synchronization Schemes

4. Fan-Out Bridge Requirements

4.1 Isochronous Mode Operation

Traditional AGP devices can demand up to the maximum bandwidth available over the AGP ports.

However, the AGP system does not guarantee to deliver the requested bandwidth, nor does it guarantee

transfers will take place within some clearly specified request/transfer latency time. This “best efforts”

arrangement works reasonably well for applications requiring low average latency and high average

throughput, as long as the device can tolerate an arbitrarily long delay now and then without losing data.

In contrast, streaming applications require the ability to transfer data on a periodic schedule and do so

continuously. Further, low cost designs must minimize the amount of request and data buffering.

Isochronous data transfer services provide a contract, established during application startup that

indicates precisely when data transfers may be requested and sets bounds on when the corresponding

data transfers will occur. This greatly alleviates the problem of arbitrarily long delays and places a strict

bound on the amount of buffering needed to avoid data loss.

This is done by the system guaranteeing to process a specified number (N) of read or write transactions

of a specified size (Y) during each isochronous time period (T). An AGP3.0 device can divide this

bandwidth between read and write traffic as appropriate. Further, the system transfers isochronous data

over the AGP3.0 Port within a specified latency (L).

Given these parameters, the system guarantees to deliver isochronous bandwidth equal to:

BW =

To simplify synchronization with the processor, isochronous write transactions are guaranteed to

become visible to the rest of the system without the need for an explicit “flush” operation. Both

consumer desktop and professional workstation systems benefit from isochronous support; however,

specific platform types may differ as to the number of transactions per period, payload and transaction

size and transaction latency.