Accelerated Graphics Port Interface Specification

AGP3.0 Interface Specification

Rev. 1.0

CLK

1 2 3 4 5 6 7 8 9

RBF

TRDY

GNT

000 000

xxx

000 000000 000 00

ST[2::0]

8-29

AD_STBF

RD1 32B | RD2 64B

STALL

RD3 32B

RD4 32B

RD5 32B

RD6

AD_STBS

Figure 2-9: Use of RBF in Read Transaction Control

In Figure 2-9, RBF is not able to stop the two pipelined GNTs from completing their transactions. (These

are labeled RD1 and RD2 in the diagram.)

RD1 performed a 32-byte transfer followed by an RD2 that performed a 64-byte transfer (too small to

initiate IRDY flow control). RBF managed to block the third data transfer (labeled RD3) from starting by

inserting two wait states.

2.3.3.4 Buffer-Full Flow Control on Core-logic Fast Writes

The target of the Fast Writes (AGP device) also has a signal called WBF that is used, if required, to

prevent the start of a Fast write transaction. The arbiter in the core-logic checks the status of WBF

before initiating a Fast Write transaction. If WBF is asserted at this time, the start of the write is

suppressed.

The only change from AGP is that the target of the Fast Writes (AGP Device) must have twice the

minimum buffering specified for AGP2.0 data rates before a fast write stall takes effect. This amounts

to 160 bytes for AGP3.0 versus 80 bytes for AGP2.0.

2.3.4 Ordering Rule Changes

Ordering rules specify the relationship between concurrently active transactions as they progress to

completion through the platform. AGP3.0 follows the same ordering rules as AGP2.0 except for AGP

writes (formerly known in AGP as low priority writes).

An AGP write is considered to have completed if it has reached a point of global visibility where any

future read access to the system memory location targeted by this write gets the modified data. Note

that the definition for completion does not require that the actual physical memory location be updated –

only that the new data is globally visible.

AGP requires that the completion of low priority writes occur in the same order as the issued requests

from the Master. The AGP3.0 specification, however, does not require that the core-logic complete the