845GL datasheet(118/193 Pages) INTEL

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Functional Description

118

Intel® 82845G/82845GL/82845GV GMCH Datasheet

Reads

• Read cycle is immediately retried (the GMCH retries the read cycle in three PCI clocks from

GFRAME# driven active) due to a pending processor-AGP or hub interface-AGP write

transaction. It is further handled using the Delayed Transaction mechanism described in a later

section. This can occur as a result of the processor posting memory write cycles to the AGP or

the GMCH storing a processor to AGP write cycle in the deferred queue. The SDRAM read

cycle is immediately retried and the GMCH initiates the Delayed Transaction activity by

issuing a single snoop on the processor bus. The Delayed transaction cannot complete until

after the pending processor-AGP or hub interface-AGP transactions have been completed on

the AGP.

• Processor-to-AGP write or hub interface-to-AGP write is posted after the processing of AGP/

PCI to SDRAM read has started but prior to data being returned. This scenario can occur due

to the level of concurrency supported by GMCH. The AGP/PCI cycle will be retried as soon as

condition is recognized and it is further handled as a Delayed Transaction.

• Processor-to-AGP read request is internally pending when an AGP-DRAM read is issued or

processor-to-AGP read request is issued after an AGP-DRAM read request is generated. The

AGP cycle is retried based on 32-clock timeout. The timer is triggered at the point when

internally pending processor-to-AGP read request is observed.

• Processor-to-AGP write occurs after AGP-to-SDRAM memory read line or memory read

multiple data has been returned. The GMCH stops snooping ahead when the processor-to-

AGP write occurs and the GMCH disconnects when the last DWord of data is read (between 2

and 3 cachelines).

• AGP-DRAM burst is disconnected after crossing the 4-KB address boundary.

• AGP-DRAM burst is disconnected if consecutive data phase can not complete within 8 clocks

and there is an AGP non-snoopable request or host bridge-to-AGP request pending.

• AGP-DRAM burst is disconnected after crossing a 2-KB address. No snoop is generated into

next 2-KB page.

Writes

• If the AGP/PCI Inbound buffer is full, the GMCH retries initial write request in the presence of

a pending AGP request issued by the AGP master using enhanced AGP protocol (i.e., non-

snoopable) or in the presence of host bridge request for AGP ownership (when there is a

pending processor-AGP or hub interface-AGP transaction).

• If the AGP Inbound buffer is full and there is no pending AGP non-snoopable request and no

host bridge request, the GMCH inserts wait states. It retries as soon as AGP non-snoopable

request is generated or an internal host bridge to AGP request is generated.

• If AGP Inbound buffers become full during the burst, the GMCH disconnects within 8 clocks

if there is an AGP non-snoopable request or host bridge-to-AGP request present.

An AGP-DRAM burst is disconnected after crossing the 2-KB address boundary.

845GL bảng dữ liệu(PDF) 118 Page - Intel Corporation