82845PE Datasheet, PDF (118/176 Pages) Intel Corporation – 82845GE Graphics and Memory Controller Hub (GMCH) and 82845PE Memory Controller Hub (MCH)

Functional Description
5.3.2 PCI Semantic Transactions on AGP
The (G)MCH accepts and generates PCI semantic transactions on the AGP bus. The (G)MCH
guarantees that PCI semantic accesses to SDRAM are kept coherent with the processor caches by
generating snoops to the processor bus.
5.3.2.1
Intel^® (G)MCH Initiator and Target PCI Operations
Table 5-6 summarizes PCI target operation support of the (G)MCH for AGP/PCI_B bus initiators.
The cycles can be either destined to main memory or the hub interface bus.
Table 5-6. PCI Commands Supported by (G)MCH When Acting As a PCI Target
PCI Command
GC/BE[3:0]# Encoding
(G)MCH
Interrupt Acknowledge
Special Cycle
I/O Read
I/O Write
Reserved
Reserved
Memory Read
Memory Write
Reserved
Reserved
Configuration Read
Configuration Write
Memory Read Multiple
Dual Address Cycle
Memory Read Line
Memory Write and Invalidate
0000
0001
0010
0011
0100
0101
0110
0110
0111
0111
1000
1001
1010
1011
1100
1100
1101
1110
1110
1111
1111
Cycle Destination
N/A
N/A
N/A
N/A
N/A
N/A
Main Memory
Hub Interface
Main Memory
Hub Interface
N/A
N/A
N/A
N/A
Main Memory
Hub Interface
N/A
Main Memory
Hub Interface
Main Memory
Hub Interface
Response As PCI
Target
No Response
No Response
No Response
No Response
No Response
No Response
Read
No Response
Posts Data
No Response
No Response
No Response
No Response
No Response
Read
No Response
No Response
Read
No Response
Posts Data
No Response
NOTE: N/A refers to a function that is not applicable
As a target of an AGP/PCI cycle, the (G)MCH only supports the following transactions:
Memory Read: The (G)MCH issues one snoop and the entire cache line of read data is buffered.
If a memory read bursts across the cache line, another snoop is issued but the transaction will be
disconnected on the cache line boundary. Subsequent memory read transaction hitting the cache
line buffer return data from the buffer.
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Intel^® 82845GE/82845PE Datasheet