
Functional Description
112
Intel
82845G/82845GL/82845GV GMCH Datasheet
4.3.1.2
AGP Target Operations
As an initiator, the GMCH does not initiate cycles using AGP enhanced protocols. The GMCH
supports AGP target interface to main memory only. The GMCH supports interleaved AGP and
PCI transactions. The
Table 4-6
summarizes target operation support of GMCH for AGP masters.
NOTE:
N/A refers to a function that is not applicable
As a target of an AGP cycle, the GMCH supports all the transactions targeted at main memory and
summarized in
Table 4-6
. The GMCH supports both normal and high priority read and write
requests. The GMCH does not support AGP cycles to the hub interface. AGP cycles do not require
coherency management and all AGP-initiator accesses to main memory using AGP protocol are
treated as non-snoopable cycles. These accesses are directed to the AGP aperture in main memory
that is programmed as either uncacheable (UC) memory or write combining (WC) in the
processor’s MTRRs.
Table 4-6. AGP Commands Supported by GMCH When Acting As an AGP Target
AGP Command
GC/BE[3:0]#
Encoding
GMCH Host Bridge
Cycle Destination
Response As AGP Target
Read
0000
Main Memory
Low Priority Read
0000
Hub Interface
Complete with random data
Hi-Priority Read
0001
Main Memory
High Priority Read
0000
Hub Interface
Complete with random data
Reserved
0010
N/A
No Response
Reserved
0011
N/A
No Response
Write
0100
Main Memory
Low Priority Write
0100
Hub Interface
Cycle goes to SDRAM with BEs inactive
Hi-Priority Write
0101
Main Memory
High Priority Write
0101
Hub Interface
Cycle goes to SDRAM with BEs inactive -
does not go to hub interface
Reserved
0110
N/A
No Response
Reserved
0111
N/A
No Response
Long Read
1000
Main Memory
Low Priority Read
Hub Interface
Complete locally with random data - does
not go to hub interface
Hi-Priority Long
Read
1001
Main Memory
High Priority Read
Hub Interface
Complete with random data
Flush
1010
GMCH
Complete with QW of Random Data
Reserved
1011
N/A
No Response
Fence
1100
GMCH
No Response – Flag inserted in GMCH
request queue
Reserved
1101
N/A
No Response
Reserved
1110
N/A
No Response
Reserved
1111
N/A
No Response