Datasheet
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