82845PE Datasheet, PDF (117/176 Pages) Intel Corporation – 82845GE Graphics and Memory Controller Hub (GMCH) and 82845PE Memory Controller Hub (MCH)

Functional Description
5.3.1.6
5.3.1.7
5.3.1.8
4X AGP Protocol
In addition to the 1X and 2X AGP protocol, the (G)MCH supports 4X AGP read and write data
transfers and 4X sideband address generation. The 4X operation is compliant with the 4X AGP
specification as currently described in the Accelerated Graphics Port Interface Specification,
Revision 2.0.
The (G)MCH indicates that it supports 4X data transfers through RATE[2] (bit 2) of the AGP
Status register. When DATA_RATE[2] (bit 2) of the AGP Command register is set to 1 during
system initialization, the (G)MCH performs AGP read and write data transactions using 4X
protocol. This bit is not dynamic. Once this bit is set during initialization, the data transfer rate will
not change.
The 4X data transfer protocol provides 1.06 GB/s transfer rates. The control signal protocol for the
4X data transfer protocol is identical to 1X/2X protocol. In 4X mode, 16 bytes of data are
transferred during each 66 MHz clock period. The minimum throttle-able block size remains four,
66 MHz clocks which means 64 bytes of data is transferred per block. Three additional signal pins
are required to implement the 4X data transfer protocol. These signal pins are complementary data
transfer strobes for the AD bus (2) and the SBA bus (1).
Fast Writes
The Fast Write (FW) transaction is from the core logic to the AGP master acting as a PCI target.
This type of access is required to pass data/control directly to the AGP master instead of placing
the data into main memory and then having the AGP master read the data. For 1X transactions, the
protocol simply follows the PCI Local Bus Specification, Revision 2.2. However, for higher speed
transactions (2X or 4X), FW transactions follow a combination for PCI and AGP bus protocols for
data movement.
AGP 1.5 V Connector
The (G)MCH’s AGP buffers only support 1.5 V operation. Therefore, 845GE/845PE chipset
platforms only support 1.5 V AGP connectors.
Intel^® 82845GE/82845PE Datasheet
117