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Intel
®
82845 MCH for SDR Datasheet 15
1.4.1 System Bus Interface
The MCH is optimized for the Pentium 4 processor. The primary enhancements over the
Compatible Mode P6 bus protocol are:
• Source synchronous double-pumped address
• Source synchronous quad-pumped data
• System bus interrupt and side-band signal delivery
The MCH supports a 64-byte cache line size. Only one processor is supported at a system bus
frequency of 400 MHz. The MCH supports a 3:4 host-to-memory frequency ratio (using the
100 MHz clock). The MCH integrates AGTL+ termination resistors on all of the AGTL+ signals.
The MCH supports 32-bit system bus addresses, allowing the processor to access the entire 4 GB
of the MCH memory address space.
The MCH has a 12-deep In-Order Queue to support up to twelve outstanding pipelined address
requests on the system bus. The MCH supports two outstanding defer cycles at a time; however,
only one to any particular I/O interface. Processor-initiated I/O cycles are positively decoded to
AGP/PCI or MCH configuration space and subtractively decoded to the hub interface. Processor-
initiated memory cycles are positively decoded to AGP/PCI or system memory, and are again
subtractively decoded to the hub interface, if under 4 GB. AGP semantic memory accesses
initiated from AGP/PCI to system memory are not snooped on the system bus. Memory accesses
initiated from AGP/PCI using PCI semantics and from the hub interface to system memory will be
snooped on the system bus. Memory accesses whose addresses lie within the AGP aperture are
translated using the AGP address translation table, regardless of the originating interface.
1.4.2 System Bus Error Checking
The MCH does not generate parity, nor check parity for data, address/request, and response
signals on the processor bus.