Intel B940 Datasheet download pdf (Page 44)

Processor Configuration Registers

44 Datasheet, Volume 2

positions must first be read, merged with the new values for other bit positions and

then written back. Note the software does not need to perform read, merge, and write

operation for the Configuration Address Register.

In addition to reserved bits within a register, the processor contains address locations

in the configuration space of the Host Bridge entity that are marked either "Reserved"

or “Intel Reserved”. The processor responds to accesses to Reserved address locations

by completing the host cycle. When a Reserved register location is read, a zero value is

returned. (Reserved registers can be 8-, 16-, or 32 bits in size). Writes to Reserved

registers have no effect on the processor. Registers that are marked as Intel Reserved

must not be modified by system software. Writes to Intel Reserved registers may cause

system failure. Reads from Intel Reserved registers may return a non-zero value.

Upon a Full Reset, the processor sets its entire set of internal configuration registers to

predetermined default states. Some register values at reset are determined by external

strapping options. The default state represents the minimum functionality feature set

required to successfully bringing up the system. Hence, it does not represent the

optimal system configuration. It is the responsibility of the system initialization

software (usually BIOS) to properly determine the DRAM configurations, operating

parameters and optional system features that are applicable, and to program the

processor registers accordingly.

2.6 I/O Mapped Registers

The processor contains two registers that reside in the processor I/O address space -

the Configuration Address (CONFIG_ADDRESS) Register and the Configuration Data

(CONFIG_DATA) Register. The Configuration Address Register enables/disables the

configuration space and determines what portion of configuration space is visible

through the Configuration Data window.

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Intel B940 Datasheet Page 44