Intel 925 Data Sheet - Page 36
Register Description, Intel, 82925X/82925XE MCH Datasheet, Description
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Register Description R Item R/WSC R/WSC/L R/WC R/WO W Reserved Bits Reserved Registers Default Value Description Read / Write Self Clear bit(s). These bits can be read and written. When the bit is '1', hardware may clear the bit to '0' based upon internal events, possibly sooner than any subsequent read could retrieve a '1'. Read / Write Self Clear / Lockable bit(s). These bits can be read and written. When the bit is '1', hardware may clear the bit to '0' based upon internal events, possibly sooner than any subsequent read could retrieve a '1'. Additionally there is a bit (which may or may not be a bit marked R/W/L) that, when set, prohibits this bit field from being writeable (bit field becomes Read Only). Read Write Clear bit(s). These bits can be read and written. However, a write of '1' clears (sets to '0') the corresponding bit(s) and a write of '0' has no effect. Write Once bit(s). Once written, bits with this attribute become Read Only. These bits can only be cleared by a Reset. Write Only. Whose bits may be written, but will always-return zeros when read. They are used for write side effects. Any data written to these registers cannot be retrieved. Some of the MCH registers described in this section contain reserved bits. These bits are labeled "Reserved". Software must deal correctly with fields that are reserved. On reads, software must use appropriate masks to extract the defined bits and not rely on reserved bits being any particular value. On writes, software must ensure that the values of reserved bit positions are preserved. That is, the values of reserved bit 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 MCH contains address locations in the configuration space of the Host Bridge entity that are marked either "Reserved" or "Intel Reserved". The MCH 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 MCH. 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 MCH 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 MCH registers accordingly. 36 Intel® 82925X/82925XE MCH Datasheet