Intel 925 - Pentium D 925 3.0GHz 800MHz 4MB-Cache Socket 775 CPU Datasheet page 36

Register Description
Item
R/WSC
R/WSC/L
R/WC
R/WO
W
Reserved Bits
Reserved
Registers
Default Value
36
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.
Description
®
Intel 82925X/82925XE MCH Datasheet
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