On 16 Jul 98 at 6:10, Steve Coldwell wrote:

> I need some information on NVRAM.... First of all , could someone
> explain its physical characteristics.

  NVRAM just means Non-Volatile RAM -- that is, RAM that will retain
its contents when external power is removed.
  There are several ways one *could* implement this.  The most common
approach in contemporary PCs is to use a small amount of CMOS SRAM,
with a battery to provide the tiny amount of current needed to retain
data.  Because the use of this particular approach is so common, it
is most often referred to as "CMOS" and only very occasionally as
"NVRAM".

> Is it a chip? Is it situated on the motherboard?

  The same battery that provides current to the CMOS also provides
power for the RTC (Real-Time Clock).  On older motherboards, you may
be able to locate a battery (or battery pack!); on newer boards, this
is often integrated into a single package.  "Dallas" is probably the
most common marking on such packages; which usually look like plastic
(rather than ceramic) and stick up 1/4" or more from the motherboard.

> Can it be replaced?

  Yes, generally.  If the manufacturer has soldered the unit to the
board, they will usually provide a pair of contacts to which a
battery pack may be attached.

> What would make a system display the following error message?
>
>   NVRAM data Invalid, NVRAM Cleared
>   Press <F1> to enter setup, <ESC> to boot
>
> Everything seems to operate normally after pressing the esc key.

  One of the pieces of information stored in the NVRAM is a
"checksum" of the settings.  At boot time, the machine recomputes
this checksum in order to assure itself that the data are intact.
  This check, or one like it, is failing.[*]  After notifying you of
this failure, the machine is initializing the settings to their
default values, which in your case seem to be close enough for normal
operation.

[*] The most likely scenario, as I think you've surmised, is that
this check fails because not enough power from the battery has been
available.  There are a few other possibilities, such as defective
CMOS RAM, but they're far less common.

David G