Differentiating chips (Was: Newbie member saying

From: Hans B Pufal <hansp_at_digiweb.com>
Date: Fri Sep 10 22:59:22 1999

Sean 'Captain Napalm' Conner wrote:

> I don't think so. The reset vector is hardcoded (it's part of the BIOS)
> and the code in the BIOS checks a certain memory location (40:something,
> don't recall of the top of my head) and if the contents are $1234, then it's
> a software reset and not a powerup reset. I don't recall all the details of
> what happens when a soft reset happens but the PC was the first computer
> where a user program couldn't grab the reset vector.

Close but not quite correct.

At reset, one of the first things the BIOS does is to read value out of
the CMOS NVRAM, that value determines how the reset is to be treated.
There is a value you can write into CMOS to cause the machine to restart
at a user supplied vector.

I think, I'd have to check the source to be sure, that you can recover
the processor type if you use this mechanism.

The $1234 value you mentioned is used simply to skip the memory test and
other time consuming POST activities which really need only be done on
power up. In fact most modern PC's that I work with ignore the $1234
value and do a full POST anyway.

If anyone wants the details of the user reset vector let me know and
I'll look it up.

Regards


Hans B Pufal
<mailto:hansp_at_digiweb.com>
Received on Fri Sep 10 1999 - 22:59:22 BST