Differentiating chips (Was: Newbie member saying

From: Philip.Belben_at_pgen.com <(Philip.Belben_at_pgen.com)>
Date: Mon Sep 13 08:15:36 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.


I didn't catch this one the first time round. What exactly did you mean by "the
first computer where a user program couldn't grab the reset vector"? If you
mean that the vector is not in alterable memory, the PET springs to mind - 6502
has the vector in locations $FFFC and $FFFD. On the PET this is ROM, and the
routine it sends you to does not look in RAM...

If you mean the reset vector was not in the computer's address space, and so
incapable of alteration even if you put RAM there, does the Z80 not count? The
reset vector is always zero, and the machine (afaik) always starts executing
code at that address. Most Z80 machines allow this to be made RAM, but plenty
don't - Sinclair springs to mind.

In my day it was (among 8-bit home computer people) considered quite a luxury if
you could redirect the reset vector...

Or did you mean the first Intel 88/86 machine?

I am puzzled.

Philip.






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Received on Mon Sep 13 1999 - 08:15:36 BST