Date : Thu, 20 Jul 2006 10:44:18 +0100
From : "Steve O'Leary" <navalenigma@...>
Subject: Re: 6502 copro diagnostics?
>From: jgh@... (Jonathan Graham Harston)
>To: bbc-micro@...
>Subject: [BBC-Micro] Re: 6502 copro diagnostics?
>Date: 20 Jul 2006 01:01:09 +0100
>
> >Message-ID: <44BB9ACA.8070302@...>
>
>Jules Richardson <julesrichardsonuk@...> wrote:
> > the copro to initialise, but doesn't indicate whether there's a huge
>chunk
>of
> > dead RAM in the middle of the memory map (say).
>
>If there is faulty RAM, then it will be seen in the same bit all
>they way through memory, not in a single place in memory.
>
>For instance, if the D0 RAM is faulty, then bit zero of *every*
>*single* *memory* *location* will be faulty, and it is highly
>unlikely the Tube OS would get far enough to display the sign-on
>message.
Not necessarily. If the data line is at fault or a general full failure of
the RAM then yes. But there are other options (admittedly rare !). For
example as this is DRAM the hi order address latch of one of the 8 chips
could have a failed A7 bit (or effectively A15 of the full address). If this
has failed you will have a corrupted bit for 32K of RAM (but not all RAM)
and a standard RAM test (writing and reading single locations) will fail to
spot this. If it's the upper 32K and the system does not use this for
scratch space then it will appear to function as normal. What would be
required would be a more thorough test that can spot this type of error.
So you could have a big 32K hole in the RAM at the start or end. If it was
A6 of the hi order address latch that was flaky then you'd have two 16K
banks faulty separate by 16K of good RAM.
In both these scenario's if the Co-Pro OS only uses the lower few K of RAM
as workspace then you can have what appears to be a working system but which
has some large area's of RAM that are unusable.
So Jules still could do with a good RAM test if he wants to be 100% sure of
a fully working system.
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