Re: In search of bad 4164, 41256 DRAM

Den Tue, 17 Sep 2019 11:46:14 +0200 skrev Gerrit Heitsch
<gerrit_at_laosinh.s.bawue.de>:
> On 9/17/19 11:27 AM, smf wrote:
> > On 17/09/2019 09:32, Gerrit Heitsch wrote:
> >>
> >> It should be since at power on the capacitor in a DRAM cell is
> >> empty and if you stop refreshing it, it will also become empty
> >> after a while. Whether this 'empty' is read as '1' or '0' depends
> >> on the location on the die and on the manufacturer.
> > 
> > Can you explain why empty is read as 1 or 0 though? As far as I know
> > dram cells are either empty or full and it checks if the cell is
> > half full to work out the 0 or 1. So unless they randomly put
> > inverters in there, an empty cell is an empty cell.
> 
> They seem to do exactly that. Otherwise it's not possible that you
> get a manufacturer specific pattern after power on.

They can attach the fixed-voltage end of the capacitor to either +5V or
ground. That would give different default start up states.

> > My thought was that during power on the dram is going to be
> > unstable & it could generate the pattern if the dram did the
> > equivalent of a refresh and the read part of it was done when there
> > wasn't enough power to accurately determine the cell is empty
> > enough while the write was done as the power stabilised. Maybe the
> > power up ends up triggering a write without a read.
> 
> That should result in a more random pattern though.
> 
> But it should be easy to find out, hook a DRAM up to power and a CPU, 
> read it out after power on. Then stop any refresh and any access for
> a while (minutes), and read it out again. If you get all zeros then
> there are no inverters. If you get a pattern again, there are
> inverters.

That is simple to do on a PC/XT as you can control the refresh circuit.
You have to do some tricks though as a parity error on ram read will
trigger an NMI.

Btw some demos rely on being able to turn off refresh. As long as you
execute code / read data that does reads that is equivalent to doing a
refresh. That way they gain a bit more memory bandwidth. :)

-- 
(\_/) Copy the bunny to your mails to help
(O.o) him achieve world domination.
(> <) Come join the dark side.
/_|_\ We have cookies.