Date   : Thu, 19 Oct 2006 00:24:01
From   : jgh@... (Jonathan Graham Harston)
Subject: Re: Would anyone happen to have a 4116 dram chip

>Message-ID: <45367CB1.40703@...>

Tim Fardell <tim.fardell@...> wrote:
> chris whytehead wrote:
> > the top 4K(?) where they were masked by the ROM. So Sinclair was buying
> > faulty chips to keep the price down.
> > Or is that another urban myth?
>
> The upper 32K of RAM in the Speccy is actually made up of eight 4164
> chips, but with one of the address lines tied to ground, effectively
> halving their capacity. It might well be that Sinclair used RAM chips
> with faults in the unused "half" - who knows?

No myth at all, but well-documented fact, and described in the
service manual.

The top 32K of RAM used 64K RAM chips. There were a lot of cheap
64K RAM chips available at the time which were faulty in one half.
The Spectrum PCB has a link to select which half of the 64K chip
is used.

An uncertain number of Speccies actually had fully working 64K RAM
chips. A PCB modifcation and a paging register gave you access to
two banks of 32K at the top of memory.

--
J.G.Harston - jgh@...                - mdfs.net/User/JGH
BBC BASIC for the Sinclair Spectrum - http://mdfs.net/Software/Spectrum