Date   : Sat, 18 Apr 2015 21:59:47 +0100 (WET-DST)
From   : bbcmicro@... (Peter Coghlan)
Subject: EPROM versus ROM for BBC/B OS1.2

jgh wrote:
>
> Those pins are there because on the very first test machines the OS was 
> in four 4K EPROMs plugged into the four sideways ROM slots with the 16K 
> BASIC plugged into the MOS socket. Changing the pin links around swapped 
> the functionality of the five sockets.
>

I can't see why A14 and A15 being present on the MOS socket was required either
for the four 4K OS EPROMs or for BASIC to function in the MOS socket.

What use could A14 and A15 be to a 16K BASIC? I suppose they could be decoded
by a ROM with several select lines to map BASIC into 4000-7FFF for example but
was this ever done?

>
> Later test machines has the OS in two EPROMS on a header board plugged 
> into the MOS socket.
>

I could see how some logic on the header board could decode A14 and A15 if
this was necessary but I can't see why it would be necessary because they
are decoded by IC21 and the enable signal for either 8000-BFFF or C000-FFFF
is selected by S21 and fed through to pin 20.

Regards,
Peter Coghlan.