Date   : Sat, 18 Apr 2015 10:43:49 +0100
From   : jgh@... (J.G.Harston)
Subject: EPROM versus ROM for BBC/B OS1.2

Peter Coghlan wrote:
> Maybe it was just felt to just be a good place to route all the address 
> lines
> where they could be picked up by an adapter board plugged into the OS 
> ROM
> socket or something like that?  On the other hand, other very useful 
> signals
> such as R/W- are not present there.

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.

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

jgh