Date   : Thu, 28 Aug 1997 02:16:07 GMT
From   : adq@... (Andrew de Quincey)
Subject: Re: Slideways RAM Whoops!

>  It all comes down to one decoder (IC26), and two bits: one is
>  Address line 4, and the other is the R/W signal
>
>current inputs and output to this decoder are:
>
> INPUTS    OUTPUT
> --------------------      ---------------------------
>  A4       R/W=09
>  0        0       VIDPROC
>  0        1       ROMSEL
>  1        0       INTON
>  1        1       not connected
>

Whoopsie! I seem to have become a bit confused here... 
ROMSEL is purely to do with selecting which paged ROM we want to deal
with;  it doesn't matter if we are going to read or write from/to it
at this point....

(Also, if A4=1 and R/W=1, then we will be reading from wherever INTON
is stored.. probably part of sheila, but this is different from the
part ROMSEL is concerned with. I don't know for sure, since I think
the electron may have differed from the BBC here, and guess who has
the Electron advanced user guide :(

Sooooo.. I don't need to do anything about ROMSEL... All I have to do
is the extra couple of chips to generate the Write Enable and fix the
Output enable...

silly me...


Incidentally,unless the circuit diagram of the BBC, or I am wrong (oh
surely not! :), the output enable of the ROM chips is generated purely
from the address being dealt with. i.e. it doesn't matter whether you
read or write to the ROM area, you always end up reading from the
ROMs.

So, if someone DID try to write to one of the ROM chips, they'd end up
with garbage on the data bus, since the processor would be outputting
the data to be "written" to the location, and the ROM would be
outputting the data it contained at that location as well?