Date   : Fri, 18 Jul 1997 13:47:32 +0100
From   : Andy.Fox@... (Andrew Fox - Sun UK - Support Engineer)
Subject: BBC Master hardware problem....some more info....

Hi All,

Further to me previous e-mail, here is some more info.........

If anyone can help, please let me know.

When I SRLOAD a rom image into the Master that is larger than
8k (i.e. between 8-16k), then the rom image no longer functions
correctly.

If I SRLOAD an 8k (or less) rom image, then everything works fine.

The rom images in question all work perfectly well on a BBC B with
sideways ram. None of the test images are copy protected (ie. they
don't try and overwrite themselves on initialistion to see if they
are in RAM, not ROM). 

There are no problems with the 64k of main memory, only the sideways
ram.

I am beginning to believe that there is an address decoding problem
somewhere on the motherboard, but need some help writing a test
program to verify this.

My plan is to write a test image of known bytes into a sideways
ram bank, and then examine it to see at what point this data gets
corrupted.

The only problem is that when looking at &8000 from BASIC, you 
only see the BASIC rom (cause it's paged in).

How can I write a basic (or assembler) program that will allow me to
do a compare on the contents of the sideways RAM bank, ie. how do I
look at the contents of the RAM without it being paged in at &8000?

I have already changed the two 74HCT253 multiplexors that serve
the ram and rom sockets, but this has not helped.

Can anyone help, or better yet, let me have a program that already
does this kind of testing?

Thanks in advance,

Andy.