Date   : Fri, 17 May 1996 12:59:00 +0100 (BST)
From   : sf4000@... (Steven Flintham)
Subject: Writing sideways ROM software

I'm writing a sideways ROM and I'm having trouble with using zero page.
Inevitably my code needs a few zero page locations to work with and I
don't know where I can use which is 'safe'. Obviously you can mess
around stacking the locations you intend to use first, but this is
extremely messy, especially when you can have more than one exit from a
routine (such as when an error occurs).

I've got the (original, I think) BBC Advanced User Guide but it doesn't
really help. The only thing which looks a bit promising is that (from
memory) there are a few zero page locations allocated for 'operating
system commands' or some such. Am I allowed to use those, and if so,
can I use them on any service call or only on service call 4?
Alternatively, can anyone suggest either suitable zero page locations
or some 'obvious when you know how' trick to avoid corrupting zero page
without all that stacking and unstacking.

Thanks in advance.

Steven