Date   : Thu, 23 Aug 2007 08:54:23 +0100
From   : dl.harper@... (David Harper)
Subject: Multiple filing systems (was Acorn Winchester

Jonathan Graham Harston wrote:

> No, the default (and always) position is for the selected filing
> system to be active. You can't have more than one filing system
> active at once - what media would be accessed?
>
> ...
>
> You're mixing up "active" with "present".

I think a bit more clarity is needed here.

Obviously you can have several filing systems in a BBC Computer, and swap 
between them with appropriate *-commands. Such a command makes one or other 
of them into the current filing system.

How much, though, is being the "current filing system" the same as being 
"active"?

Certainly on the Master you can open a file under one filing system; if you 
then swap to another filing system then the first file remains open. If, for 
instance, you try:

    *ADFS
    H%=OPENOUT("AFILE")
    *DISC
    BPUT# H%,50

the fourth line will correctly write the byte to the ADFS file, even though 
DFS is current. It will cause ADFS to be swapped in temporarily for the byte 
to be written (because the file is open in ADFS), but it will not affect DFS 
being the "current" filing system.

In other words, you can have files open in several different filing systems 
at the same time, and the system will happily recognise which FS each one 
belongs to, and swap them in and out automatically in order for appropriate 
actions to take place. Once you have opened a file, you can forget which 
filing system it belongs to - the machine will look after it for you.

This seems to me like having several filing systems "active" at the same 
time, even though only one of them is "current".

What I cannot remember is how much of this existed on the original Beeb, and 
how much was introduced with the Master.

David Harper