Date   : Sun, 31 Oct 2004 01:43:04 +0100
From   : jgh@... (Jonathan Graham Harston)
Subject: Re: &D00

Andrew W <a.weston2@...> wrote:
> I read in one book that &D00-DFF was okay for user routines but on
 
That book is plainly wrong. &D00 upwards holds the NMI routine,
&0D9F-&0DEF holds the extended vector table and &0DFx holds the ROM
workspace flags. If you trample over the NMI area, any sprurious NMI will
crash the machine. If you trample over the extended vector area and
vectored system call could crash the machine. If you trample over the ROM
workspace flags ROMs will trample over undefined bits of memory.
 
> looking at the advanced user guide it says that the floppy disc
> controller may sometimes make use of it. Can anybody say when the
> floppy disc controller calls this area of memory and if using DFS as
 
The disk subsystem claims the NMI whenever it needs to do disk transfers.
The network subsystem claims the NMI all the time it is not claimed by
something else.
 
> the filing system is it a safe bet just not to store anything there?
 
Never store anything there unless using it for the purpose for which it is
reserved, ie NMI code, extended vector, ROM workspace flags.[1]
 
[1] There's a couple of other bits reserved in &0D00. Econet owns
&0D60-&0D7F[2], trackerball/mouse owns &0D92-&0D9E.
 
[2] And *don't* read this to mean "I don't have a network, therefore *I*
will trample all over the network space. I am *fed up* to the *back teeth*
fixing programs that die as soon as a network packet goes past just
because some brain-dead moron has trampled all over the network space.
 
-- 
J.G.Harston - jgh@...                - mdfs.net/User/JGH
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