Date   : Tue, 26 Jul 1994 18:19:23 EST
From   : Stephen Quan <quan@...>
Subject: Re: Erk! I've got stuck again! (fwd)

>From Stephen_Youell.wgc-e@... Tue Jul 26 18:13:03 1994

> In-Reply-To: <9407252255.AA06390@...> from
> "quan@..." at Jul 26, 94 02:51:12 pm
> 
> > > No sooner than I've started, I've come across an STA &FE30.
> >
> > Doesn't some hardware attach itself to these locations?
> > I remember there were a few bytes up there that you set which gave
> > you direct access to the user port and perhaps a few other things.
> 
> SHEILA, FRED and JIM are what you might call interesting. Firstly, does
> anyone have any plans for supporting software emulated peripherals? For
> instance two beeb emulators running at once with an RS423 'cable' between
> them? What if some other hardware were emulated that could be 'plugged into'
> the beeb emulator's user port? How sick are we going to get?
> 
> Is anyone keeping in mind someone writing something like a ram expansion
> board or Replay ;-)
> 
> FE30, if I remember right, is the same as FE31, FE32 and FE33, as in the
> address decoding the bottom two bits of the address bus are ignored.
> 
> These areas of memory are messy and this is where the emulators are going to
> lose the speed. With my experience of James B's emulator, and I assume with
> the others too, any special case memory locations are going to slow things
> down. It may be sensible to have a second process for bits of this area which
> aren't synchronous and use shared memory to communicate. Obviously the 6522
> timers and the like can't be supported this way.
> 
> FRED (FC00-FCFF) and JIM (FD00-FDFF) are a doddle. I'm guessing here a bit, I
> can't truly remember without the Adv' User Guide, but JIM and FRED are for
> the 1MHz bus. Both are 256 byte areas used for memory mapped IO. Most of the
> bytes in JIM have been reserved for specific use by particular pieces of
> hardware and FRED is an area to which nothing is mapped in a beeb - the
> /dev/null of beebs if there is nothing plugged in.
> 
> As an example I built an eprom emulator which can emulate upto a 32k eprom. I
> used FCFF in JIM (it's an unassigned byte) as a register (a bit like FE30) to
> switch each 256 block into FRED. I used the top bit of FCFF to take control
> of the memory in the eprom emulator and the bottom 7 bits to map each 256
> block. (7 bits * 256bytes = 32k bytes). Could anyone emulate this?
> 
> I've got a good background in the goings on of the hardware of the beeb. My
> beeb and soldering iron live together.
> 
> If anyone wants anymore info or clarification of anything in the Adv' User
> Guide I'll be happy to help. At some point I'm going to be writing the 8271
> for James B's emulator. It's simpler than playing with the DFS, there's not
> much to the 8271.
> 
>   Steve Youell

Sorry this didn't make it to the list, so I thought I'd just
forward it.  Could people make beeb mail is forwarded to

    cs0081@...

Thanks!