Date   : Tue, 25 Mar 2008 23:49:55 +0000
From   : jgh@... (Jonathan Graham Harston)
Subject: New 6809 TUBE

Johan Heuseveldt wrote:
> That's how I designed the FLEX version, although I've used 219s, which is
> more sensible with the big RAM chips available these days. Later I
discovered
> the SWTPC boards that indeed use the 189 for that.
 
Yes, the '189 inverts the outputs compared to the '89, but as far
as the processor is concerned, everything "looks" the same, as
long as the address lines are connected appropriately.
 
> > Or toggle A7 to put vectors at &FF7x.
> 
> The hardware is assigned to &FEF0 to &FEFF. The RAM there is thus unused, so
> I made that available to the 16 locations for the HW vectors. That the
> most logical thing to do, and most simple as well. Why making an extra
> gap in the software space?
 
That's the usual arrangement when you don't have a seperate I/O
space, eg BBC MOS with a 'gap' for the hardware at &FC00-&FEFF,
6502 Tube has a gap at &FEF0-&FEFF, 320126 Tube has a gap at
&FF0000-&FFFFFF, the C64 has a gap at &D000-&DFFF.
 
> These locations for the vectors can still be accessed by putting a vector
> number 0-7 into location &FEFD and the two vector locations are available at
> &FEFE and &FEFF. The real discrete logic is no fun at all, but the boolean
 
Ah, I was wondering how you'd differentiate between STA &FEFx
being an access of Tube Register or an access to a vector. The
hardware's easier if you simply "abandon" the RAM that's lost
underneath the hardware: A15-A4=&FEEx - select hardware,
A15-A4<>&FEEx - select memory.
 
Later edit:
I've also just remembered that FLEX expects to be told where
SWI3 and IRQ vectors are so it can change them by just poking,
ie something like LDD #swi3handle : LDX swi3vector : STD ,X
 
> Register. Other locations are for bank select registers, and something
secret
 
The SWTPC has 16 bank select registers accessed at 16 I/O
locations, which is why my notes sketch in 16 registers. With 16
8bit registers that only gives access to 1M of memory, but I put
together my design in about 1985 when 1M was unheard-of luxery :)
 
> (I'll explain later when the schematics are published)
 
Oo! I look forward to it.
 
-- 
J.G.Harston - jgh@...                - mdfs.net/User/JGH
Sheffield Boundary Review at http://mdfs.net/User/JGH/Docs/Politics/ParlReview