Date   : Sun, 09 Mar 2008 23:57:58 +0100 (GMT)
From   : johan@... (Johan Heuseveldt)
Subject: TUBE chip, accessing 'Parasite' side

Hi,

On Sun 09 Mar, Jonathan Graham Harston wrote:
> > Message-ID: <Marcel-1.53-0308144655-b49xSBG@...>
> Johan Heuseveldt wrote:

> >   BA  BS   description
> >    0   1   Interrupt or Reset aknowledge
> > 
> > So BA/BS=01 can be seen as a Vector Pull. I'll use that to switch to the
> > appropiate RAM locations, only needing A8 to toggle; FFFx > FEFx, as these
> > 16 bytes are not used because I/O is mapped there, including the TUBE.
>  
> Ah! That's useful. Similar to the Z80 CoPro. When NMI occurs the
> ROM is paged in at the NMI address at &0066, so the NMI interupt
> fetches a jump to the proper NMI code in high memory.

Ah, didn't know that. It's different than leaving out the ROM all te time!

> Does 'interupt acknowledge' include the software interupts, so
> it's valid for accessing any of the vectors at &FFF0-&FFFF.

Yes.

> Obviates the need to provide an interface with SWIs, as the JSR
> entries are all available.

I think it is the SBUG ROM, or similar, that has the SWIx subdivided,
and a vector table is setup for that. Didn't look too well for now,
when I stumbled onto that.

>   Hmmm. Shame JMP [addr] needs four bytes in 6809.

Yeah. For quite a while I didn't know if it even exists. Opcode tables don't
show that as a native adressing mode - it's a feature of the indexed mode.


> Doing a bit more digging around I've found that the most usual
> memory layout for a 6809 system is as follows:
>  
> 0000-DFFF   Program RAM
>  
> E000-EFFF   Monitor workspace/IO ports/etc
>  
> F800        Monitor ROM (or ROM copied to RAM)
> F800-F812   Standard entry vectors
> F814...     Additional entry vectors
>  
> Flex-09 or OS/9 lives at &C000-&DFFF, leaving &0000-&BFFF for user
> programs.

In a FLEX version all the device drivers need to be rewritten, taken
the TUBE into account. Not sure I'll fancy that. Then there is the issue of
getting the software in the machine. Does taht need any additional Beeb code?
Well, so much in that field to find out yet. That's why more or less I choose
a more Acorn approach with some bank switching.


Greetings,
Johan

-- 
Johan Heuseveldt <johan@...              >
  aka  waarland

  The best place is a Riscy place
 
Let a fool hold his tongue and he will pass for a sage.