Re: Difference between R65C02 and original 6502 (wrt 6509 emulation)

From: Mia Magnusson <mia_at_plea.se>
Date: Wed, 17 Apr 2019 02:44:35 +0200
Message-ID: <20190417024435.000048f2_at_plea.se>
Den Tue, 16 Apr 2019 16:37:43 +0200 skrev MichaƂ Pleban
<lists_at_michau.name>:
> Mia Magnusson wrote:
> 
> > Something that Michal and you might want conteplate: Would there be
> > any benefit for the new 8088 support software if it were possible
> > to hide the $0/$1 registers in '816 native mode?
> 
> I am wondering about this. That would necessarily mean that the IPC
> library would run in '816 mode, which would mean that there could be
> no KERNAL calls unless the KERNAL is rewritten for the '816?
> 
> > (For example, when emulating an MDA display adapter, the first 
> > char and its corresponding attribute can
> > afaik not be read by the 65xx processor so any transfer from the
> > emulated MDA display to the CBM-II native display would need at
> > least two bytes copied to some other location by the 8088
> > processor. 
> 
> Yes, this is how it's currently done.
> 
> > Also disk operations that read/write memory feely can't access the
> > first two bytes in every 64k block. Maybe the benefit of being able
> > to hide those two registers is less than the cost (in work and chip
> > usage) to actually implement this though. Sorry if I'm bringing up
> > stuff that we've already discussed.
> 
> But disk reads call the KERNAL IEEE routines extensively, so it would
> be very nontrivial to use them in '816 mode?

How "expensive" is it to switch to/from '816 mode? If it's just one
instruction it might not be much of an issue, but if it's more
complicated it might not be worth the effort.

On a system with plenty of ram parts of the ieee routines could be
duplicated in ram.

Btw, recently the blog Pagetable.com got some really nice articles
about the Commodore peripheral buses.

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Received on 2020-05-29 21:33:16

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