Re: Big-PET (2)

From: Andre Fachat (
Date: 1998-05-02 13:09:30 wrote:
> Hallo Andre,
> > You don't clock the output of the MMU. When you access the MMU in the 

> Yes, if in MAP-MODE! But the point is to disable the map-mode for that moment. 

Yes, I understand that. I just think that you then cannot access the MMU
in the lowest bank (where the MMU is active).

> Simple question: If the 612 w/o latch will cause problems IYO, why is it used 
> in the AT and worked w/o any problem??? :-)

I have no idea how it is used in the PC :-))

> I hope youunderstand the above is not meant to be a "I'm right, You're wrong 
> discusion". As I intend to build Big-PET using a 612, we soon or later will 
> find out anyway who was right.

No problem with that :-))  I don't feel offended or so, and I hope you
you don't feel as well. I think that's just a technical discussion between 
techies :-)

> > But: when the opcodes take more/less time and/or the clock frequency
> > is different, there is a problem.
> Only in software dependent timeloops.

> > There should not be part-cycle out of phase, as both CPUs should be
> > coupled by Phi0/Phi2. If the 65816 is clocked at higher speed, it should
> > be phase-locked to Phi0/Phi2.
> That I do understand but which Phi0/Phi2; the original one of the PET or the 
> one of the coupled 1541???

I am now putting the schematics of my emulation boards on the net 
(CPU and keyboard), you might want to look at it. 
I use the CS/A65 clock Phi2 for the emulated CPU in the VC1541.
It worked. However, if you have circuitry depending on phase-locks 
between Phi0 and Phi2 I can (by jumper) use Phi0 from the emulated
6502, although I have never needed/tested it.

I am also putting a "write-protect" diff for my CPU board on the web:
using the MMU to map memory pages write-protected!
> > One more comment: Is it that useful to use the MMU in the lowest 
> > bank $00****?  As far as I know this bank has a special meaning for Stack
> > etc in the 65816. 
> No. OK, the stack can be place anywhere within Bank 00 but I don't see that as 
> a problem. My mean reason to enable it in Bank 0 is that I can address it using 
> the BASIC or the onboard Monitor of the PET. And both are NOT capable of 
> addressing another Bank.

I don't know much of the 65816. Can one not run 6502 programs in another
bank without the 6502 knowing it (probably except for the stack)?

I mean, if you speed up the 65816, each stack access would be slowed
down because you mapped the PET memory there... - but with the MMU 
you can after all map "fast" RAM even in bank0, right?


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Andre Fachat, Institute of physics, Technische Universitšt Chemnitz, FRG

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