[Coco] Interesting reading

[sales at gimechip.com]

It would be so awesome if you did generate a cycle accurate 6809 CPU core. 
:-)

----- Original Message ----- 
From: <jdaggett at gate.net>
To: "coCoList for Color Computer Enthusiasts" <coco at maltedmedia.com>
Sent: Saturday, June 19, 2010 7:34 PM
Subject: [Coco] Interesting reading


>A while back I was doing some patent searches and came across several 
>Motorola Patents
> on the 6800 series processors. I also came across two other patents that 
> covered the FIRQ
> function.
>
> Well today I really started in depth reading of one patent. Well needless 
> to say it is been very
> educational. Figure #7 of the design and the corresponding description of 
> it is enlightening on
> how the hardware inputs work. From everything I read the embodiement 
> processor for the
> invention is the MC6809. No other processor of that time had Reset, Halt, 
> NMI, FIRQ and
> IRQ external hardware inputs. Patent US-4200912, Processor Interrupt 
> System, describes
> much of the logic behind the MC6809 interrupt system and how it works.
>
> I have gathered this from the patent. There is a priority assigned to the 
> external inputs. They
> are as follows:
>
> RESET   1 (highest)
> HALT    2
> NMI    3
> FIRQ      4
> IRQ        5  (lowest)
>
> Also the priority circuit allows for an asychronous reset to occur and if 
> in HALT mode, once
> the HALT input is released a RESET occurs.
>
> So far I have gained even a better insight on the vernable old processor. 
> All this started
> recently because I wanted a cycle accurate 6809/6309 processor for a 
> project I have. I also
> want to put it in an FPGA for increased speed and lower voltage operation. 
> Hopefully battery
> operation capable.
>
> This has been very educational and fun read. I still need to decipher 
> more.  Also if figure #2 is
> correct in the patent, then the 6809 has two 8 bit data buses along with 
> the two  8 bit address
> buses. That would explain a lot of how movement of data could be done 
> without a huge
> amount of buss switches or multiplexors.
>
> Now I have a better idea on how to code the HDL and hopefully a cycle 
> accurate processor
> can come out of it. Eventually a 6309 capability also. I think I am about 
> to move this project
> up the priority ladder now. ( no pun intended)
>
> more as I get further along.
>
> james
>
> --
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> Coco at maltedmedia.com
> http://five.pairlist.net/mailman/listinfo/coco


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