[Coco] 6309/6809 opcodes with mixed 8/16 bit registers

[Mike Pepe]

> -----Original Message-----
> From: coco-bounces at maltedmedia.com [mailto:coco-
> bounces at maltedmedia.com] On Behalf Of jdaggett at gate.net
> Sent: Thursday, November 22, 2012 2:29 PM
> To: CoCoList for Color Computer Enthusiasts
> Subject: Re: [Coco] 6309/6809 opcodes with mixed 8/16 bit registers
> 
> On 22 Nov 2012 at 12:36, Harry Hurst wrote:
> 
> > I've heard about the RESET. I've never tried it. There appear to be 3
> > HCF's on the 6809, opcodes $14, $15, and $CD. I've tried all of those
> > and their page $10 and $11 counterparts. They all work.
> >
> > Go to this page:
> > https://sites.google.com/a/aaronwolfe.com/cococoding/home/docs
> >
> > Download the "6x09_Instruction_Sets.pdf" there. It has all of this and
> > more, and it's laid very neatly.
> >
> > Thanks,
> > HH
> 
> Since the 6800's HCF opcode is $DD, I would suspect that the 6809 could be
> $CD. This would force into a test mode in which the address bus becomes
> essentially a 16 bit counter.
> In the 6800 the HCF mode does not do a fetch cycle. So it takes a hard
reset
> or power on reset to get it out of that test mode. Opcodes $14 and $15 may
> do other internal test that one would have to have a logic analyzer on the
> pins to determine more what they do.
> 
> The internal workings of the 6809 determine instruction and addressing
> mode from a one byte opcode. The often called prebyte is actually a page
> switch byte or instruction modifier byte. The upper nibble of the opcode
> determines addressing mode with the exception of $1 and $3. Take for
> expample Indexed mode. The upper nibble for indexed mode are $6, $A and
> $E.
> 
> here is a better look at the bits individually
> 
> $6	0110
> $A	1010
> $E	1110
> 
> Only the two MSbits change so that to decode indexed addressing mode you
> only need an equation of
> 
> (bit3 or bit2) and bit1 and not(bit0)
> 
> This yeilds a two bit OR gate and an inverter and a three bit AND gate.
> 
> All the decode logic is done essentially the same way. There is no
microcode.
> Thus the speed limitation of this type design is based on the geometery of
> the gates and the propogation through the gates. The page switch byte or
> instruction modifier byte will alter the instruction and/or the
destination
> register. This can be seen in page one opcodes $8C, $9C, $AC and $BC.
These
> are all CMPX in four addressing modes. Add the page switch byte in front
of
> the above opcodes and the register acted on goes from X to Y. Similarly
the
> page two byte changes the register acted on to S. This is essentially the
heart
> of the patent that is called the Instruction Set Modifier Register. The
> instruction modifier byte is loaded first and then the output of the
register is
> fed to the decode logic to alter the instruction and/or the register acted
on.
> 
> Depending on how complete the decode of the unused opcodes are they
> will do something or do nothing. That is the one fault of the 6809. It
would
> have been nice if they funneled them into a fault vector. Looking at the
> vector map there seemed to be intended 16 pairs of bytes for the vector
> table originally. Only 7 were ever implemented. Hitachi in the 6309 did
> implement the fault vector.  A wise move on their part.
> 
> just giving a bit of insight on my opinion of how the beloved processor
works.
> Much of it comes from working with the sister of the 6809, the HC11. Also
I
> did and extensive patent search several years ago and downloaded a few.
> Also I gleened some info from the Byte magizine article that describes
some
> of the work done on the processor. By far I am no expert or claim to be. I
> found that it was an interesting design and having worked for Motorola for
> 23yrs, it was neat to try and figure out how the actual circuit design was
done.
> It was a remarkable design that probably greyed a few engineers hair back
> then. To do that complex of a circuit with no modern tools to simulate and
> verify the design is a chore. Even breadboading the IC itself had to be
labor
> intensive project if that was done.
> 
> james
> 

I could have sworn the HCF on the 6809 was $FF. I recall that if you were to
take a 6809, stick it on a breadboard, give it power and clock it'll run the
address bus test routine- the data bus should float to $FF if it's not
connected...

Hm, I should (re?) try that. I'm certain I've done this before.

-Mike