[Coco] Why do we need a CoCo 4? (Long irrelevant rant)

[jdaggett at gate.net]

Frank

The HC12 supports almost all of the addressing modes that the 6809 does. It even 
allows auto pre and post increment and decrement of the index register. Pre/post auto 
increment/decrement can be done by one to 8 bits. HC12 has the LEA instruction like 
the 6809. 

I would venture than most 6809 code could very easily be ported over to an HC12. Any 
instructions that deal with the "U" register would have to be changed. The PSHU/PULU 
are not defined. Also Indirect Indexed offset with the PCR I believe is not supported. 

>From a brief look a few years ago, the 6809 source code is about 80% or more 
compatable with the HC12. 


james


On 30 Dec 2008 at 18:02, Frank Swygert wrote:

> Am I reading this right? Could a more or less CoCo compatible machine
> be made with a HC12? Something that would be at least 75% compatible?
> I know that a lot of compatibility issues with the CoCo 3 are in the
> GIME chip, but you're working that issue! Of course I'd like to see
> something backward compatible with DECB, but I think it should be
> acceptable if CC1/2 compatibility suffered or was gone (such as
> support for PMODE screens) IF the space were needed for graphics
> enhancements or coding could be seriously simplified. There would be
> some software "loss" without those old graphics modes, but I
> personally think that would be an acceptable compromise -- can't be
> many "popular" programs out there that don't have some newer CC3
> replacement. Some of the kids games, most likely, would be lost. DECB
> could even be patched if necessary. As long as CC3 specific games ran
> I don't think many would object. 
> 
> ------------
> Date: Mon, 29 Dec 2008 20:41:32 -0500
> From: jdaggett at gate.net
> 
> Probably would be better to do something like what the HC11/HC12
> approach. Then again if the HC12 had a U register then this thread
> would be mute. A Coco4 would better be done with an HC12. 
> 
> -- 
> Frank Swygert
> Publisher, "American Motors Cars" 
> Magazine (AMC)
> For all AMC enthusiasts
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> 
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