[Coco] Spare GIME's (I think) giveaway

jdaggett at gate.net jdaggett at gate.net
Sun Nov 11 22:34:05 EST 2007

On 11 Nov 2007 at 21:12, Joel Ewy wrote:

> Hasn't there been some speculation that the two may differ also in the
> amount of remaining residual functionality they might have pertaining
> to the 256 color mode, if it even existed at all?


Since there has been no one to duplicate the alledge function, at this time I have 
to believe that the function does not exist on the production parts. It may have 
been apart of some early prototype of the part. Back in the 80's most ICs used 
between 3 and 5 passes to get to production levels. The function could have 
been removed to reduce die area for final production. 

Still if it is there or not the functionality of the two parts are essentially the same 
otherwise. The 1987 part more than likely cleaned up some timing issues that the 
1986 part had. Just from the service manual and the fact that the Coco3 uses 
150 nS dram means that the timing between IDMA reads are very  tight at 
1.78MHz processor speeds. 150 Ns DRAM require about 260nS total cycle time. 
One half the processor cycle time at 1.78 MHz is 279nS. That leaves a rather 
slim margin for switching between video and program section of the shared ram.  


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