[Coco] 6x09 read and write strobes.

[jdaggett at gate.net]

On 19 Dec 2011 at 8:48, john dumas wrote:

> The 6809 die plots that I saw did not have a PLA for the decode logic, 
> but used random logic gates.
> IIRC Bryant and Dennis busted their buns cramming in the logic, 
> maintaining that array logic
> like PLAs would eat up too much die area. As an aside, the push to keep 
> the smallest die
> size was the central element of a continuing - friendly - battle between
> Terry and Joel on
> one side and Bryant and Dennis on the other. ....A common tension 
> between "features"
> and "producibility" in those days..........
> Been there, done that.... on both sides myself!
> 
> In the end , the 09 ended up like the 00, with random logic. Later 
> designs went to array logic,
> microcoded, etc...
> 
> cheers,
> johnd

PLA may not be exactly the best way to describe the input circuitry. 

>From the 6800 patents and the schematic in there, the instruction opcode goes to a register 
and then there are 16 lines which are each bit and its associated inverted level. Those 
sixteen lines feed the random logic and the equations for each instruction execution. In a 
rudimentary form it is a PLA. Though not like a common PLA which are ands and ors to 
make up the product terms. 

The way I understand the Instruction Modiication Register patent, the Page1 and Page 2 
opcodes modify the lines that feed random logic for instruction execution. 

james