BigEd wrote:
The rest of Jeff's sentence should help you. Without wishing to discourage you, it does appear that you need to read more carefully and think more deeply: this is not easy stuff, it requires study and consideration. What you're aiming to do is build up a mental model of the cycle by cycle behaviour of an MPU in its system, so you know what's happening. This isn't going to work if you just jump from one guess to another: the questions are harder than you think.
What's more, you risk running everyone out of patience: there are half a dozen people here with enough goodwill to help you, but unfortunately that's not an unlimited resource.
Here's what Jeff said. Get paper and pencil and work through cycle by cycle what happens when an MPU reads an instruction. This is going to take you at least a couple of hours.
Dr Jefyll wrote:
BTW in case anyone's wondering, in this thread the term NOP doesn't necessarily imply $EA, the official NOP. $EA isn't ideal for the job of manipulating the '816 to read (but ignore) a series of bytes at PC because it only accesses one byte every 2 cycles. A better choice is to feed the '816 WDM ($42) -- a two-byte NOP that executes in 2 cycles.
I did, and I do (understand, that is). I'm just asking for a simple clarification - the
object of the term "feed." Simply put, do you "feed" it on the 'c816 side, in which case that would mean simply running no-op or the WDM instruction, until the co-processor sends back some signal, like an interrupt (stalling the processor from the inside)? Or, is there some external mechanism by which the co-processor sends the $EA or WDM
to the 'c816 to execute, stalling the 'c816
externally? Or, if either of those are the case, and what you need is to stall the processor, could one not just hold RDY and (maybe) BE
low until the co-processor is ready to return data?
Ed, your suggestion that these are frivolous questions is spurious. If one carefully reads beyond the superficial, one will see that these are actually very pointed questions that just need a little closer read. But, maybe I assume too much, and I should provide further qualification. And, I
do appreciate the help. If I need to be more clear, just say that.
Jon
Edit: your previous post would make Socrates blush
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