I guess this is still on-topic for PIC....

Actually, with few exceptions even CISC processors will take a consistent
number of cycles to execute code. Unfortunately figuring out how many is
_extremely_ difficult with all the caches and such scattered about.

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

----- Original Message -----
From: Dan Michaels <oricom@LYNX.SNI.NET>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Monday, August 21, 2000 12:01 PM
Subject: Re: [PIC]: Sine table


> Scott wrote:
> .........
> >
> >I think the level of "determinism" needs to be defined. In many
applications, a
> >timer interrupt can be used to trigger events that can then be processed
by
> >high-level C-code. I had an application like this to control a bank of 64
> relays
> >once. It just didn't need to be deterministic to the microsecond, 10's of
> >milliseconds was good enough. That was written in C. If you need 10's of
> >microseconds of resolution, then you'd be hard pressed to get determinism
> out of
> >C-compiler. You'd have to either resort to inline assembly and/or careful
tune
> >the way C-code is written.
> >
>
> Yeah, I think this is the answer. PIC and Scenix have been marketed
> as having deterministic timing, in the sense that you could know
> "exactly" how many cycles both your code and your interrupts will
> take. This is converse to the std CISC processors, where the same
> instruction can take vastly different #cycles, depending upon the
> arguments and addressing modes.
>
> With C, I think, you can only a priori "code" deterministic timing
> if you know **exactly** how many asm instructions a C instruction
> will compile into - which is probably much more trouble than is
> worth thinking about.
>
> Even if you can read the asm produced by the C, all you know is
> what the compiler gave you, and you are not necessarily in
> control of the output - plus you have to read the asm to tell
> what you have anyways.
>
> Trying to fine-tune the C to get the "desired" asm output sounds
> like a losing proposition, for several reasons. And obviously,
> using inline asm to get determinism is not coding in C.
>
> C = determinism = I doubt it [polite form of NO!]
>
> best regards,
> - danM
>
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