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From: "Rod Pemberton" <buzzoff@nonamewhichexists.cmm>
Newsgroups: comp.lang.forth
Subject: Re: Clarity vs. Efficiency
Date: Fri, 17 Oct 2014 21:48:06 -0400
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On Wed, 15 Oct 2014 11:54:53 -0400, <forthisit@gmail.com> wrote:

> I recently wrote some code (for an embedded Forth processor) to convert
> a user's desired frequency into control register values to be written
> to Xilinx's PLL (which they call MMCM, for mixed mode clock manager).
> In this way the frequency output can be changed dynamically to any
> desired value (within certain resolution limits, of course) without
> changing the FPGA's hardware design. It was a fun project.
>
> Here are two of the words developed along the way:
>
> \ calculate "M"
> : fbdiv  ( -- n ) \ aka M (multiplier)
>     drp_freq $1FF and    \ strip off [12:9]
>     2 rshift
>     >606? IF 2/
>     ELSE >400? not IF 2/ THEN
>     THEN ;
>
> \ calculate value for CLKFBOUT's CLKREG1 register
> : fbreg1  ( -- n )
>     fbdiv 2/           ( n )
>     frac? IF 1 - THEN  ( n )  \ fractional divide adds undocumented '2'
>     dup                ( n n )\ (ie. 1 in frac bits gives M of 2.125,not  
> 0.125)
>     fbdiv odd? +       ( h l )\ if M is odd LOW_TIME = HIGH_TIME + 1
>     swap 6 lshift or   ( n )
> ;
>

Should you have posted these publicly?  (Just a thought. No reply needed.)

> [...]
>
> So fbdiv basically retrieves the desired frequency setting, returned by
> drp_freq, and calculates and returns "M". The value of "M" is required
> in the calculation of more than one control register value. From an
> efficiency perspective it would be best to only calculate this once,
> store it in a variable or on the stack, and retrieve it as needed.
>
> But from a clarity perspective I found that I liked the expressiveness
> of just calling the fbdiv word whenever I needed the value of "M" in
> the calculations (as in the fbreg1 word above). It saves variable space,
> reduces the so called "stack juggling" that Forth is often criticized
> for, and it really helped when checking my words interactively using
> Gforth as I coded.
>
> It got me to thinking that if more code was written like this, especially
> when making the case for the acceptance of Forth, the code might be more
> easily understood and better appreciated; even though it sacrifices some
> efficiency. I felt that this embedded Forth processor running Forth code
> is so fast already that I'm willing to sacrifice a little efficiency for
> clarity.
>
> Agree or disagree?
>

By "like this," do you mean both 1) not using the stack, and
2) not using variables?  That kind of neuters Forth doesn't it?

I can see avoiding "stack juggling," but just how much variable
space do you save by _not_ retrieving a variable?  2 bytes? 4 bytes?

Of course, fbdiv could just retrieve that variable, not compute it.
I.e., shouldn't your code should probably be written that way already?


Rod Pemberton