[Oberon] Use of DIV and MOD for bit manipulation

[Jörg]

Srinivas

Good question. The intention of Oberon was to be on one side a high-level
language but nevertheless be able to write low-level code wherever
interaction with HW is needed (device drivers, compilers...)

In the past, Wirth found that it is not necessary to foresee special
language constructs for this kind of low level programming. If bitwise AND
and OR are needed, use the type SET instead of INTEGER. He thought that
almost all needed bit manipulations can be done by applying standard
language constructs like DIV, MOD and *. 
In his former languages Modula-2 and Oberon the only special language
construct for bit manipulation was the standard procedure ASH.

Over the time it seems he changed his way of thinking. In Oberon-07 these
special language constructs were introduced: LSL, ASR and ROR. And if
urgently needed, you can even make use of SYSTEM.BIT, SYSTEM.LSH and
SYSTEM.ROT.

So, basically using DIV, MOD and * (with powers of two) is for Oberoners the
"normal" way to shift bits.

In ORG.Mod you will find in procedures MulOp, DivOp the special handlings
for power of two.
  DivOp 
    div --> ASR
    mod --> AND
  MulOp --> LSL

Jörg

-----Original Message-----
From: Oberon [mailto:oberon-bounces at lists.inf.ethz.ch] On Behalf Of Srinivas
Nayak
Sent: Samstag, 9. Juli 2016 15:20
To: ETH Oberon and related systems <oberon at lists.inf.ethz.ch>
Subject: [Oberon] Use of DIV and MOD for bit manipulation

Dear All,

While reading compiler code(OBG.Mod.txt),
I see that in many places we have used DIV, MOD, multiplication etc.
No doubt, one can argue that, we want to do a mathematical operation there,
but to be honest, the interest there is not to do a mathematical operation;
rather to do some bit manipulation.
And, since we know that DIV and MOD can help us there, we employ them.

How shall we justify it (to obscure the natural meaning)?
Isn't the code clarity being compromised there?

I would like to know, how Oberoners think about it...


With thanks and best regards,

Yours sincerely,
Srinivas Nayak

Home: http://www.mathmeth.com/sn/
Blog: http://srinivas-nayak.blogspot.in/
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