[Oberon] Assumed issue in the Lola-2 compiler

Joerg joerg.straube at iaeth.ch
Mon Apr 15 16:53:30 CEST 2019



In my point of view the Oberon spec is rather clear:

- q = x DIV y, r = x MOD y, x = q*y + r, 0 <= r < y

- MOD is always positive ( 0 <= r < y)

- It follows this pattern


For x = -1 and y = 8 the only integers fulfilling the defining equation set
is q = -1, r = 7.

DIV is rounding down to nearest integer.





-----Original Message-----
From: Oberon <oberon-bounces at lists.inf.ethz.ch> On Behalf Of
rochus.keller at bluewin.ch
Sent: Monday, April 15, 2019 4:13 PM
To: oberon at lists.inf.ethz.ch
Subject: Re: [Oberon] Assumed issue in the Lola-2 compiler


@ Paul Reed:


>> -1/16 is -0.0625 not 0.  (I agree there are indeed multiple integer 

>> solutions if you don't also add the range restriction on r.)


>From the language report we can conclude that the operators and result of
DIV and MOD (i.e. x, y, q and r) are (algebraic) integers.

We can also assume that in the equation 'x = q*y + r' the multiplication and
addition are proper mathematical operators.

Since the given equation uses mathematical operators we can expect that
rounding also follows proper mathematical rules. Therefore the rational
number -1/16 is rounded to the algebraic integer 0, which would be the
mathematically proper solution of  q = x DIV y = ( x - r ) / y with x = -1
and y = 16.


But as mentioned: I have a viable solution which re-uses features of C to
simulate the Oberon implementation.





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