[Oberon] RISC emulator and floating-point

[Peter De Wachter]

On 25-03-14 22:11, Paul Onyschuk wrote:
> I think I found issue.  Right now in Oberon running under emulator:
>
>   -1 DIV 10 = -1
>   -1 MOD 10 = 9
>
> Is that indented behavior?

Yes, see the Oberon07 report, section 8.2.2:

The operators DIV and MOD apply to integer operands only. Let q = x DIV
y, and r = x MOD y. Then quotient q and remainder r are defined by the
equation:
x = q * y + r
0 <= r < y

> Now version of procedure WriteInt() with
> comments:
>
>   PROCEDURE WriteInt* (VAR W: Writer; x, n: LONGINT);
>     VAR i: INTEGER; x0: LONGINT;
>       a: ARRAY 10 OF CHAR;
>   BEGIN i := 0;
> (* Taking this branch, since x = 80000000H = -2147483648 *)
>     IF x < 0 THEN
>       IF x = 0 THEN WriteString(W, " -2147483648")
> (* Negating x results in integer overflow and x0 wraps
>                                    around to -2147483648 *)
>       ELSE DEC(n); x0 := -x
>       END
>     ELSE x0 := x
>     END;
>     REPEAT
>       a[i] := CHR(x0 MOD 10 + 30H); x0 := x0 DIV 10; INC(i)
> (* Loop never terminates, since -1 DIV 10 = -1 and x0 is never 0 *)
>     UNTIL x0 = 0;
>     WHILE n > i DO Write(W, " "); DEC(n) END;
>     IF x < 0 THEN Write(W, "-") END;
>     REPEAT DEC(i); Write(W, a[i]) UNTIL i = 0
>   END WriteInt;

The intention was obviously for -2147483648 to be a special case, so the
procedure should be rearranged into something like this:

PROCEDURE WriteInt* (VAR W: Writer; x, n: LONGINT);
  VAR i: INTEGER; x0: LONGINT;
    a: ARRAY 10 OF HAR;
  BEGIN
    IF x = 80000000H THEN WriteString(W, " -2147483648")
    ELSE
      (* the rest of the code *)
    END
  END WriteInt;