NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/lang/c-part1/4412

Internet Message Format | 1996-08-05 | 2.1 KB

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From: Lawrence Kirby <[e-mail]>
Newsgroups: comp.lang.c
Subject: Re: quick decision: is n a power of 2?
Date: Sun, 04 Feb 96 13:13:04 GMT
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References: <[e-mail]> <[e-mail]> <4e6rs0$[e-mail]> <[e-mail]> <[e-mail]> <4er59u$[e-mail]>
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In article <4er59u$[e-mail]>
           [e-mail] "G. Herrmannsfeldt" writes:

>Are there any ones-complement machines (I think CDC used to make them),
>with C compilers that generate the two's complement for -u on an unsigned int?

The bit manipulations required to perform unsigned arithmetic are different
for those required to perform 1's complement arithmetic. The compiler would
have to generate instructions to perform the correct bit manipulations for
-u and these would have to be a different sequence to those generated for -s
(on a signed int), unless the compiler simulates a 2's complement environment
for the C program.

The nice thing about 2's complement is that in most cases the bit
manipulations required are the same as for unsigned arithmetic. The
difference is how the result is interpreted (for example how condition
codes are tested).

>I suppose it makes a little sense, but are there really any?
>
>And, for that matter, what about BCD machines?  Maybe they are rare, too.

That is a different case because C doesn't support BCD as a representation
model. The compiler for such a system wuould therefore have to jump through
significant hoops to simulate a 'pure' binary environment for the C
program to run in.

--
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Lawrence Kirby | [e-mail]
Wilts, England | [e-mail]
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