From mboxrd@z Thu Jan 1 00:00:00 1970 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me X-Spam-Level: * X-Spam-Status: No, score=1.3 required=5.0 tests=BAYES_00,INVALID_MSGID, MSGID_RANDY autolearn=no autolearn_force=no version=3.4.4 X-Google-Language: ENGLISH,ASCII-7-bit X-Google-Thread: 103376,7844279822ce7c28 X-Google-Attributes: gid103376,public From: jdla@my-deja.com Subject: Re: Newbie question : types , representation Date: 1999/08/24 Message-ID: <7ptsgn$nvp$1@nnrp1.deja.com>#1/1 X-Deja-AN: 516515959 References: <37BFC251.601ADF8F@village.uunet.be> <7ppdon$j3p$1@nnrp1.deja.com> X-Http-Proxy: 1.0 proxyfun.eunet.be:80 (Squid/2.2.STABLE3), 1.0 x35.deja.com:80 (Squid/1.1.22) for client 195.207.101.123, 193.74.208.160 Organization: Deja.com - Share what you know. Learn what you don't. X-Article-Creation-Date: Tue Aug 24 10:35:03 1999 GMT X-MyDeja-Info: XMYDJUIDjdla Newsgroups: comp.lang.ada X-Http-User-Agent: Mozilla/4.61 [en] (Win95; I) Date: 1999-08-24T00:00:00+00:00 List-Id: In article <7ppdon$j3p$1@nnrp1.deja.com>, Robert Dewar wrote: > In article <37BFC251.601ADF8F@village.uunet.be>, > Jos De Laender wrote: > > In C I could do it within 5 minutes. > > Probably by making unwarranted assumptions :-) For sure. But I don't think I would have much to assume except for the char being 8 bits. The remainder of operations I could do on that level. Of course I would also have to include some #ifdef 's ;-) > > > It would involve some bitmanipulation (shifting , anding , > > oring ) on > > the character itself and then outputting. That's it. > > > > However, what's unclear is how to do this in ADA types : A > > character > > cannot be bit manipulated , and so needs some explicit > > translation to a > > 'byte' or something. > > How is this usually done ? I would guess to use a modular type > 2**8 ? If > > I declare such a type , and I want a byte , is this guaranteed > by 2**8 > > or do I have explicitly to attribute it with a 'for Byte'size > use 8' ? > > Is char guaranteed to be 8 bits in C, answer no ... No, I know. The point is however that when for a compiler/architecture it is 8 bits, it would be 8 bits for all instances of a char , isn't it ? Does this same property hold for modular types like 2**8 , i.e. is it allowed that one instance takes 8 bits and another 16 bits , due to other optimization ? How does the use 'size influence this ? > > > Also I will need some kind of assignment to that modular type, > say aVar > > := 2#00000011#. This will assign the value 3 to aVar ? Is this > > guaranteed to have that bit pattern ? > > nope, and of course there is no such guarantee in C either Again I know. But in C I would start from characters with known representation (i.e. ascii) and do bitlevel operations. I would not need the assignment on that level. > > > Or could I imagine a (probably > > exotic) machine on which 3 is represented with another bit > pattern ? > > If you want to imagine such machines, your imagination is > equally relevant in C or Ada. For me this isn't _that_ imaginery. I constructed already (small) processing units in which the representation was quite different for power reasons. Luckily I didn't have to construct compilers for it ... > > > Does ADA guarantee ? > > If I output aVar with a write function , is the bitpattern > guaranteed ? > > nope, and neither does C > > > Thanks for all clarifications. > > Bottom line here: Don't get carried away by the formalism here. > Write reasonable code with reasonable assumptions, as you would > in C. Your low level bit twiddling stuff may be a target > dependent, at least from a theoretical point of view, but that's > not a terrible crime (and certainly no worse a crime in Ada > than in C :-) Thanks for the answer. ADA has as least the advantage that it makes you _think_ about those issues. But I didn't see however too much of practical answers how to do, in a ADA way and without spoiling to much processor cycles in converting hence and back. But I'll find out for sure ... > > Sent via Deja.com http://www.deja.com/ > Share what you know. Learn what you don't. > Sent via Deja.com http://www.deja.com/ Share what you know. Learn what you don't.