OT: Requesting C advice

[Mike McCarty]

Chris Schumann wrote:
> Late reply; sorry.
> 
> 
>>Date: Thu, 24 May 2007 08:43:26 -0700
>>From: Les <hlhowell at pacbell.net>
> 
> 
>>Embedded applications today
>>are mostly 8 bit, but many, many designers have already begun the
>>transition to 16 bit, and soon 
>>will be moving to 32 bit.  The reasons are much the same as 
>>the reasons
>>that general computing has moved from 8 to 16 to 32 and now 
>>to 64, with
>>the cutting edge already 
>>looking at 128 bit and parallel processing, along with dedicated
>>processors running 32 or 64 bit floating point math.  Also 
>>the length of
>>the integer used in C, which is a virtual 
>>machine is independent of the word length of the processor, 
>>except the C
>>language designers (originally Kernigan and Ritchie) made the language
>>somewhat flexible to simplify migration.  That is why there were some
>>undefined situations in the original specification.  Remember 
>>that C is
>>a virtual machine language, whose processor only has 24 
>>instructions (I
>>think the Ansi committee added a couple, but they have specific uses
>>that were not foreseen in the original usage of the language) 
>> It can be
>>ported to any machine currently extant by only writing about 1K of
>>machine code, and even that can be done in another available higher
>>level language if you so desire, as long as it is compiled for
>>efficiency.
> 
> 
> Having used C since the original K&R version, I have to ask WHAT?!?
> 
> Since when is C a virtual machine language?

I believe that the reference is to this language from the Standard:


5.1.2.3  Program execution

        [#1]   The   semantic  descriptions  in  this  International
        Standard describe the behavior of  an  abstract  machine  in
        which issues of optimization are irrelevant.

[snip]

> (and because it is a virtual machine language...)
> 
>>    That is why even the 8 bit implementations of C used a 16 bit
>>integer.
> 
> 
> No it's not. They used 16 bit integers because you can't do much

They used 16 bit integers because the original compiler was for
the PDP series of machines. The architecture of that machine
influenced several aspects of the language. As the compiler evolved,
some of the architectural aspects of that were removed, but
not all by any means.

> of anything useful with only 8 bit integers. The compiler designers
> for those systems (like the Apple II) had to work around the 8 bit
> registers. Looking at the assembly-language source for some of the
> libraries was not pleasant.

The argument goes the wrong way. The reason the PDP series used
16 bit integers is because not much can be done with 8 bit
integers. This is what influenced the compiler.

Mike
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