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From: haynes@ucscc.UCSC.EDU (Jim Haynes)
Newsgroups: comp.arch
Subject: Re: String lengths
Message-ID: <6239@saturn.ucsc.edu>
Date: 7 Feb 89 04:48:12 GMT
References: <8876@alice.UUCP>
Sender: usenet@saturn.ucsc.edu
Reply-To: haynes@ucscc.UCSC.EDU (Jim Haynes)
Organization: California State Home for the Weird
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In article <8876@alice.UUCP> dmr@alice.UUCP writes:
>The question arose: why does C use a terminating character for
>strings instead of a count?
>
Since dmr has spoken I probably shouldn't even think of trying to
add anything, but ...

If we think of machine architecture there are historically several
different ways that have been used to represent strings.
1) Put the characters in adjacent storage locations, and use a special
character to delimit the end of the string.  This goes back to at least
IBM 702 or thereabouts.  It is basically what is used in C; IBM just
happened to use a character other than null.
2) Put the characters in adjacent storage locations, and reserve a bit
per storage location to delimit string boundaries.  This is found in
IBM 1401-1410-7010 family and 1620.
3) Put the characters in adjacent storage locations, don't delimit the
boundaries at all, and absorb the information about string length into
the instruction stream.  This is what's done in IBM 360 and a lot of
other machines.  In contrast to 1) and 2) it requires something like a
simulation of 4) below to deal with varying length strings and dynamic
allocation of storage.
4) Put the characters in adjacent storage locations and store information
about starting address and length in a data structure for the purpose,
a string descriptor.  This is used in Burroughs B6500 and later
machines of that family.  The string is virtually delimited, in that
you can't access it, or aren't supposed to, without going through
the descriptor and observing the boundary.

What's the difference between a string and an array of characters?
Is it anything other than the set of operations that are provided
to operate on it?
haynes@ucscc.ucsc.edu
haynes@ucscc.bitnet
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"Any clod can have the facts, but having opinions is an Art."
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