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From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.lang.c++
Subject: Re: What does the "L" mean
Date: Fri, 06 Dec 2019 22:36:20 -0800
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Ben Bacarisse <ben.usenet@bsb.me.uk> writes:

> Vir Campestris <vir.campestris@invalid.invalid> writes:
> <cut>
>
>>>>> Vir Campestris<vir.campestris@invalid.invalid>  writes:
>>>>> <cut>
>>>>>
>>>>>> The one that really strained C was the one where incrementing a
>>>>>> pointer by 1 went to the next bit.  Not byte or word.  (TI GPU)
>
> <cut>
>
>> BTW the Dec10 - the 36 bit one I used - an int was 36 bits, but a
>> character string was stored as set of 5 7-bit ASCII characters plus
>> one pad bit in each word... Never used C on it though.
>
> It would be very expensive to implement C using that packed format for
> character strings.  Some C implementations on other word addressed
> machines decided to make char one word;  on others arithmetic was needed
> to convert between word and "char" pointers.  It's notable that,
> programmer assumptions aside, word addressed machines are probably more
> awkward for C the bit-addressed ones.

The PDP-10 had a special representation for pointing to, and
accessing, "bytes" within a word, and instructions for doing loads,
stores, and pointer arithmetic in terms of "bytes" rather than
words.  A nice thing about the PDP-10 byte pointers is the width in
bits of a "byte" is part of the stored representation in each byte
pointer.  A C implementation on a PDP-10 could choose 9-bit bytes
for 'char' without too much trouble[*].  Alternatively, for a bit of
fun, bytes could be 12 bits, with two bytes for int's, and three
bytes for long (no longer works for C11, naturally), and two 36-bit
words for long long.  But 9-bit bytes would work just fine, even for
C11.  (Sorry folks - no uint32_t in those implementations.)

[*] IIRC, "bytes" in the PDP-10 were treated as unsigned, so
signed types would need sign extension on loading.