From: lasner@watsun.cc.columbia.edu (Charles Lasner)
Newsgroups: alt.folklore.computers
Subject: Re: What was the intended advantage of using 36bits CPU?
Date: 13 Oct 92 22:56:20 GMT
Reply-To: lasner@watsun.cc.columbia.edu (Charles Lasner)
Organization: Columbia University

In article <14930@auspex-gw.auspex.com> guy@Auspex.COM (Guy Harris) writes:
>(Followups to "alt.folklore.computers".)
>
>>The PDP-6 was designed and built before the days of 8 bit character codes -
>
>But was it designed and built before the days of 7-bit character codes?
>The PDP-10's OS from DEC used ASCII as the character set for text (it
>may have used SIXBIT, or whatever it was called, for symbols in
>programming languages and object files, but it used ASCII for text); did
>the PDP-6's OS do so as well?

The instruction set of the 6/10/20 series allows any-width bytes, even a
single bit.  Any size of character set is as easy to handle as another.

The only hardware restriction is that if there aren't enough bits left
in the latest underlying 36-bit word, the rest of that word is not used
automatically, thus a theoretical waste for non submultiples of 36 bits
per character.  Filenames help un 6-bit fit into a single 36 bit word if
a 6 char name, etc.  7-bit ASCII fits 5 chars/36 bit word and an indicator
bit used to signify that the rest of the word isn't chars at all, but rather
is a binary line number.

Compare this so-called inefficiency with the vast majority of 8-bit byte
machines holding chars that are primarily 7-bit expressable ASCII chars.

2 PDP-10 words= 10 ASCII chars and two wasted bits in 72 bits.

9 8-bit bytes= 9 ASCII chars and 9 wasted bits in the same 72 bits.

Moreover, 8-bit bytes was created for EBCDIC, an admittedly highly inefficient
char code virtually none of us use anyway.  It has many problems such as
gaps in the alpha set, and printing characters too spread out in the set to
allow a 7-bit subset, yet there aren't even 128 printing codes.  Too many
unneeded special-purpose codes that could have been overlapped to mean 
different things to different hardware that cared about it.

Moreover still, ASCII includes extensions such as SI and SO, so you can
create added features, and the rise of ESC sequences which can still mostly
be expressed in a 7-bit version.  (Think DEC's CSI or ESC [ which are the 
same thing except the first has to have an 8-bit connection.

Early on, the 8-bit was just a nuisance, and there was that copout about
parity which usually is meaningless in most applications.  Most of the
transactions that really use these codes (even today) is expressable in
7-bit.  It's all too easy to claim the 8th bit is necessary with largely
a copout merely because it's there.

Today there is call for extended character sets.  It appears that 7 bits
may not be enough for a truly universal charset.  But 8 bits aren't enough
either, with some people calling for 9 or more bits/char.  PDP-10-style
architecture is up to handling that trivially, while all 8-bit systems are
going to have to waste the difference between the needed amount and the
rounded up value of 16 bits/char.  (Or is someone going to justify a
replacement character code set with 65536 unique combinations?)

cj "Baudot is 5-bit code" l