Re: HELP ! need to insert value in array !!

[miker3@ix.netcom.com (Mike Rubenstein)]

dewar@merv.cs.nyu.edu (Robert Dewar) wrote:

> iRichard O'Keefe said
> 
> <<Note that 5 ASCII characters per word was a software convention<<
> 
> I think that is wrong, there were hardware instructions on the PDP10 that
> used this format, I am pretty sure of this, but it is a while ago ...
> 

Not really, at least not in the sense that there were instructions
that handled 7 bit characters specially.  The PDP10 (like its
predecessor the PDP6) had a byte pointer construction that could be
used to access any byte in a word where the byte size was specified as
part of byte pointer.  There was an instruction that would get a byte
and increment the pointer, handling word boundaries properly (a byte
could not be split across words).  Byte pointers and the associated
instructions worked equally well with any byte size up to 36 bits,
though, of course, some word sizes resulted in more wasted bits than
others.

The reason for 7 bit characters was simple -- it was the smallest
standard character size that provied both upper and lower cast.  9 bit
characters would have wasted more memory in text applications of the
time and standard peripherals couldn't handle more than 7 bit ASCII
anyways.  While the maximum memory on early models was 256K 36 bit
words, early machines that large were rather rare on because of the
high cost of memory.

Michael M Rubenstein