[ntp:questions] 500ppm - is it too small?

David Woolley david at ex.djwhome.demon.invalid
Thu Nov 12 23:09:14 UTC 2009


Joseph Gwinn wrote:

> No, 8 bits isn't arbitrary.  
> 
> Computer hardware is simplified if the various word lengths are all 
> powers of two.

Not significantly.  Early machines commonly did not use 8 bit multiples, 
and they would have been much more sensitive to efficient use of 
hardware.  About the only place where it might be of advantage in modern 
systems is if the machine instructions allow addressing individual bits, 
because it wouldn't waste bit offset codes.

I suspect a major factor in IBM using 8 bits was binary coded decimal 
arithmetic.  Some very early machines worked in BCD rather than binary, 
as they were intended for doing commercial arithmetic, and this results 
in a 4 bit unit.  8 bits is the smallest multiple of this that handles 
characters well.

The Manchester University Atlas architecture used 6 bit sub-units of its 
words for characters (with shift codes).  The Digital PDP7 used 18 bit 
words - that was definitely a discrete transistor design, so efficient 
use of hardware would be particularly important.

Making memory sizes powers of two does have real advantages.




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