hancock4@[EMAIL PROTECTED]
wrote:
> John W. Kennedy wrote:
>> It is very well known that the entire 360 FP feature could have used
>> some input from numerical analysts; it's shot full of design defects.
>
> Could you elaborate on those design defects?
>
> How did S/360 compare with its predecessor machines (ie 709x) regarding
> those defects? What differences did competitors machines--those
> available in 1965--have compared to S/360 regarding these defects?
To start with, the S/360 word was four bits shorter than the 704 word.
This was, at least, a strategic error, because it meant that /up/grading
to a 360 meant, in this area, a /down/grading in function.
But the hexadecimal base further meant that the effective length of the
fraction was essentially 21 bits (single precision) or 53 bits (double
precision), rather than the superficial 24 or 56, and this was not
clearly understood at first.
Other problems were corrected in a massive Engineering Change, which
added a guard digit to double precision, added postnormalization to the
halve instructions HER and HDR, and changed the results returned in
cases of overflow and underflow.
The early competitors generally had words longer than 32 bits, but I am
not familiar with any of them in detail.
--
John W. Kennedy
"But now is a new thing which is very old--
that the rich make themselves richer and not poorer,
which is the true Gospel, for the poor's sake."
-- Charles Williams. "Judgement at Chelmsford"
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