"John W. Kennedy" <jwkenne@[EMAIL PROTECTED]
> wrote in message
news:E1Uxf.1208$l03.452@[EMAIL PROTECTED]
> 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.
Yes and no. Double precision gave 28 extra bits.
But for most work, little difference between 36 bits and 32 bits.
But that's no measure, anyhow. The appropriate mesaure is
the number of mantissa bits and range of exponent.
And as for a "strategic error", the S/360 was the only architecture
that was copied around the world [apart from the PC],
and is the only architecture that survives from the 1960s and earlier
[albeit updated].
> 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.
I never had any difficulty with that, and I suspect
that nobody else did either.
How would you have done it better?
With binary, you would have, say, 21 bit mantissa plus sign
and 9-bit exponent plus sign (or biased 10 bits).
The reason for chosing the 8-bit exponent field was influenced by
byte-orientation, which, among other things, permitted instructions
like IC and STC to manipulate the exponent.
Then there was the question of performance during pre- and
post-normalising ****fts of 4 bits at a time (maximum of 6 ****fts
for single precision) for hex is a lot quicker than 1 bit at a time
for binary (maximum 24 ****fts) [single precision, and corresponding
values for double precision].
The choice gave a range of 10**-78 thru 10**75 IIRC,
while some competitors had a less-accommodating range of
10**-35 to 10**35.
And if you chose 24 bit mantissa, that would give you 7 biased
exponent bits, or 6 real bits. Which doesn't give you an
exciting range of exponents, to put it mildly.
> 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.
Are you sure of that? The 1964 Principles of Operations
states that a zero word is returned for underflow,
which it always did.
> The early competitors generally had words longer than 32 bits, but I am
> not familiar with any of them in detail.
Competitive equipment had 32 bits, 48 bits, 36 bits, 60 bits
but in the main, more than 32 bits was scarcely the rule.
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