Path: utzoo!attcan!uunet!ingr!garth!smryan
From: smryan@garth.UUCP (Xxxxxx Xxxx)
Newsgroups: comp.arch
Subject: Re: Quadruple-Precision Floating Point ?
Message-ID: <2273@garth.UUCP>
Date: 21 Dec 88 23:55:36 GMT
References: <8561@alice.UUCP> <3688@s.cc.purdue.edu> <285@loligo.fsu.edu> <6053@louie.udel.EDU>
Reply-To: smryan@garth.UUCP (Xxxxxx Xxxx)
Organization: Xxxxxxxxxx (XXX) -- Xxxx Xxxx, XX
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>If I remember correctly from my days at CDC working with the 205, the
>floating point hardware actually produced 128 bit results.  There were
>different versions of the floating point instructions that returned
>different (64-bit) parts of this 128 bit result.  The normal instructions

The 205 uses `significant zeros.' The machine is set up to treat, say,
.123-.122 as .001 rather .1xxe-2, where xx are insignificant bits from
normalisation.

So there add-upper which returns the 16-bit exponent+48-bit top half of
the accumulator after such an add, and an add-lower which returns the
bottom half. An add-normalise does normalising shifts and returns the
upper 48 bits of that.

So if you want to do signficant adds rather than normalised adds, it's
very easy. Unfortunately someone (customers?) decided that double
precision adds must be normalised, so that what would have been a very
simple routine ended up being very complicated, a dozen instructions and
still maybe not quite right.

>add and multiply multiply (maybe divide ?) there were instructions to

Divide-upper (used for integer divides) and divide-significant.

>I am quite sure that these instructions existed in both scalar and
>vector forms.

Yes.

>               These instructions would allow you to add 128 bit
>numbers with four additions, and this certainly vectorizes.

Double precision is not vectorised because:

- the compiler could only produce one temporary vector for each operator
  and normalised double precision would require many more.

- no library vector routines existed; these did exist for complex, hence
  complex does vectorise.

- insufficient apparent demand from the customers to change either of the
  above.
-- 
The maggot spawn of Ymir's flesh                                     -- s m ryan
whose lust of gold is trapping mesh,                            
have lost their hoard to gods and men,                          -- Andwari's Gem
now hid in river, mine, or fen.                                           -- 1/1