"James J. Weinkam" <jjw@[EMAIL PROTECTED]
> wrote in message
news:SY_pf.26191$Hl4.15544@[EMAIL PROTECTED]
> glen herrmannsfeldt wrote:
> > robin wrote:
> >
> >> "John W. Kennedy" <jwkenne@[EMAIL PROTECTED]
> wrote in message
> >> news:vYypf.39224$L7.37622@[EMAIL PROTECTED]
> >
> >>> The guard digit was added to double precision, postnormalization was
> >>> added to the HER and HDR instructions, and the behavior of overflow
and
> >>> underflow was altered.
> >
> >> The HE, HER, HD, HDR set was a glaring design error - a faux pas.
> >> That it failed to post-normalise meant that it couldn't be used
> >> in a loop to divide by, say, 32.
> >
> > Well, first there are no HE or HD instructions.
> >
> > I am pretty sure that HER and HDR will, and always have, done a
> > one digit ****ft when needed. It might be that they won't normalize
> > a previously unnormalized number, but in that rare case using AER
> > or some other that will normalize should be fine.
> >
> > (snip)
> >
> > -- glen
> >
> Unfortunately, I no longer have any S/360 manuals and there don't seem
to be
any
> free downloadable versions. However, according to GA22-7000-8, IBM
System/370
> Principles of Operation (1981), HER and HDR do the following:
>
> The second operand is divided by 2 and the normalized quotient is placed
in
the
> first operand location.
>
> The manual goes to describe the exact operation of the instruction in
detail,
> covering every conceivable eventuality. Under "Programming Notes" it
states:
>
> 3. The result of HALVE is zero only when the second operand fraction is
zero,
or
> when exonent underflow occurs with the exponent underflow mask set to
zero. A
> fraction with zeros in every bit position, except for a one in the
rightmost
bit
> position , does not become a zero after the right ****ft. This is
brecause the
> one bit is preserved in the guard digit and, when the result is not made
a
true
> zero because of underflow, becomes the leftmost bitafter normalization
of the
> result.
>
> So much for not fully normalizing a previously unnormalized number.
>
> As I recall the S/360, all floating pointing operations procuced
normalized
> results except for the various load instructions and the unnormalized
> instructions.
Except, of course, initially for HER and HDR. The 1964 Principles of
Operation
makes this clear.
Indeed, the RCA Spectra (and the EE System 4 which was a licenced
copy)
did not normalize in the case of HER and HDR. That never changed for the
EE Systrem 4 (I don't know what RCA subsequently did for the Spectra.)
Nor did the System 4 retrofit a guard digit on d.p.
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