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| "James J. Weinkam" <jjw@cs.sfu.ca> wrote in message
news:SY_pf.26191$Hl4.15544@clgrps13...
> glen herrmannsfeldt wrote:
> 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 shift. 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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