Re: S/360

Dan Nagle wrote:

(snip)

> No Cray-1 ever had hardware divide.
> Floating point divide was always a reciprocal approximation,
> followed by correcting multiplications.  A special 2-a.b instruction
> applied part of the correction.

Well, it was used to implement divide in high-level languages.

I thought I remembered that multiply was not accurate out to the
last bit, the reciprocal approximation obviously isn't.  Maybe the
corrected reciprocal is pretty close.

Back to the 360, the 360/91 uses a divide algorithm similar to Cray,
except that it does the multiply at the same time.  It generates a
rounded quotient, unlike the truncated quotient that S/360 specifies.
As a result, it violates the Principles of Operation by being closer to 
the right answer.

-- glen