[comp.arch] The curious floating-point arithmetic on the IBM RS6000

seznec@parasite.irisa.fr (Seznec Andre) (05/31/90)

There has already been a discussion on the floating-point MAF (multiply-add) on the IBM RS6000.
Since this discussion, I have  studied the implementation of the floating unit in the RS6000 (see paper "RISC System 6000 Floating-Point Unit" by B.Olsson and al).

When executing a MAF, the result flowing out from the the multiplier is not rounded when entering the adder (106 bits of mantissa), but the other operand of the addition (or substract) is a normal 64 bits floating-point data; authors of the paper t