Path: utzoo!utgpu!water!watmath!clyde!rutgers!rochester!PT.CS.CMU.EDU!FAS.RI.CMU.EDU!schmitz
From: schmitz@FAS.RI.CMU.EDU (Donald Schmitz)
Newsgroups: comp.arch
Subject: Re: base 10 float hardware
Message-ID: <647@PT.CS.CMU.EDU>
Date: 14 Jan 88 18:35:03 GMT
References: <8801141342.AA15537@decwrl.dec.com>
Sender: netnews@PT.CS.CMU.EDU
Organization: Carnegie-Mellon University, CS/RI
Lines: 22

>By definition, base 10 hardware must waste some circuitry, because you are
>consciously deciding not to store the binary values 11-15, which you could
>otherwise do. So you lose range, but gain precision for decimal calculations.

This assumes representing the mantissa and/or exponent in BCD, I don't think
this is what is meant by base 10 floating point.  Rather, the mantissa and
exponent would still be an unsigned binary int, however the fp number would
represent mantissa * 10^exponent.  From a hardware stand point, this would
complicate pre and post justification; rather than simply use a barrel
shifter, a multiplier/divider would be needed.

If the same number of bits are used for the exponent as a base 2
implementation, the range will increase.  Similarly, precision will decrease
somewhat given the same number of mantissa bits, (this may not be obvious,
but more unique fp values can not be represented in the same number of bits,
so the expanded range must be spanned by a more spread out set of values).
I'm not sure what the real win is, since (as mentioned) there are still
numbers which can't be represented exactly, the hardware implemented gate
count is almost sure to increase, and the format will be incompatible with
the rest of the world.

Donald.Schmitz	schmitz@fas.ri.cmu.edu