From: anton@mips.complang.tuwien.ac.at
MitchAlsup writes:
>So, would it not be easier and faster to simply make a densely-packed
>128-bit Fixed-Point decimal function unit ?!?
128-bit binary integers are mostly good enough, and support for that
is ok in current architectures; division support might be better,
though, but see below. Rescaling with a power of 10 is something that
may merit additional hardware support if it occurs often enough; but I
am not convinced that it occurs often enough:
You usually don't need it for addition and subtraction, because the
operands have the same scale factor, and the same scale factor as the
result.
For multiplication, one common operation is to multiply a price with a
number of pieces resulting in a price, and no rescaling is necessary
there. Another common operation is to compute a percentage; you do
have rescaling there.
For division, it seems to me that the most common case is division by
a percentage that is applied to many dividends (maybe not in the USA,
but certainly in Europe it is common to compute the price without VAT
(sales tax) from the price with VAT; but there are only few VAT rates
in each country); that can be turned into a two-stage operation that
might include any necessary rescaling: compute an inverse that can
then be used for a cheap multiplication-and-rounding operation (e.g.,
where a power-of-2 scale factor is involved for computing something
below the least significant digit, in order to implement rounding).
And yes, support for several rounding modes is needed when an inexact
result is involved. Hardware may be helpful here.
I have not done much financial programming, so maybe somebody else can
complement my views.
- anton
--
'Anyone trying for "industrial quality" ISA should avoid undefined behavior.'
Mitch Alsup,
--- SoupGate-Win32 v1.05
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