Decimal versus binary arithmetic was Re: J4 - presentation/discussion on "Future

On Tue, 18 Mar 2008 19:34:51 -0600, Robert <no@e.mail> wrote:

>On Tue, 18 Mar 2008 09:09:44 -0600, Howard Brazee <howard@brazee.net> wrote
:
> 
>
>Lots of muddled thinking on numeric errors has been published. Some talk ab
out 'binary
>arithmetic' without making a distinction between integer and floating point
. Here's an
>example:

Try doing a simple divide like calculate the value of 1 / 5 in binary.
You get a never ending fraction.
>
>" Why did computers use binary in the first place?
>
>    Many early computers (such as the ENIAC, or the IBM 650) were in fact d
ecimal
>machines. In the 1950s, however, most computers turned to binary representa
tions of
>numbers as this made useful reductions in the complexity of arithmetic unit
s (for example,
>a binary adder requires about 15% less circuitry than a decimal adder). Thi
s reduction in
>turn led to greater reliability and lower costs.
>
>    Storing decimal integers in a simple binary coded decimal (BCD) form, r
ather than a
>pure binary form, also uses up to 20% more storage than the binary form, de
pending on the
>coding used.
>
>    Decimal arithmetic, therefore, is inherently less efficient than binary
 arithmetic,
>and at the time this justified the switch to binary floating-point arithmet
ic (just as a
>two-digit representation for the year in a date was justifiable at the time
). However, the
>programming and conversion overheads and other costs of using binary arithm
etic suggest
>that hardware decimal arithmetic is now the more economical option for most
 applications.
>
> Has any company formally announced hardware decimal floating-point support
?
>
>    Yes. IBM announced on 18 April 2007 hardware decimal floating point fac
ilities for IBM
>z9 EC and z9 BC:
>
>Since then, IBM has also announced support for decimal floating-point in th
e Power6
>processors, and has released details of the decimal floating-point unit in 
the z6
>microprocessor.
>http://www2.hursley.ibm.com/decimal/decifaq3.html#calcs
>
>Cowlishaw accidentally overlooks the fact that every Intel x86 processor/FP
U made since
>1980 has supported 17 digit hardware decimal floating point.  Now that IEEE
 754r is on the
>horizon, Intel is dropping it (I think they already did in Itanium).
>
>"based on Intel’s investigation on the frequency of
>use and performance requirements for decimal processing in existing applica
tions, the
>potential benefits of hardware-implemented decimal arithmetic do not justif
y its cost in
>terms of dedicating transistors and power consumption to it on a processor 
core.
>Consequently, Intel is proposing the use of software emulation instead.
>
>Just as the computing landscape changed dramatically from the ratification 
of the first
>floating point standard to its current revision, equally big changes are in
 store for the
>next 15 to 20 years. The multi-core processor revolution has begun and is p
oised to make
>even greater leaps in performance (and performance per watt) for personal c
omputers.
>Intel’s research and development plans suggest the possibility of a 256-cor
e processor by
>the 2015 time frame."
>http://www.intel.com/standards/floatingpoint.pdf