Re: Packed vs Integer (Was RE: V4R5?)

Bob wrote:
>I always thought the "native" format of the AS/400 was packed.  And
>operations against packed numbers where the fastest.  Is this not true?
>
>Or is it more like: Packed is faster than zoned, but binary/integer is
>faster than packed?
>
>When did this change?

Others  have already provided good answers.  I'll just
summarize and add a few points.

The AS/400, like the S/38, and practically every other
machine out there performs arithmetic fastest in two's
complement integer format.  RPG II and RPG III offered
only decimal  numeric data types:  Packed, zoned,  and
binary decimal.   For the  arithmetic to work the same
for all three of these types, the  arithmetic was done
in packed numeric format. As a result, arithmetic with
packed operands was fastest since it didn't involve an
additional conversion - the data was already packed.

In RPG IV, the situation is a bit more complicated. We
have basically four different types of numerics:

1) floating point
2) decimal
3) integer
4) unsigned

For the decimal type, there are three formats:  packed,
zoned, and  binary  decimal.   Altogether six different
formats,  but conceptually you can put  them into these
four groups.

The way arithmetic works  in RPG IV is that if you have
operands of different types, the operation is performed
in the format of the "bigger" of the two types.  If you
add together unsigned and  decimal fields, for example,
the compiler converts both to decimal and the result of
the operation is decimal.

So basically the rule of thumb for arithmetic in RPG IV
is different than in RPG III.  If possible, define your
numeric fields as  integer or unsigned for performance.
Use decimal fields only when you're dealing with values
with decimal places, such as monetary amounts. Yes, you
can store the most precision using  decimal, but now if
you're on V4R4, you can use 8-byte integer and unsigned
fields, which give you about 19 digits.   And since the
machine is 64-bit, you'll  find that the 8-byte integer
and unsigned types perform practically the same as  the
4-byte I/U types.

Cheers!  Hans

Hans Boldt, ILE RPG Development, IBM Toronto Lab, boldt@ca.ibm.com


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