Re: Bit Operations

[Simon Coulter <shc@xxxxxxxxxxxxxxxxx>]

On 31/05/2005, at 10:12 PM, Fisher, Don wrote:

> I'm actually just trying to determine the status of a specific bit.  
> As I
> understand the a.p.i. documentation, this one character field contains 
> bit
> data that may look like '01110000' or '01001000' or something else 
> depending
> on what the a.p.i. detects.  The third bit (bit number 2), may be on 
> or off
> and I need to determine which.

Then you simply define a mask which has the third bit set on and use 
%BITAND to perform a logical AND operation between the original value 
and the mask and then compare the result of the AND operation to the 
mask.

In IBM systems (not PCs) bits are numbered from left to right starting 
with bit 0 (being the most significant bit) so an 8-bit value with only 
bit 2 set on is 00100000 which is x'20' in hexadecimal.

Thus:
                IF              ( %BITAND(value : x'20') = x'20' )

although using named constants for the mask values would be better.

If you need this explained in detail then you should convert the input 
value to binary and perform boolean arithmetic with the mask. Thus:

Original value:		01110000		x'70'	determined by adding the value of the 
set bit positions
AND Mask:			00100000		x'20'
					--------
Result:				00100000		x'20'	See how the result of an AND is the same as 
the mask if the
											mask bits are on in the original value? A bit is set on in
											the result if that bit is on in both input values (i.e.,
											the bit is ON in the original value AND in the mask).

Here's the same operation with a different input value.
Original value:		01011111		x'5f'
AND Mask:			00100000		x'20'
					--------
Result:				00000000		x'00'	See how the result of an AND is 0 when the 
mask bits are
											not set in the original value?

The same principle holds for multi-bit masks such as x'55'.

Regards,
Simon Coulter.
--------------------------------------------------------------------
   FlyByNight Software         AS/400 Technical Specialists

   http://www.flybynight.com.au/
   Phone: +61 3 9419 0175   Mobile: +61 0411 091 400        /"\
   Fax:   +61 3 9419 0175                                   \ /
                                                             X
                 ASCII Ribbon campaign against HTML E-Mail  / \
--------------------------------------------------------------------