Logical shift negative numbers

Question

So, I think i have found a bug in Labview.  I am using Labview 8.5
I have a very very simple Vi Where i want to Right shift a negative number.  For example:
-16 >> 2 = -4
What i get is 16380.  If I mismatch the data types.  Meaning I right shift from a 16 integar and place the result into a 8 bit integar I get -4.  
It would appear that the function is padding with leading zeros and not ones when handling negative numbers.   
Is this an issue that has been fixed already?  Or am i using the function incorrectly?
Attachments:
Logical Shift bug.vi ‏7 KB

Answer 1

Gruntboy,
     What I think you're looking for is an arithmetic shift function, while the function in Labview is a logical shift.  The difference is in the leading bit of the result when they shift a number to the right.  A logical shift right pads the numbers with '0's and an arithmetic shift right pads the numbers with their copies of their most significant bit.  This is very important when dealing with signed numbers, because the most significant bit (in standard two's complement notation) denotes whether or not a number is negative. 
     The number four (4) represented in six bits would be: 000100, and in two's complement negative four (-4) would be 111100.
A logical shift right, which pads the numbers with a '0' on the left would be:
000100 --> 000010  (4 --> 2)
111100 --> 011110  (-4 --> 30)
An arithmetic shift right, which pads the numbers with whatever their most significant bit is would be: 
000100 --> 000010  (4 --> 2)
111100 --> 111110  (-4 --> -2) 
I don't see  an arithmetic shift function in Labview, but you could build one by converting the number to an array of booleans and shifting to your heart's content.
When operating on signed numbers Arithmetic shift operations tend to be more useful, but I'm sure there would be quite a bit of confusion if the unsigned number 60 ("111110") gets right shifted and becomes 127! ("111111").
Finally, there is "one" (heh I'm so punny) special case for arithmetic shifting.  Consider how right shifting (logical and arithmetic) 8 results in 4, then 2, then 1, then 0.  What about arithmetic right shifting of -8?  The results would be -4, then -2, then -1...... then -1, then -1, then -1, etc. 
Worthwhile resources:
Two's compliment notation for negative binary numbers: http://en.wikipedia.org/wiki/Two's_complement
Logical shift explanation (w/ pictures!): http://en.wikipedia.org/wiki/Logical_shift
Arithmetic shift explanation (w/ pictures!): http://en.wikipedia.org/wiki/Arithmetic_shift
I hope this helps!
Hugs,
memoryleak 
P.S. I noticed that you mentioned you were modeling number operations of an ASIC.  This book (ISBN: 978-0534378042) was used in the Intro to Digital Logic class I took way too long ago, and I remember it hitting very hard on binary, signs and bit operations.  Looks like a used copy can be had on Amazon for less than $40 
Message Edited by memoryleak on 10-09-2009 11:00 AM