Bitwise operations with Oxygenbasic

[Arnold]

Hi Charles,

I had a great break last month. There are too many changes of weather which kill me down a little bit. My editor project is delayed at the moment (but not stalled).

In the meantime I do some exercises which I find at Rosetta Code. There is a task which deals with bitwise operations and this seems to work with Oxygenbasic like in other 32 bit languages. However I am not quite sure when it comes to arithmetic / logical right shift using negative integers (positive numbers give the same result). Is there a better way in Oxygenbasic to get the arithmetic right shift value for negative numbers?

Roland

Code: [Select]

'Bitwise operations (32 bit)

include "$/inc/console.inc"
print "Bitwise operations (32 bit)" & cr


sub bitwise(int a, b)   
   printl "a       = " + a
   printl "b       = " + b
   printl
   printl "a and b = " + a and b 
   printl "a or b  = " + a or b 
   printl "a xor b = " + a xor b 
   printl "not a   = " + not a 
   printl "a << b  = " + a << b     'left shift 
   printl "a / (2^b) (arithmetic) = " + floor(a / (2^b)) 'right shift
   printl "a >> b    (logical)    = " + a >> b           'right shift
   printl "a <<< b = " + a <<< b    'rotate left
   printl "a >>> b = " + a >>> b    'rotate right   
   printl
end sub


int x, y

sub prompt()
   printl "Enter an integer ? " : x = input()
   print "Enter another integer ? " : y = input()
end sub


while 1
   prompt()
   bitwise(x, y)
   printl "Another test? (y/n) "
   ans = ltrim rtrim input()
   if lcase(ans) != "y" then exit while
wend



.

[Charles Pegge]

Hi Roland,

In assembler sar is the arithmetic counterpart of shr

thus:

Code: [Select]

int a=-16
sar a,2 'SAR shift arithmetic right
print a   '-4

You can also do it in Basic by inverting the bits, but you must first test whether the number is negative:

Code: [Select]

int a=-16
print not(not(a)>>2)   '-4


[Arnold]

Hi Charles,

would this little function be ok? I get the expected results but I am not sure if I will need a pushad / popad.

Code: [Select]

function arshift(int d, c) 
   mov   eax, d     'dest
   mov   ecx, c     'count
   sar   eax, ecx   'SAR shift arithmetic right 
   return eax
end function

Using the second (not(not..) approach it seems to me that I only get the logical value for right shift e.g. 536870910 instead of -2 for a=-15, b=3. Am I doing something wrong?

Roland

[Charles Pegge]

Yes but it can be a little simpler:

the final value of eax is returned by default
cl is the lowest 8 bits of ecx

Code: [Select]

function arshift( int d, c)
   mov   eax, d     'dest
   mov   cl, c     'count
   sar   eax, cl   'SAR shift arithmetic right
end function

int b=arshift( -16,2)
print b

The not(..) technique is rather contorted and only works on negative integers. You will also need to ensure that it assigns to an int, not a dword, to display the negative value.

obfuscators delight

Code: [Select]

int a=-16
int b
b= ( a>0 ? a>>2 : not(not(a)>>2) )
print b


[Arnold]

Thank you Charles for your help. Everything works fine now. And I think it is a very nice thing that it is possible now to use the ternary operator ? : with Oxygenbasic too.