Programming Forums - least significant 1 bit

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- - least significant 1 bit (http://www.programmingforums.org/showthread.php?t=9096)

eax	Mar 28th, 2006 11:44 PM

least significant 1 bit

please give me any clue for:

How i can write a code in assembly language (80*86 ) to find number of the least significant 1 bit set and total number of 1 bits set.

andro

Mar 29th, 2006 2:45 AM

Here's some psuedo code...

lsbit = 0

pattern = 0b00000001

count = 0



for (loopcount = 1; loopcount <= 8; loopcount++)

        num = num && pattern

        num = num - pattern



        if ZeroFlag is set

                count = count + 1

                if lsbit == 0

                        lsbit = loopcount



        pattern = pattern << 1

Narue

Mar 30th, 2006 2:46 PM

>find number of the least significant 1 bit set

; ebx has the value

mov ecx,0 ; Only needed if ebx won't be zero

bsf ecx,ebx

; ecx has the bit position of the LSB

>total number of 1 bits set
There's not a convenient instruction for that, just shift and count until the value is zero for the simplest solution.

eax	Mar 31st, 2006 2:08 AM

I wrote this code(pasted below) and collecting total number of 1 bits in 00fffff0h
but when i run my program it goes in infinite loop.

I am not able to figure out why its going in infinite loop.
help me with this.

mov ecx,32
mov eax,00fffff0h

check: shl eax,1
jc setcount
loop check

setcount: inc count
loop check

DaWei

Mar 31st, 2006 8:16 AM

You have no statement to take you out of the loop.

eax	Mar 31st, 2006 1:56 PM

i used "loop" inctruction which automatically decrement value in ecx
and when ecx =0 it fall out of the loop

And since i want to process all 32 bits integer (which is input for program) , i am using loop instruction for both labels : check and setcount.

Does using loop instruction for both labels can cause some problem ?

DaWei

Mar 31st, 2006 2:04 PM

Suppose ecx goes to zero at the upper loop statement. Then you fall to "inc count", do that, then execute the last loop statement, which decrements ecx to a non-zero value, so off you go, around the mulberry bush again.

eax	Mar 31st, 2006 2:31 PM

yes, i think i got your point.

But when i change my code (pasted below) it still goes into infinite loop

mov ebx,31
mov eax,00fffff0h

dojob: cmp ebx,0
jnz doshift

doshift: shl eax,1
dec ebx
jc setcount
jmp dojob

setcount: inc count
jmp dojob

DaWei

Mar 31st, 2006 3:13 PM

Been a long time since I wrote this stuff, check the syntax.

mov ecx, 32

mov eax, theValueToCheck



check:

shl eax, 1

jnc dontCountTheThang

inc count



dontCountTheThang:

loop check

eax	Mar 31st, 2006 3:23 PM

yeh !! it is working perfectly fine. Thanks a lot for the code.

BUT , in the mean time i wrote a test code to see if instruction "jc" is working :

mov eax,00fffff0h

dojob: shl eax,1
jc print_value_after_shift
jnc exit

print_value_after_shift: dtoa no,eax
output lable

and when i run the program it was jumping to label "print_value_after_shift"
even when cf <> 1 , i.e i got the output as 33554400 .

This is really surprising for me now,

DOES IT MEAN instruction "jc" IS NOT WORKING ??

Quote:

Originally Posted by DaWei

Been a long time since I wrote this stuff, check the syntax.

mov ecx, 32

mov eax, theValueToCheck



check:

shl eax, 1

jnc dontCountTheThang

inc count



dontCountTheThang:

loop check

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