Programming Forums > Application Development > Assembly

eax · Mar 28th, 2006, 11:44 PM

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...

(Toggle Plain Text)

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

(Toggle Plain Text)

; 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.

(Toggle Plain Text)

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.

(Toggle Plain Text)

mov ecx, 32
mov eax, theValueToCheck

check:
shl eax, 1
jnc dontCountTheThang
inc count

dontCountTheThang:
loop check

Mar 28th, 2006, 11:44 PM	#1
eax Newbie Join Date: Mar 2006 Posts: 17 Rep Power: 0	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.

Mar 29th, 2006, 2:45 AM	#2
andro Professional Programmer Join Date: Oct 2005 Location: California Posts: 312 Rep Power: 3	Here's some psuedo code... (Toggle Plain Text) 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 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

Mar 30th, 2006, 2:46 PM	#3
Narue Professional Programmer Join Date: Sep 2005 Posts: 419 Rep Power: 4	>find number of the least significant 1 bit set (Toggle Plain Text) ; 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 ; 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. __________________ Even if the voices aren't real, they have some pretty good ideas.

Mar 31st, 2006, 8:16 AM	#5
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	You have no statement to take you out of the loop. __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

Mar 31st, 2006, 2:04 PM	#7
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	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. __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

Mar 31st, 2006, 2:08 AM	#4
eax Newbie Join Date: Mar 2006 Posts: 17 Rep Power: 0	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

Mar 31st, 2006, 1:56 PM	#6
eax Newbie Join Date: Mar 2006 Posts: 17 Rep Power: 0	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 ?

Mar 31st, 2006, 2:31 PM	#8
eax Newbie Join Date: Mar 2006 Posts: 17 Rep Power: 0	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

Mar 31st, 2006, 3:13 PM	#9
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	Been a long time since I wrote this stuff, check the syntax. (Toggle Plain Text) mov ecx, 32 mov eax, theValueToCheck check: shl eax, 1 jnc dontCountTheThang inc count dontCountTheThang: loop check mov ecx, 32 mov eax, theValueToCheck check: shl eax, 1 jnc dontCountTheThang inc count dontCountTheThang: loop check __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

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