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Thread: One bit

  1. Feb 15th, 2003, 06:11 AM #1
    CornedBee
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    One bit

    How do I find out in a very fast way if exactly one bit of a byte is set?
    I currently use this (inline assembly):
    Code:
    	mov al, bit_depth ; bit_depth is what I want to test
shift:
	shr al
	jnc shift
	test al, al
	jz just_one_bit
    Anyone got a better idea?
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    CornedBee

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  2. Feb 15th, 2003, 06:39 AM #2
    CornedBee
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    Here's another piece I've come up with:
    Code:
    	movzx ax, bit_depth
	bsf bx, ax
	bsr cx, ax
	cmp bx, cx
	je just_one_bit
    With the advantage that it doesn't need any jumps.
    All the buzzt
    CornedBee

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  3. Feb 15th, 2003, 04:03 PM #3
    AirScape17
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    how about and'ing it?
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  4. Feb 15th, 2003, 04:32 PM #4
    CornedBee
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    Yeah, keda gave me that piece by jim:
    Code:
    bool just_one_bit = (to_test & (to_test-1)) == 0;
    All the buzzt
    CornedBee

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  6. Apr 25th, 2003, 07:55 PM #5
    AndySoft
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    I know that the Z80 has a BIT command, don't know if x86 does though...

    Code:
    BIT     [BitNum],[What]
    For the Z80... I think. Maybe parms are backwards.
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  7. Jun 18th, 2003, 06:19 PM #6
    Darkwraith
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    I don't think the 80x86 instruction set contains a BIT instruction.

    Also, what would the code segment look like in this problem?
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  8. Jul 6th, 2003, 09:55 PM #7
    please
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    Why don't you and your byte with 00010000 (the one in the location you want your bit tested) and chek if the result is zero?

    (just a thought)
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  9. Jul 6th, 2003, 10:44 PM #8
    Darkwraith
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    It depends if this is a check to see if this is a certain bit or any bit.
    "Can't" and "shouldn't" are two totally separate things.

    All questions should be answered. All answers should be true. That is why I post.
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  11. Jul 7th, 2003, 01:04 AM #9
    CornedBee
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    No, this is for any single bit, not matter where.
    So, acceptable for a byte would be
    00000001
    00000010
    00000100
    00001000
    00010000
    00100000
    01000000
    10000000
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    CornedBee

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  12. Jul 8th, 2003, 11:34 PM #10
    Darkwraith
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    Isn't bsf and bsr sorta expensive? I think the first alogrithm is faster.
    "Can't" and "shouldn't" are two totally separate things.

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  13. Jul 9th, 2003, 01:12 AM #11
    CornedBee
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    The third (the line of C code) is the fastest.
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    CornedBee

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  14. Jul 9th, 2003, 04:03 PM #12
    Darkwraith
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    What would the assembly look like on that one?
    "Can't" and "shouldn't" are two totally separate things.

    All questions should be answered. All answers should be true. That is why I post.
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  15. Jul 10th, 2003, 01:38 AM #13
    CornedBee
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    Code:
    mov eax, to_test
dec eax
and eax, to_test
jz just_one_bit
    All the buzzt
    CornedBee

    "Writing specifications is like writing a novel. Writing code is like writing poetry."
    - Anonymous, published by Raymond Chen

    Don't PM me with your problems, I scan most of the forums daily. If you do PM me, I will not answer your question.
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