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

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Anyone got a better idea?

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.

how about and'ing it?

Yeah, keda gave me that piece by jim:

Code:

---

bool just_one_bit = (to_test & (to_test-1)) == 0;

---

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.

I don't think the 80x86 instruction set contains a BIT instruction.

Also, what would the code segment look like in this problem?

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)

It depends if this is a check to see if this is a certain bit or any bit.

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

Isn't bsf and bsr sorta expensive? I think the first alogrithm is faster.

The third (the line of C code) is the fastest.

What would the assembly look like on that one?

Code:

---

mov eax, to_test
dec eax
and eax, to_test
jz just_one_bit

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