Author Topic: Checking the amount of set bits. (Read 2604 times)

ralphdspam · « **on:** July 07, 2011, 10:55:05 pm »

Hi, is there an efficient way to check if a byte has only one reset bit?

Example:
%11111011
True, because bit 2 is reset.

%01110111
False, because bit 3 and 7 are reset.

Thanks for all of the suggestions.

Iambian · « **Reply #1 on:** July 07, 2011, 11:01:35 pm »

Code: [Select]

;Input: A=byteToCheck
;Destroys B.
CheckOneBitUnset:
 cpl
 ld b,8
 rlca
 jr c,$+6
 djnz $-3
 scf
 ret  ;returns with a carry if no bits are set
 and %11111110
 ret  ;zero if only one bit is unset, nonzero if others are set. No carry regardless.

Check to see if that actually works. Just made it up on the spot.

calc84maniac · « **Reply #2 on:** July 07, 2011, 11:04:06 pm »

This method tests for zero or one reset bits (you'd have to check the $FF case manually)

Code: [Select]

;Outputs Z if 0-1 bits of A are reset
ld b,a
inc a
or b
inc a

ralphdspam · « **Reply #3 on:** July 07, 2011, 11:07:58 pm »

Quote from: calc84maniac on July 07, 2011, 11:04:06 pm

This method tests for zero or one reset bits (you'd have to check the $FF case manually)
Code: [Select]
;Outputs Z if 0-1 bits of A are reset ld b,a inc a or b inc a

Thanks! That is awesomeness! A winner is you!

Quigibo · « **Reply #4 on:** July 07, 2011, 11:41:03 pm »

This is the smallest I could come up with:

Code: [Select]

;Input: a = byte to check
;Flags: z = Exactly 1 reset bit
 cpl
 ld b,a
 dec a
 xor b
 add a,1
 rra
 xor b

Edit: Nevermind, calc wins! Although you do have to special case with his, but even with that I think it would still be smaller by one byte.

calc84maniac · « **Reply #5 on:** July 07, 2011, 11:45:27 pm »

Hmm, I just realized a fairly simple way to handle the special case:

Code: [Select]

;Outputs Z if exactly 1 bit of A is reset
  ld b,a
  inc a
  jr z,special
  or b
special:
  inc a

Author Topic: Checking the amount of set bits. (Read 2604 times)

ralphdspam

Checking the amount of set bits.

Iambian

Re: Checking the amount of set bits.

calc84maniac

Re: Checking the amount of set bits.

ralphdspam

Re: Checking the amount of set bits.

Quigibo

Re: Checking the amount of set bits.

calc84maniac

Re: Checking the amount of set bits.