Reading Individual Bits in Variables?
coryco2
Posts: 107
Does anyone know a simple way to test the state of an individual bit in a one-byte variable? I would like to be able to test, for example, whether the third bit in the byte is 1 or 0 regardless of the states of the other bits. Any suggestions?
Comments
PUB CheckBit( value, bitNr )
return value & |< bitNr
where value = 0 .. 7 in your case. As parameters in SPIN are LONGs you can also go from 0..31
Explaination:
|< is an operator which shifts a 1 bit set at the least significant bit as much to the left as given by the parameter. For example |< 3
%00000000_00000000_00000000_00000001 shifted left 3 times gives you
%00000000_00000000_00000000_00001000
The next operation is a logical and which gives you back all the bits where the bit of both operands are set to one. If one operand only has 1 bit set, the result shows you if this one bit is set in the other operand regardless of all other bits. For example:
%01111010_11010001_00011001_11011000 &
%00000000_00000000_00000000_00001000 gives you
%00000000_00000000_00000000_00001000
If you use this number in an expression which expects a boolean value, it's the same as TRUE because all values <>0 are TRUE and only 0 is FALSE. So, it's safe to use this function in an IF statement for example.
pub bitcount(value) | bc '' Returns # of bits set in value bc := 0 ' clear count repeat 32 ' test all bits bc += (value & %1) ' add bit value to count value >>= 1 ' next bit return bc pub bitpos(value, mode) | pos '' Returns position of 1st bit set '' -- mode 0 to scan from lsb, mode 1 to scan from msb '' -- -1 = no bits set if (value == 0) ' if no bits return -1 ' return -1 else if (mode == 0) ' check from LSB value ><= 32 ' flip for >| return (32 - >|value) else return (>|value - 1) pub getnib(target, pos) '' returns nib value (0..15) of target.nib[pos] if ((pos => 0) and (pos =< 3)) return (target >> (pos << 2)) & %1111 else return 0 pub getquart(target, pos) '' returns quarternary value (0..3) of target.quart[pos] if ((pos => 0) and (pos =< 7)) return (target >> (pos << 1)) & %11 else return 0 pub getbit(target, pos) '' returns bit value (0..1) of target.pos if ((pos => 0) and (pos =< 31)) return (target >> pos) & 1 else return 0 pub putbit(target, pos, value) '' writes value.0 to target.pos if (value & 1) return target | (1 << pos) else return target & !(1 << pos) pub togglebit(target, pos) '' toggles target.pos return target ^ (1 << pos)...