Software - Counting Bits

Here are two ways to count the number of bits that are set (i.e. the number of bits that are ones) in a byte:

This routine returns the result in the accumulator.

   LSR
   BCC .1
   ADC #$3F
.1 LSR
   BCC .2
   ADC #$1F
.2 LSR
   BCC .3
   ADC #$0F
.3 LSR
   BCC .4
   ADC #7
.4 LSR
   BCC .5
   ADC #3
.5 LSR
   BCC .6
   ADC #1
.6 LSR
   ADC #0

This routine returns the result in the X register.

   LDX #$FF
.1 INX
.2 ASL
   BCS .1
   BNE .2