Invented by Hoogo. Written by Frantic.
Sometimes you've got a byte value and for each bit you want to perform some action if the bit is set to 1 and do nothing, or something else, if the bit is set to 0. Hoogo came up with a nice way of doing that on CSDb which doesn't clobber any registers except for the status register.
asl pattern bcc no_action inc pattern ;Perform action here ... no_action:
This works fine because ASL always shifts in a 0 as the new lsb in the byte and the INC, which changes the lsb to 1, is only executed if a 1 was shifted out into the carry flag by the ASL instruction.
In some circumstances it might be a better optimization to use 0 as flag for “take action” instead of 1, because then the INC will only be needed when no action is to be taken, but whether that is desirable obviously depend on a number of factors.