Rotating: rol and ror

A running light: one lamp is lit, the next takes over, and after the last one it starts again at the front. With sll the bit falls out on the left and is gone. Rotating holds on to it: what falls out at the top comes back in at the bottom.

rol and ror#

In the game both rotate within 54 bits, not 64. A bit that goes past bit 53 on the left lands in bit 0.

rol r0 %0001 1    # %0010 = 2
ror r1 %0010 1    # %0001 = 1
rol r2 %1000 2    # %10_0000 = 32

The 54-bit trap#

If you rotate bit 0 right by one, it lands right at the top in bit 53. As a number that is gigantic and completely useless for four lights. Going left it also takes 54 steps until a bit is back at the front. For small running lights you therefore need a small ring of your own.

Building small rings yourself#

For a ring of four bits you bring the bits that fell out back yourself: rotate left, shift everything from bit 4 to the right, put it back in and cut down to four bits:

rol pattern pattern 1
srl tmp pattern 4        # bit 4 slides to bit 0
or pattern pattern tmp
and pattern pattern $F    # keep only bits 0 to 3

Going right uses the same code: in a ring of four bits “1 to the right” is the same as “3 to the left”. With eight bits it would be 7 to the left, and instead of $F and 4 you use $FF and 8.

Running light#

Each lamp gets a bit: lamp 0 is lit when bit 0 is set, and so on. You get the bit out with srl and and 1 – tomorrow that works in one line with ext.