ld hl,($c1be) ; Get high two bytes
  ld a,l        ; xor low byte with high byte of doubled number
  add hl,hl
  xor h
  ld b,a        ; -> b
  ld hl,($c1bd) ; Then read again one byte lower, and repeat
  ld a,l
  add hl,hl
  xor h
  ld c,a        ; -> c
  ld hl,($c1bc) ; copy last value read to high 2 bytes
  ld ($c1be),hl
  ld ($c1bc),bc ; and save new value to low 2 bytes
  ret