: ACIA$
    H@ [ ACIA( LITN ] ! 0 [ ACIAR> LITN ] C!
    1 [ ACIAW> LITN ] C! ( write index starts one pos later )
    [ ACIA_BUFSZ LITN ] ALLOT
( setup ACIA
  CR7 (1) - Receive Interrupt enabled
  CR6:5 (00) - RTS low, transmit interrupt disabled.
  CR4:2 (101) - 8 bits + 1 stop bit
  CR1:0 (10) - Counter divide: 64 )
    0b10010110 [ ACIA_CTL LITN ] PC!
    (im1) ;