0x80 CONSTANT ACIA_CTL ( IO port for ACIA's control register )
0x81 CONSTANT ACIA_IO ( IO port for ACIA's data registers )
0x20 CONSTANT ACIA_BUFSZ ( SZ-1 must be a mask )
( Address in memory that can be used variables shared
  with ACIA's native words. 4 bytes used. )
CREATE ACIA_MEM SYSVARS 0x70 + ,
( Points to ACIA buf )
: ACIA( ACIA_MEM @ 2+ ;
( Read buf idx Pre-inc )
: ACIAR> ACIA_MEM @ ;
( Write buf idx Post-inc )
: ACIAW> ACIA_MEM @ 1+ ;
( This means that if W> == R>, buffer is full.
  If R>+1 == W>, buffer is empty. )