0x80 CONSTANT SIO_ACTL   0x81 CONSTANT SIO_ADATA
0x82 CONSTANT SIO_BCTL   0x83 CONSTANT SIO_BDATA
0x20 CONSTANT SIO_BUFSZ ( SZ-1 must be a mask )
( Address in memory that can be used variables shared
  with SIO native words. 4 bytes used. )
CREATE SIO_MEM SYSVARS 0x70 + ,
( Points to SIO buf )
: SIO( SIO_MEM @ 2+ ;
( Read buf idx Pre-inc )
: SIOR> SIO_MEM @ ;
( Write buf idx Post-inc )
: SIOW> SIO_MEM @ 1+ ;
( This means that if W> == R>, buffer is full.
  If R>+1 == W>, buffer is empty. )