Description of the internal UART:
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The UART occupies four addresses in the DIY calculator address space:
  $F034 : Tx data register (write)
  $F014 : Rx data register (read)
  $F035 : control register (write)
  $F015 : status register  (read)


These are the bits of the control register:
	tx empty interrupt enable: $04
	rx ready interrupt enable: $08
	RTS signal: $10
  automatic RTS: $80

These are the bits of the status register:
  tx empty: $01
  rx ready: $02
  tx empty interrupt enable: $04
  rx ready interrupt enable: $08
  RTS: $10
  CTS: $20
  automatic RTS: $80