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  1. DESKTOP x1k1
  2.                                 ╔═════════════════╗
  3.                                 ║     MC6850      ║
  4.                                 ║ bit assignments ║
  5.                                 ╚═════════════════╝
  6.                                  Motorola Corporation 6850
  7.                                  Asynchronous Communication Interface Adaptator
  8.               ┌─────────────────┐
  9.               │ Status Register │    b15=interrupt requested
  10.               │                 │        0:no
  11.               │                 │        1:yes
  12.               │                 │    b13=overrun error
  13.               │                 │        0:no
  14.               │                 │        1:yes
  15.               │                 │     b9=transmit register empty
  16.               │                 │        0:no
  17.               │                 │        1:yes
  18.               │                 │     b8=receive register full
  19.               │                 │        0:no
  20.               │                 │        1:yes
  21.               └─────────────────┘
  22.              ┌──────────────────┐
  23.              │ Control Register │    b15=interrupt when receive register full
  24.              │                  │        0:no
  25.              │                  │        1:yes
  26.              │                  │    b13=interrupt when transmit register empty
  27.              │                  │        0:no
  28.              │                  │        1:yes
  29.              │                  │  b9-b8=11:reset
  30.              └──────────────────┘
  31.              ┌──────────────────┐
  32.              │ Receive Register │ b15-b8=[b7-b0]=receive register
  33.              └──────────────────┘
  34.             ┌───────────────────┐
  35.             │ Transmit Register │ b15-b8=[b7-b0]=transmit register
  36.             └───────────────────┘
  37.