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  1. PRT_LINE x1k1                                ╔═════════════════╗
  2.                                 ║     MC68901     ║
  3.                                 ║ bit assignments ║
  4.                                 ╚═════════════════╝
  5.                                  Motorola Corporation 68901
  6.                                  Multi-Fonction Peripheral
  7.  ┌──────────────────────────────┐
  8.  │ General Purpose input output │     b7=monochrom monitor                 (NOT)
  9.  │ Interrupt Port               │     b6=auxiliary ring indicator          (NOT)
  10.  │                              │     b5=FDC/HDC                           (NOT)
  11.  │                              │     b4=MC6850_IKBD/MC6850_MIDI           (NOT)
  12.  │                              │     b3=blitter busy                      (NOT)
  13.  │                              │     b2=auxiliary clear to send           (NOT)
  14.  │                              │     b1=auxiliary data carrier detected   (NOT)
  15.  │                              │     b0=printer busy                      (NOT)
  16.  └──────────────────────────────┘
  17.          ┌──────────────────────┐
  18.          │ Active Edge Register │  b7-b0=(see GPIP)              (0:low  1:high)
  19.          └──────────────────────┘
  20.       ┌─────────────────────────┐
  21.       │ Data Direction Register │  b7-b0=(see GPIP)          (0:input  1:output)
  22.       └─────────────────────────┘
  23.   ┌─────────────────────────────┐                                 Priority  &
  24.   │ Interrupt Enable Register A │                                 Vector [b3-b0]
  25.   │                             │     b7=monochrom monitor                 1111
  26.   │                             │     b6=auxiliary ring indicator          1110
  27.   │                             │     b5=timer A                           1101
  28.   │                             │     b4=auxiliary receive register full   1100
  29.   │                             │     b3=auxiliary receive error           1011
  30.   │                             │     b2=auxiliary transmit register empty 1010
  31.   │                             │     b1=auxiliary transmit error          1001
  32.   │                             │     b0=timer B                           1000
  33.   ├─────────────────────────────┤
  34.   │ Interrupt Enable Register B │     b7=FDC/HDC                           0111
  35.   │                             │     b6=MC6850_IKBD/MC6850_MIDI           0110
  36.   │                             │     b5=timer C                           0101
  37.   │                             │     b4=timer D                           0100
  38.   │                             │     b3=blitter busy                      0011
  39.   │                             │     b2=auxiliary clear to send           0010
  40.   │                             │     b1=auxiliary data carrier detected   0001
  41.   │                             │     b0=printer busy                      0000
  42.   │                             │                          (0:disable  1:enable)
  43.   └─────────────────────────────┘
  44. ┌───────────────────────────────┐
  45. │Interrupt Pending Register A/B │  b7-b0=(see IERA/B)      (0:nothing 1:pending)
  46. └───────────────────────────────┘
  47. ┌───────────────────────────────┐
  48. │Inter. In Service Register A/B │  b7-b0=(see IERA/B)   (0:nothing 1:in service)
  49. └───────────────────────────────┘
  50.   ┌─────────────────────────────┐
  51.   │ Interrupt Mask Register A/B │  b7-b0=(see IERA/B)    (0:masked 1:authorized)
  52.   └─────────────────────────────┘
  53.               ┌─────────────────┐
  54.               │ Vector Register │  b7-b4=[b7-b4]=non auto-vector high
  55.               │                 │     b3=software interrupt mode
  56.               │                 │        0:automatic end of interrupt
  57.               │                 │        1:software  end of interrupt
  58.               └─────────────────┘
  59.    ┌────────────────────────────┐
  60.    │ Timer A/B Control Register │     b4=reset
  61.    │                            │        0:no
  62.    │                            │        1:yes
  63.    │                            │  b3-b0=mode
  64.    │                            │        0000:stop
  65.    │                            │        0001:delay  fr./4
  66.    │                            │        0010:delay  fr./10
  67.    │                            │        0011:delay  fr./16
  68.    │                            │        0100:delay  fr./50
  69.    │                            │        0101:delay  fr./64
  70.    │                            │        0110:delay  fr./100
  71.    │                            │        0111:delay  fr./200
  72.    │                            │                   (frequency=2.4576 MHz)
  73.    │                            │        1000:event count
  74.    │                            │             (timer A input=printer busy  )
  75.    │                            │             (timer B input=display enable)
  76.    │                            │        1001:measure fr./4
  77.    │                            │        1010:measure fr./10
  78.    │                            │        1011:measure fr./16
  79.    │                            │        1100:measure fr./50
  80.    │                            │        1101:measure fr./64
  81.    │                            │        1110:measure fr./100
  82.    │                            │        1111:measure fr./200
  83.    └────────────────────────────┘
  84.    ┌────────────────────────────┐
  85.    │ Timer C+D Control Register │  b6-b4=timer C mode (see timer A/B b2-b0)
  86.    │                            │  b2-b0=timer D mode (see timer A/B b2-b0)
  87.    │                            │        (serial clock=timer D output)
  88.    └────────────────────────────┘
  89.   ┌─────────────────────────────┐
  90.   │ Timer A/B/C/D Data Register │  b7-b0=data
  91.   └─────────────────────────────┘
  92. ┌───────────────────────────────┐
  93. │Synchronous Character Register │  b7-b0=character
  94. └───────────────────────────────┘
  95. ┌───────────────────────────────┐
  96. │ Universal                     │     b7=serial frequency
  97. │ synchronous asynchronous      │        0:frequency
  98. │ receiver transmitter          │        1:frequency/16
  99. │ Control Register              │  b6-b5=character size
  100. │                               │        00:8 bits
  101. │                               │        01:7 bits
  102. │                               │        10:6 bits
  103. │                               │        11:5 bits
  104. │                               │  b4-b3=frame
  105. │                               │           transmission    start-bit  stop-bit
  106. │                               │        00:synchronous          0         0
  107. │                               │        01:asynchronous         1         1
  108. │                               │        10:asynchronous         1         1.5
  109. │                               │        11:asynchronous         1         2
  110. │                               │     b2=parity
  111. │                               │        0:no
  112. │                               │        1:yes
  113. │                               │     b1=parity type
  114. │                               │        0:odd
  115. │                               │        1:even
  116. └───────────────────────────────┘
  117.      ┌──────────────────────────┐
  118.      │ Receiver Status Register │     b7=buffer  full               (0:no 1:yes)
  119.      │                          │     b6=overrun error              (0:no 1:yes)
  120.      │                          │     b5=parity  error              (0:no 1:yes)
  121.      │                          │     b4=frame   error (async.)     (0:no 1:yes)
  122.      │                          │     b3=found (sync.) / break     (async.)
  123.      │                          │        0:found         0:no detecded
  124.      │                          │        1:search        1:detected
  125.      │                          │     b2=match (sync.) / character (async.)
  126.      │                          │        0:no            0:nothing
  127.      │                          │        1:yes           1:in progress
  128.      │                          │     b1=synchronous strip
  129.      │                          │        0:disable
  130.      │                          │        1:enable
  131.      │                          │     b0=receiver
  132.      │                          │        0:disable
  133.      │                          │        1:enable
  134.      └──────────────────────────┘
  135.   ┌─────────────────────────────┐
  136.   │ Transmitter Status Register │     b7=buffer   empty             (0:no 1:yes)
  137.   │                             │     b6=underrun error             (0:no 1:yes)
  138.   │                             │     b5=auto turnaround            (0:no 1:yes)
  139.   │                             │     b4=end of transmission        (0:no 1:yes)
  140.   │                             │     b3=break (async.)             (0:no 1:yes)
  141.   │                             │  b2-b1=low and high bits
  142.   │                             │        00:high impedance
  143.   │                             │        01:low
  144.   │                             │        10:high
  145.   │                             │        11:high (loop back)
  146.   │                             │     b0=transmitter
  147.   │                             │        0:disable
  148.   │                             │        1:enable
  149.   └─────────────────────────────┘
  150.           ┌─────────────────────┐
  151.           │ USART Data Register │  b7-b0=data
  152.           └─────────────────────┘
  153.