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 Keyboard layout
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  AD - Ctrl A     ST - Ctrl S     SST on - ]
  DA - Ctrl D     RS - Ctrl R     SST off- [
  PC - Ctrl P     GO - Ctrl G
          
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 Useful ROM Routines
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----------------------------------------------
 Usage:    define GETKEY $1F6A
           JSR GETKEY
----------------------------------------------

  AK       $1EFE Check for key depressed. A non-zero: no key down. A equal 0, key down.
  SCAND    $1F19 Display address and contents.
  SCANDS   $1F1F Output six hex characters on display. Stored in $00F9, $00FA, $00FB.
  GETKEY   $1F6A Get key from keypad
  CONVD    $1F48 Output HEX digit
  OUTPUT   $1F4E Output 7 segment code stored in A register (to lit up a custom segment)
  KEYIN    $1F40 Open up keyboard channel. Call before using GETKEY (or call SCANDS).
  INCPT    $1F63 Increment display address.
  GETKEY   $1F6A Return key from keyboard. Value 0-F, 10(AD), 11(DA), 12(+), 13(GO), 14(PC), 15 (no keypress).
  TABLE    $1FE7 Table of 7-segment patterns.
  SAVE     $1C00 Normal interrupt entry point.
  RST      $1C22 Reset return to monitor.
  START    $1C4F Return to monitor entry

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 I/O addresses
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  $1740 SAD   7 segment output / keypad code
  $1741 PADD  6530 RIOT data direction
  $1742 SBD   display digit / keypad row

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 Machine Context (saved/restored by ST/GO):
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  00EF PC low
  00F0 PC high
  00F1 Status Register (flags)
  00F2 Stack Pointer
  00F3 A
  00F4 Y
  00F5 X

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 NMI Initialization for Single Step and Stop:
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  (Pre-loaded into RAM)
  17FA 00
  17FB 1C

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 IRQ Initialization for BRK:
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  (Pre-loaded into RAM)
  17FE 00
  17FF 1C