Electron internal key matrix, accessed via sideways ROM 8

     A0  A1  A2  A3  A4  A5  A6  A7  A8  A9  A10 A11 A12 A13
    +---+---+---+---+---+---+---+---+---+---+---+---+---+---+
 D0 |ESC| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 0 | - |<- |-> |
    +---+---+---+---+---+---+---+---+---+---+---+---+---+---+
 D1 |CAP| Q | W | E | R | T | Y | U | I | O | P | UP|DWN|CPY|
    +---+---+---+---+---+---+---+---+---+---+---+---+---+---+
 D2 |CTL| A | S | D | F | G | H | J | K | L | ; | : |RET|   |
    +---+---+---+---+---+---+---+---+---+---+---+---+---+---+
 D3 |SFT| Z | X | C | V | B | N | M | , | . | / |   |DEL|SPC|
    +---+---+---+---+---+---+---+---+---+---+---+---+---+---+

A0-A13 fed to keyboard
D0-D3 read from keyboard

&A000   1010 0000 all columns    &BF00,X 1011 1111 0111 11111
                                 &BF00,X 1011 1111 1011 11111
&9FFF   1001 1111                &BF00,X 1011 1111 1101 11111
&AFFF   1010 1111                &BF00,X 1011 1111 1110 11111
&B7FF   1011 0111                &BF00,X 1011 1111 1111 01111
&BBFF   1011 1011                &BF00,X 1011 1111 1111 10111
&BDFF   1011 1101                &BF00,X 1011 1111 1111 11011
&BEFF   1011 1110                &BF00,X 1011 1111 1111 11101
                                 &BF00,X 1011 1111 1111 11110


Electron internal key number

    +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
    |x0 |x1 |x2 |x3 |x4 |x5 |x6 |x7 |x8 |x9 |xA |xB |xC |xD |xE |xF |
    +===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+
    |   |   |   |   || \|[ ]|   |   |^ ~|_ {|   |   |   |£ }| * |   |
 0x |   |   |   |   | ->|CPY|&00|SPC|<- |dwn|RET|DEL| - | up| : |&00|
    +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
    | @ |   | + | ? | ) |   |   | > | ( |   |   | < | ' |   |   |   |
 1x | 0 | P | ; | / | 9 | O | L | . | 8 | I | K | , | 7 | U | J | M |
    +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
    | & |   |   |   | % |   |   |   | $ |   |   |   | # |   |   |   |
 2x | 6 | Y | H | N | 5 | T | G | B | 4 | R | F | V | 3 | E | D | C |
    +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
    | " |   |   |   | ! |   |   |   |   |FNC|   |   |   |   |   |   |
 3x | 2 | W | S | X | 1 | Q | A | Z |ESC|CAP|CTL|SFT|   |   |   |   |
    +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+


Internal key number converted to keyboard column/row address with:
  (keynum DIV 4)-1 = column address line
  (keynum MOD 4)   = row bit position


Table overrun in MOS results in:
        &3C: &FE58,&01 -> &BF58,&01 %1011111101011000 
-126 -> &3D: &FE58,&02 -> &BF58,&02 %1011111101011000 Y U I O P DWN UP  CPY
-124 -> &3E: &FE58,&04 -> &BF58,&04 %1011111101011000 H J K L ;  :  RET
        &3F: &FE58,&08 -> &BF58,&08 %1011111101011000 

If MOS keyboard table modified, unused keys &06 and &0F could be used, for
example for TAB key.