Electron internal key matrix, accessed via sideways ROM 8

   +-------+-------+-----+-----+-------+-------+-----+-----+
   |  x0   |  x1   | x2  | x3  |  x4   |  x5   | x6  | x7  |
   |   x8  |   x9  |  xA |  xB |   xC  |   xD  |  xE |  xF |
   +=======+=======+=====+=====+=======+=======+=====+=====+
0x |       |       |     |     | |\ -> | []CPY |     | SPC |
   | ^~ <- | _{ dn | RET | DEL |   -   | £} up |  :  |     |
   +-------+-------+-----+-----+-------+-------+-----+-----+
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  |
   +-------+-------+-----+-----+-------+-------+-----+-----+
3x |   2   |   W   |  S  |  X  |   1   |   Q   |  A  |  Z  |
   |  ESC  |CAP/FUN| CTRL|SHIFT|       |       |     |     |
   +-------+-------+-----+-----+-------+-------+-----+-----+

54 individual keys plus BREAK

&A000   1010 0000 all columns

&9FFF   1001 1111
&AFFF   1010 1111
&B7FF   1011 0111
&BBFF   1011 1011
&BDFF   1011 1101
&BEFF   1011 1110
&BF00,x 1011 1111


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

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

&04-&07: &BFFE %1011111111111110
&08-&0B: &BFFD %1011111111111101

table overrun 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 

So, selects column 0,1,2,5,7