Various Keyboard Matrix Options
                     ===============================

                           8x8 keyboard matrix
                           ===================

                     D0  D1  D2  D3  D4  D5  D6  D7
                     |   |   |   |   |   |   |   | 
                   +-+---+---+---+---+---+---+---+-+
IORQ--+---\        |                               |
 RD---+ OR >-+-----+EN         74LS245             |
PORT--+---/  |     |                               |
             |     +-+---+---+---+---+---+---+---+-+
             |       |   |   |   |   |   |   |   | 
          +--+--+    |   |   |   |   |   |   |   |     Part-processed keypress  
 A8 -->---+ EN  +-- SHF CTL             TAB CAP CPY    00 01          05 06 07
 A9 -->---+  7  +-- LFT RGT DWN UP  DEL RET ESC SPC    08 09 0A 0B 0C 0D 0E 0F
A10 -->---+  4  +--  0   1   2   3   4   5   6   7     30 31 32 33 34 35 36 37
A11 -->---+  L  +--  8   9   :   ;   .   -   ,   /     38 39 3A 3B 3C 3D 3E 3F
A12 -->---+  S  +--  @   A   B   C   D   E   F   G     40 41 42 43 44 45 46 47
A13 -->---+  2  +--  H   I   J   K   L   M   N   O     48 49 4A 4B 4C 4D 4E 4F
A14 -->---+  4  +--  P   Q   R   S   T   U   V   W     50 51 52 53 54 55 56 57
A15 -->---+  5  +--  X   Y   Z   [   \   ]   ^   _     58 59 5A 5B 5C 5D 5E 5F
          +-----+

A8-A15 -> Select of row 0 to row 7
D0-D7  <- keypress state from the selected row

Raw keypress:
        A15-A8    D7-D0
+--+--+--+--+--+--+--+--+
|  |  |  :  :  |  :  :  |  -> gives (ASCII code)-32, then
+--+--+--+--+--+--+--+--+      some minor processing

Raw keypress gives nearly ASCII code of keypress, minor processing of keycode gives
ASCII key. Typically, key &5F is '_' and key &30 is '0', or key &5F is DEL and key
&30 is '0' and Shift-0 is '_'.


                        8x10 keyboard matrix
                        ====================

      D0  D1  D2  D3  D4  D5  D6  D7     Part-processed keypress
       |   |   |   |   |   |   |   | 
 R0 - SHF CTL                            00 01
 R1 -     TAB     CAP     RET ESC SPC       09    0B    0D 0E 0F
 R2 -  0   1   2   3   4   5   6   7     30 31 32 33 34 35 36 37
 R3 -  8   9   :   ;   .   -   ,   /     38 39 3A 3B 3C 3D 3E 3F
 R4 -  @   A   B   C   D   E   F   G     40 41 42 43 44 45 46 47
 R5 -  H   I   J   K   L   M   N   O     48 49 4A 4B 4C 4D 4E 4F
 R6 -  P   Q   R   S   T   U   V   W     50 51 52 53 54 55 56 57
 R7 -  X   Y   Z   [   \   ]   ^  DEL    58 59 4A 5B 5C 5D 5E 5F
 R8 - f0  f1  f2  f3  f4  f5  f6  f7     80 81 82 83 84 85 86 87
 R9 - f8  f9  f10 CPY LFT RGT DWN UP     88 89 8A 8B 8C 8D 8E 8F


                        8x10 keyboard matrix
                        ====================

      0  1  2   3   4  5  6  7  8  9
      |  |  |   |   |  |  |  |  |  |
0 -- SFT f0 f8      0  8  @  H  P  X
1 -- CTL f1 f9      1  9  A  I  Q  Y
2 -- SLK f2 f10     2  *  B  J  R  Z
3 -- CAP f3 f11 CPY 3  +  C  K  S  [
4 -- SPC f4 f12 LFT 4  <  D  L  T  \
5 -- ENT f5 f13 RGT 5  =  E  M  U  ]
6 -- TAB f6 f14 DWN 6  >  F  N  V  ^
7 -- ESC f7 f15 UP  7  ?  G  O  W DEL


Multiplexed 8-bit 8x8 keyboard matrix
=====================================
  This is similar to the Acorn Atom

D0--------+---------------------------+
D1--------+                           |
D2--------+---+---+---+---+---+---+---+
          |   |   |   |   |   |   |   |
    +-+--SPC             ESC         DEL
    | +--LFT RGT DWN UP      RET
D3--+ +-- 0   1   2   3   4   5   6   7
D4--+ +-- 8   9   :   ;   <   =   >   ?
D5--+ +-- @   A   B   C   D   E   F   G
    | +-- H   I   J   K   L   M   N   O
    | +-- P   Q   R   S   T   U   V   W
    +-+-- X   Y   Z   [   \   ]   ^   _

D6--SHIFT
D7--CTRL

&00-&0F      controls
&10-&1F  0123456789:;<=>?
&20-&2F  @ABCDEFGHIJKLMNO
&30-&3F  PQRSTUVWXYZ[\]^_