AY-3-4592 keyboard ROM matrix mapping
                     =====================================
                               Inverted data bus

          Y0      Y1      Y2      Y3      Y4      Y5      Y6      Y7
       +-------+-------+-------+-------+-------+-------+-------+-------+
    C  | 000   | 000   | 000   | 000   | 000   | 000   | 000   | 331 1 |
 X0 S  | 000   | 000   | 000   | 000   | 000   | 000   | 000   | 321 ! |
    -  | 3FE   | 3FD   | 3FC   | 3FB   | 3FA   | 3F9   | 3F8   | 331 1 |
       | SHIFT | SHIFT | SLOCK | CTRL  | ALPHA | X15   | D10   |       |
       +-------+-------+-------+-------+-------+-------+-------+-------+
    C  | 21B   | 332 2 | 332 2 | 217   | 211   | 213   | 201   | 21A   |
 X1 S  | 21B   | 240 @ | 322 " | 257 W | 251 Q | 253 S | 241 A | 25A Z |
    -  | 21B   | 332 2 | 332 2 | 277 w | 271 q | 273 s | 261 a | 27A z |
       +-------+-------+-------+-------+-------+-------+-------+-------+
    C  | 280   | 334 4 | 333 3 | 212   | 205   | 204   | 218   | 203   |
 X2 S  | 280   | 324 $ | 323 # | 252 R | 245 E | 244 D | 258 X | 243 C |
    -  | 280   | 334 4 | 333 3 | 272 r | 265 e | 264 d | 278 x | 263 c |
       +-------+-------+-------+-------+-------+-------+-------+-------+
    C  | 281   | 282   | 335 % | 214   | 206   | 207   | 216   | 202   |
 X3 S  | 281   | 282   | 325 % | 254 T | 246 F | 247 G | 256 V | 242 B |
    -  | 281   | 282   | 335 5 | 274 t | 266 f | 267 g | 276 v | 262 b |
       +-------+-------+-------+-------+-------+-------+-------+-------+
    C  | 283   | 337 7 | 336 6 | 219   | 208   | 20E   | 336 6 | 320   |
 X4 S  | 283   | 326 & | 326 & | 259 Y | 248 H | 24E N | 33C < | 320   |
    -  | 283   | 337 7 | 336 6 | 279 y | 268 h | 26E n | 336 6 | 320   |
       +-------+-------+-------+-------+-------+-------+-------+-------+
    C  | 284   | 338 8 | 337 7 | 215   | 20A   | 20B   | 20D   | 32C , |
 X5 S  | 284   | 32A * | 327 ' | 255 U | 24A J | 24B K | 24D M | 33C < |
    -  | 284   | 338 8 | 337 7 | 275 u | 26A j | 26B k | 26D m | 32C , |
       +-------+-------+-------+-------+-------+-------+-------+-------+
    C  | 285   | 339 9 | 338 8 | 209   | 20F   | 20B   | 20C   | 20D   |
 X6 S  | 285   | 328 ( | 328 ( | 249 I | 24F O | 25B [ | 24C L | 25D ] |
    -  | 285   | 339 9 | 338 8 | 269 i | 26F o | 26B k | 26C l | 26D m |
       +-------+-------+-------+-------+-------+-------+-------+-------+
    C  | 286   | 330 0 | 329 9 | 287   | 210   | 33B ; | 20C   | 32E . |
 X7 S  | 286   | 329 ) | 329 ) | 287   | 250 P | 33A : | 25C \ | 33E > |
    -  | 286   | 330 0 | 339 9 | 287   | 270 p | 33B ; | 26C l | 32E . |
       +-------+-------+-------+-------+-------+-------+-------+-------+
    C  | 32D - | 20E   | 210   | 21B   | 327 ' | 33B ; | 32F / | 288   |
 X8 S  | 25F _ | 25E ^ | 240 @ | 25D ] | 322 " | 32B + | 33F ? | 288   |
    -  | 32B - | 26E n | 270 p | 25B [ | 327 ' | 33B ; | 32F / | 288   |
       +-------+-------+-------+-------+-------+-------+-------+-------+
    C  | 33D = | 33A : | 289   | 21C   | 28A   | 21B   | 20D   | 21D   |
 X9 S  | 32B + | 32A * | 289   | 37C | | 28A   | 37B { | 20D   | 37D } |
    -  | 33D = | 33A : | 289   | 25C \ | 28A   | 25B [ | 20D   | 25D ] |
       +-------+-------+-------+-------+-------+-------+-------+-------+
    C  | 37F   | 28B   | 21F   | 28C   | 20A   | 200   | 21E   | 25F _ |
X10 S  | 37F   | 28B   | 25F _ | 28C   | 20A   | 25C \ | 37E ~ | 33D = |
    -  | 37F   | 28B   | 32D - | 28C   | 20A   | 240 @ | 25E ^ | 25F _ |
       +-------+-------+-------+-------+-------+-------+-------+-------+
    C  | 28D   | 209   | 32D - | 28E   | 20F   | 25B [ | 208   | 29F   |
X11 S  | 28D   | 209   | 33D = | 28E   | 25F _ | 25D ] | 208   | 29F   |
    -  | 28D   | 209   | 32D - | 28E   | 26F o | 25B [ | 208   | 29F   |
       +-------+-------+-------+-------+-------+-------+-------+-------+
    C  | 28F   | 337 7 | 20B   | 290   | 334 4 | 32C , | 331 1 | 330 0 |
X12 S  | 28F   | 337 7 | 20B   | 290   | 334 4 | 32C , | 331 1 | 330 0 |
    -  | 28F   | 337 7 | 20B   | 290   | 334 4 | 32C , | 331 1 | 330 0 |
       +-------+-------+-------+-------+-------+-------+-------+-------+
    C  | 291   | 339 9 | 338 8 | 335 5 | 336 6 | 332 2 | 333 3 | 32E . |
X13 S  | 291   | 339 9 | 338 8 | 335 5 | 336 6 | 332 2 | 333 3 | 32E . |
    -  | 291   | 339 9 | 338 8 | 335 5 | 336 6 | 332 2 | 333 3 | 32E . |
       +-------+-------+-------+-------+-------+-------+-------+-------+

D0-D7: ASCII code
 D8:   0=letter key, external CapsLock can modify code if D6=1
       1=non-letter key, CapsLock must not modify
 D9:   0=no action
       1=active keypress

The key matrix allows mapping for:
* bit-paired keyboard with keys, eg 6& 7' 8( 9) etc.
* typewriter keyboard with keys, eg 6^ 7& 8* 9( 0) etc.
* teletype keyboard with keys, eg K[ L\ M] N^ O_ P@ etc.
* calculator numberpad with number keys not modified by Shift
* keys that return &80-&91 and &9F to allow additional special keys, such
  as function keys.