The Keyboard.

   Firstly, it is important to once again recognise that the VIC 20 is very
  similar to the C64. Both machines have very similar keyboards. They don't
  function entirely the same though. The difference is described later on.
   Almost all my VIC 20 keyboard knowledge is based on the information and
  code provided by VICE.


  THE VIC 20 KEYBOARD
  AND FEATURES

    The keyboard is treated as a set of switches organized into a matrix
  of 8 columns by 8 rows. The keyboard matrix is scanned for key switch-
  closures by the KERNAL using the VIA #2 chip (MOS 6522 Versatile Interface
  Adapter). Two VIA registers are used to perform the scan:
  register #0 at location 37152 ($9120) for keyboard columns and
  register #1 at location 37153 ($9121) for keyboard rows.
    Bits 0-7 of memory location 37152 correspond to the columns 0-7. Bits
  0-7 of memory location 37153 correspond to rows 0-7. By writing column
  values in sequence, then reading row values, the KERNAL decodes the
  switch closures into the CHR$ (N) value of the key pressed.
    Eight columns by eight rows yields 64 possible values. 



 Below is shown the keyboard layout of a real VIC-20 and C64 computers.
 They both use the same keyboard mechanism, only wiring on the motherboeard
 differs. The keyboard mechanism is interchangeable between VIC-20 and the C64.
 However, on the main circuit board (motherboard), the VIC-20 has all the lines
 connected to the VIA chip in order, whereas C64 has Column3 and Column7
 swapped. That's why the C64 gives different keyboard matrix. It also affects
 to the keycodes returned.


	VIC-20 / C64 Standard:

  |  <-  1!  2"  3#  4$  5%  6&  7'  8(  9)  0   +   -  GBP HOME DEL |  | F1 |
  | CTRL   q   w   e   r   t   y   u   i   o   p   @   *  ^~ RESTORE |  | F3 |
 | RUN LOCK a   s   d   f   g   h   j   k   l   :[  ;]  =   RETURN  |   | F5 |
 | C=  SHIFT  z   x   c   v   b   n   m   ,<  .>  /?  SHIFT DWN RGT |   | F7 |
 -------------|_______________SPACE_______________|------------------

	'~' is 'pi'


 VIC-20 Keyboard Matrix
              
 9121   9120: (Write column value to this address)

        7f      bf      df      ef      f7      fb      fd      fe

  fe    RunStop q       CBM     Space   2       Control Lft_arr 1       
  fd    Shift_L e       s       z       4       a       w       3       
  fb    x       t       f       c       6       d       r       5       
  f7    v       u       h       b       8       g       y       7       
  ef	n	o	k	m	0	j	i	9
  df	,	@	:	.	-	l	p	+
  bf	/	Up_arr	=	Shift_R	Home	;	*	GBP	
  7f	Down	F5	F3	F1	F7	Right	Return	Del

 Note: the values above are the complements of the values 1, 2, 4, 8, 16
 32, 64, 128. 

 The VIC 20 uses one of its VIA chip timers to generate regular interrupts
to check on such things as its keyboard. This procedure used to check the
keyboard is described below.

 The KERNEL begins by deciding if any key has been pressed. It does this by
writing $00 to $9120 which is the code to scan all keyboard columns. If it
receives back the value $FF, then it knows that no key is being pushed. If it
receives a different value then a key is currently being pushed down. When
this happens the KERNEL scans each row separately to determine which keys are
being pushed down. For example, if the KERNEL sends the value $FE to the
address $9120 and it receives the value $7f back at address $9121, then it
would know that the Del key is being pushed down. 


EMULATING THE KEYBOARD

 Every time your emulator drops out of executing 6502 code to check on
interrupts, update timers and draw a new scanline, you must also check if
a key has been pushed on the PC keyboard. If it has, then the emulator should
translate this key into a VIC key press and store it in some sort of buffer
structured as an 8x8 matrix representing the state of the VIC keyboard.
Ideally this matrix will only retain this value until the key is released.
(In practice this is hard to achieve because of the difficulty in detect