THE VIC 20 KEYBOARD
Written by Lance Ewing
(27 Oct 96)

 Note: This text has been corrected and added to since the original version.

INTRODUCTION

 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 ITS 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. 


KEYBOARD LAYOUT

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


HOW DOES IT KNOW WHEN TWO KEYS ARE BEING PUSHED ON THE SAME COLUMN

 The way I understand it is that the kernel selects a column and the data that 
comes back is for all 8 keys on that column. The return value is an 8 bit number
where each bit position represents one of the eight keys on that column. If a 
bit position is "0" then that key is being pushed down. If a bit position is 
"1" then that key isn't being pushed down. The return value read from $9121 can 
be any value from 0 to 255. If the value 0 is read from this address, then 
every key on the selected column is being held down. 


FAMILIARITY WITH THE VIC 20 KEYBOARD

 I'm aware that some of you may not have used the original VIC 20. For this 
reason I'll mention what a few of the key combinations do. When the VIC 20 is 
turned on it is in upper/graphics mode. This means that if you push a letter 
like the "X/x" key, then you will get a capital X. If you hold down the SHIFT 
key and push X, you will get a graphics character which looks like a filled in
circle. If you hold down the CBM key and push the X key, you will get a different
graphics character.
 If you push the CBM key and the SHIFT key at the same time, the mode is 
changed from upper/graphics to lower/upper. This means that when you now push
the "X/x" key, a lower case x will be displayed. If you hold down the SHIFT key
and push X, you will get a capital X.
 The way the KERNEL determines which character to display is by scanning the
keyboard to see what combinations of keys are being held down.