Programming/Assembly
To read keyboard/joystick inputs, a selector value must be written to both the keyboard latch at $FD30 and the joystick latch at $FF08, with the resulting answer being read from $FF08.
The keyboard matrix is arranged in 8 groups of 8 keys, as shown in the table below:
| The Plus/4 Keyboard controller | | $FD30 selector value | | $FE | $FD | $FB | $F7 | $EF | $DF | $BF | $7F | | $FF08 answer value | $FE($01) | Ins/Del | 3 | 5 | 7 | 9 | Down | Left | 1 (one) | | $FD($02) | Return | W | R | Y | I | P | * | Clr/Home | | $FB($04) | Pound | A | D | G | J | L | ; / ] | Controls | | $F7($08) | Help/F7 | 4 | 6 | 8 | 0 (zero) | Up | Right | 2 | | $EF($10) | F1/F4 | Z | C | B | M | . / > | Escape | Space | | $DF($20) | F2/F5 | S | F | H | K | : / [ | = | Commodore | | $BF($40) | F3/F6 | E | T | U | O | - | + | Q | | $7F($80) | @ sign | Shift keys | X | V | N | , / < | / | Run/Stop |
Each bit in the selector value represents a group of 8 keys. For example, bit 0 represents keys Inst/Del, Return, Pound, the Function keys and the @ key, bit 1 represents keys 3, W, A, 4, Z, S, E and Shift, and so on.
To read a group of keys, the corresponding bit must be zero in the selector value. The answer value read from $FF08 indicate which of the keys in that group was pressed. If a key was pressed, it's corresponding bit in the answer value will be zero.
For example, to check if the Space bar was pressed, the selector value would be $7F (zero in bit 7), and if Space is pressed, the answer value read from $FF08 will be $EF (zero in bit 4).
Multiple groups of keys can be checked by clearing multiple bits in the selector value. To check if any key on the keyboard was pressed, use $00 as the selector value. If no keys were pressed, the answer value will be $FF, if it is not $FF, then at least one key was pressed.
For joystick inputs, bit 1 of the selector value reads Port 2, and bit 2 reads Port 1. The selector and answer values for the joysticks are as follows:
| Joystick controller | | $FF08 selector value | | $FD | $FB | | $FF08 answer value | $FE($01) | Joy2 Up | Joy1 Up | | $FD($02) | Joy2 Down | Joy1 Down | | $FB($04) | Joy2 Left | Joy1 Left | | $F7($08) | Joy2 Right | Joy1 Right | | $EF($10) | | | | $DF($20) | | | | $BF($40) | | Joy1 Fire | | $7F($80) | Joy2 Fire | |
If the same selector value is written to both $FD30 and $FF08, then both keyboard and joystick will be read simultaneously. For example, if the selector value written to both $FD30 and $FF08 is $FB, and the answer is $FD, then either key R was pressed, or Joystick 1 was pushed down.
To read keyboard inputs separately from the joysticks, write the desired selector value to the $FD30 keyboard latch, and write $FF to the $FF08 joystick latch.
Likewise, to read the joysticks separately from the keyboard, write $FF to the $FD30 keyboard latch, and the desired selector value to the $FF08 joystick latch.
Check if space key is pressed
LDA #$7F Check column 7
STA $FD30 Write to Keyboard Latch
STA $FF08 Write to Joystick Latch
LDA $FF08 Read answer
AND #$10 Query for "Space" bit
BNE not_pressed
space is pressed
not_pressed
Check if 'T' is pressed, but not checking if Joystick 1 Fire is pressed:
LDA #$FB Check bit 2
STA $FD30 Write to Keyboard Latch
LDA #$FF
STA $FF08 Write $FF to Joystick Latch
LDA $FF08 Read answer
AND #$40 Query for "T" bit
BNE not_pressed
'T' is pressed, but not J1 Fire
not_pressed
Check if Joystick 2 Up is pressed, but not checking if key '3' is pressed
LDA #$FF
STA $FD30 Write $FF to Keyboard Latch
LDA #$FD Check bit 1
STA $FF08 Write to Joystick Latch
LDA $FF08 Read answer
AND #$01 Query for J2 Up bit
BNE not_pressed
J2 Up is pressed, but not '3'
not_pressed
BSZ and TLC have recently (early 2021) conducted some analyses about the role of the joysticks' reading beaking into the other TED's locations, and how this works in broken TED's ICs . Read the dedicated forum's thread.
Keyboard Joystick Matrix |
