;; This example shows the correct method to read the keyboard and
;; joysticks on the CPC, CPC+ and KC Compact.
;;
;; This source is compatible with the CPC+.
;;
;; The following is assumed before executing of this algorithm:
;; - I/O port A of the PSG is set to input,
;; - PPI Port A is set to output
;;

;;--------------------------------------------------------------------------------
;; example code showing how to use read_matrix.

org &4000
nolist

.main_loop
;; wait for vsync
ld b,&f5
.v1 in a,(c)
rra
jr nc,v1

call read_matrix

;; NOTE: Consult the 'matrix' table in the
;; document 'Scanning the Keyboard & Joysticks' to find the keyboard line
;; and bit for the keys you want to check.

;; test the 'return' key has been pressed.
;; (line 2, bit 2).
ld a,(matrix_buffer+2)
bit 2,a
jr z,not_pressed

;; return key pressed

.not_pressed

jp main_loop


;;--------------------------------------------------------------------------------

.read_matrix 
ld hl,matrix_buffer        ; buffer to store matrix data

ld bc,&f40e                ; write PSG register index (14) to PPI port A (databus to PSG)
out (c),c

ld b,&f6
in a,(c)
and &30
ld c,a

or &C0                     ; bit 7=bit 6=1 (PSG operation: write register index)
out (c),a                  ; set PSG operation -> select PSG register 14

;; at this point PSG will have register 14 selected.
;; any read/write operation to the PSG will act on this register.

out (c),c                  ; bit 7=bit 6=0 (PSG operation: inactive)

inc b
ld a,&92
out (c),a                  ; write PPI control: port A: input, port B: input, port C upper: output
                           ; port C lower: output
push bc
set 6,c                    ; bit 7=0, bit 6=1 (PSG operation: read register data)

.scan_key 
ld b,&f6 
out (c),c                 ;set matrix line & set PSG operation

ld b,&f4                  ;PPI port A (databus to/from PSG)
in a,(c)                  ;get matrix line data from PSG register 14

cpl                       ;invert data: 1->0, 0->1
                          ;if a key/joystick button is pressed bit will be "1"
                          ;keys that are not pressed will be "0"

ld (hl),a                 ;write line data to buffer
inc hl                    ;update position in buffer
inc c                     ;update line

ld a,c
and &0f
cp &0a                    ;scanned all rows?
jr nz,scan_key            ;no loop and get next row

;; scanned all rows
pop bc

ld a,&82                  ;write PPI Control: Port A: Output, Port B: Input, Port C upper: output, Port C lower: output.
out (c),a

dec b
out (c),c                 ;set PSG operation: bit7=0, bit 6=0 (PSG operation: inactive)
ret

;; This buffer has one byte per keyboard line. 
;; Each byte defines a single keyboard line, which defines 
;; the state for up to 8 keys.
;;
;; A bit in a byte will be '1' if the corresponding key 
;; is pressed, '0' if the key is not pressed.
.matrix_buffer
defs 10