freq_and_duty_modulation.asm

* makes beeps
   
         ORG   $300
         LDA   #$FF
         STA   FREQ
         LDA   #$80
         STA   PLSWIDTH
PLAY     LDA   $C030
         LDY   PLSWIDTH
PULSE    DEY
         BNE   PULSE
         LDA   $C030
         LDX   FREQ
COUNTDN  DEX
         BNE   COUNTDN
         JSR   READKB
         JMP   PLAY

FREQ     EQU   $350
PLSWIDTH EQU   $352

INCR     INC   FREQ
         RTS

DECR     DEC   FREQ
         RTS

PULSEINC DEC   PLSWIDTH
         INC   FREQ
         RTS

PULSEDEC INC   PLSWIDTH
         DEC   FREQ
         RTS

READKB   LDA   $C000
         STA   $C010   
         CMP   #$88
         BEQ   INCR
         CMP   #$95
         BEQ   DECR
         CMP   #$C1
         BEQ   PULSEINC
         CMP   #$DA
         BEQ   PULSEDEC
         RTS