2018-09-17 23:39:46 +00:00
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;Lightpen Assembly Language Module for VIC-20
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2018-09-12 13:54:54 +00:00
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LGTPNS EQU #$FF ;Light Pen Status (Supported)
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;Read Light Pen
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LGTPEN: LDX #0 ;Initialize X to FALSE (0)
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LDA $9009 ;Check Paddle 1 (Magnum Light Phaser)
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BMI LGTPEJ ;If Trigger Pulled
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DEX ; Set X to TRUE ($FF)
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BNE LGTPEP ;Else Check Joystick Inputs
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LGTPEJ: LDA $9113 ; Get DDR for Port A Status
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PHA ; and Save It
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LDA $9122 ; Set DDR for Port B Status
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PHA ; and Save It
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STX $9113 ; Set DDR for Port A to All Inputs
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STX $9122 ; Set DDR for Port B to All Inputs
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LDA $9120 ; Read Port B (S2)
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ASL ; Shift Bit 7 into Carry
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LDA $9111 ; Read Port A
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ROR ; Rotate Carry Back In
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EOR #$FF ; Invert Bits
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AND #$9E ; and Mask Joystick Inputs
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BEQ LGTPES ; If Any Bits Set
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DEX ; Set X to TRUE ($FF)
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LGTPES: PLA ; Restore Port B DDDR
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STA $9122
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PLA ; Restore Port A DDDR
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STA $9113
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LGTPEP: LDY $9007 ;Read Y Position into Y
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LDA $9006 ;Read X Position into A
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RTS
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