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58 lines
2.4 KiB
Plaintext
58 lines
2.4 KiB
Plaintext
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; VIC-20 Assembly Language Routines for VIC 6560 Chip
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;VIC Chip Registers
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VICSCH EQU $9000 ;Interlace Mode + Horizontal Screen Origin
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VICSCV EQU $9001 ;Vertical Screen Origin
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VICCOL EQU $9002 ;Screen Memory Location + Number of Video Columns
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VICROW EQU $9003 ;Raster Value + Number of Video Rows + Character Size
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VICRST EQU $9004 ;Raster Value
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VICLOC EQU $9005 ;Screen Memory Location + Character Memory Location
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VICLPH EQU $9006 ;Light Pen - Horizontal
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VICLPV EQU $9007 ;Light Pen - Vertical
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VICPD1 EQU $9008 ;Paddle 1
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VICPD2 EQU $9009 ;Paddle 2
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VICBSF EQU $900A ;Bass Sound Switch and Frequency
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VICASF EQU $900B ;Alto Sound Switch and Frequency
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VICSSF EQU $900C ;Soprano Sound Switch and Frequency
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VICNSF EQU $900D ;Noise Switch and Frequency
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VICAVL EQU $900E ;Auxillary Color + Sound Volume
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VICCLR EQU $900F ;Screen Color + Reverse Mode + Border Color
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;Set Screen Origin and Interlace Mode
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;Args: A = Horizontal Screen Origin
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; Y = Vertical Screen Origin
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; X = Interlace Mode, 0=Off, !0=On
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SETORG: AND #$7F ;Clear Accumulator High Bit
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CPX #0 ;Test Interlace Mode
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BEQ SETORH ;If Not Zero
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ORA #$80 ; Set Accumulator High Bit
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STA VICSCH ;Write to Horizontal Origin Register
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STY VICSCV ;Write Y to Vertical Origin Register
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RTS
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;Set Memory Locations and Number of Columns
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;Args: A = Screen Memory Address Offset
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Y = Character Address Memory Offset
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X = Number of Video Columns
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SETSCN: LSR ;Roll Screen Offset Low Bit into Carry Flag
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PHA ;And Save Top Four Bits on Stack
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TXA ;Copy Columns into Accumulator
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AND #$7F ;Clear High Bit
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BCC SETSCC ;If Carry Set from Roll
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ORA #$80 ; Set High Bit
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STA VICCOL ;Write to Video Columns Register
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PLA ;Retrieve Screen Memory Offset
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ASL ;Shift to High Bit
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ASL
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ASL
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ASL
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PHA ;And Push onto Stack
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TYA ;Put Character Address Offset in Accumulator
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AND #$0F ;Clear High Nybble
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TSX ;Get Index to Screen Address Offset
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ORA $101,X ;Load Into Accumulator High Nybble
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STA VICLOC ;Write to Memory Locations Register
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PLA ;Clean up Stack
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RTS
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