; VIC-20 Assembly Language Routines for VIC 6560 Chip

;VIC Chip Registers
VICSCH  EQU $9000 ;Interlace Mode + Horizontal Screen Origin
VICSCV  EQU $9001 ;Vertical Screen Origin
VICCOL  EQU $9002 ;Screen Memory Location + Number of Video Columns
VICROW  EQU $9003 ;Raster Value + Number of Video Rows + Character Size
VICRST  EQU $9004 ;Raster Value  
VICLOC  EQU $9005 ;Screen Memory Location + Character Memory Location
VICLPH  EQU $9006 ;Light Pen - Horizontal
VICLPV  EQU $9007 ;Light Pen - Vertical
VICPD1  EQU $9008 ;Paddle 1
VICPD2  EQU $9009 ;Paddle 2
VICBSF  EQU $900A ;Bass Sound Switch and Frequency
VICASF  EQU $900B ;Alto Sound Switch and Frequency
VICSSF  EQU $900C ;Soprano Sound Switch and Frequency
VICNSF  EQU $900D ;Noise Switch and Frequency
VICAVL  EQU $900E ;Auxillary Color + Sound Volume
VICCLR  EQU $900F ;Screen Color + Reverse Mode + Border Color

;Set Screen Origin and Interlace Mode
;Args: A = Horizontal Screen Origin
;      Y = Vertical Screen Origin
;      X = Interlace Mode, 0=Off, !0=On
SETORG: AND #$7F        ;Clear Accumulator High Bit
        CPX #0          ;Test Interlace Mode
        BEQ SETORH      ;If Not Zero
        ORA #$80        ;  Set Accumulator High Bit
        STA VICSCH      ;Write to Horizontal Origin Register
        STY VICSCV      ;Write Y to Vertical Origin Register
        RTS

;Set Memory Locations and Number of Columns
;Args: A = Screen Memory Address Offset
       Y = Character Address Memory Offset
       X = Number of Video Columns
SETSCN: LSR             ;Roll Screen Offset Low Bit into Carry Flag
        PHA             ;And Save Top Four Bits on Stack
        TXA             ;Copy Columns into Accumulator
        AND #$7F        ;Clear High Bit
        BCC SETSCC      ;If Carry Set from Roll
        ORA #$80        ;  Set High Bit
        STA VICCOL      ;Write to Video Columns Register
        PLA             ;Retrieve Screen Memory Offset
        ASL             ;Shift to High Bit
        ASL
        ASL
        ASL
        PHA             ;And Push onto Stack
        TYA             ;Put Character Address Offset in Accumulator
        AND #$0F        ;Clear High Nybble
        TSX             ;Get Index to Screen Address Offset
        ORA $101,X      ;Load Into Accumulator High Nybble
        STA VICLOC      ;Write to Memory Locations Register
        PLA             ;Clean up Stack
        RTS