mirror of
https://github.com/sehugg/8bitworkshop.git
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60 lines
799 B
NASM
60 lines
799 B
NASM
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;;;;; SUBROUTINES
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ClearRAM: subroutine
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lda #0
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tax
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.clearRAM
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sta $0,x
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cpx #$fe ; don't clear last 2 bytes of stack
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bcs .skipStack
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sta $100,x
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.skipStack
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; skip $200-$2FF, used for OAM display list
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sta $300,x
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sta $400,x
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sta $500,x
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sta $600,x
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sta $700,x
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inx
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bne .clearRAM
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rts
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; fill video RAM
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FillVRAM: subroutine
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txa
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ldy #$20
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sty PPU_ADDR
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sta PPU_ADDR
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ldy #$10
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.loop:
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sta PPU_DATA
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adc #7
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inx
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bne .loop
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dey
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bne .loop
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rts
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; wait for VSYNC to start
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WaitSync:
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bit PPU_STATUS
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bpl WaitSync
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rts
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;;;;; RANDOM NUMBERS
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NextRandom subroutine
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lsr
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bcc .NoEor
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eor #$d4
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.NoEor:
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rts
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; Get previous random value
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PrevRandom subroutine
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asl
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bcc .NoEor
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eor #$a9
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.NoEor:
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rts
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