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65 lines
1.1 KiB
NASM
65 lines
1.1 KiB
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 $700,x
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inx
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bne .clearRAM
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rts
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; wait for VSYNC to start
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WaitSync:
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bit PPU_STATUS
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bpl WaitSync
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rts
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;;;;; RANDOM NUMBERS
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NextRandom subroutine
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lsr
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bcc .NoEor
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eor #$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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;;;;; CONTROLLER READING
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ReadJoypad0 subroutine
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ldy #0
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ReadJoypadY
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lda #$01
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sta JOYPAD1,y ; set strobe bit
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lsr ; now A is 0
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sta JOYPAD1,y ; clear strobe bit
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ldx #8 ; read 8 bits
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.loop:
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pha ; save A (result)
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lda JOYPAD1,y ; load controller state
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lsr ; bit 0 -> carry
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pla ; restore A (result)
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rol ; carry -> bit 0 of result
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dex ; X = X - 1
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bne .loop ; repeat if X is 0
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rts ; controller bits returned in A
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