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; "Vertical Auto Scroll" generator
; By Jimmy52905
; Fixed by yoshicookiezeus
; Uses first extra bit: YES
; If extra bit is set, the generator makes the screen scroll downwards instead of upwards
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STOP dcb $10 ; screen number the sprite should stop at when scrolling downwards

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dcb "INIT"
dcb "MAIN"
PHB
PHK
PLB
JSR Main
PLB
RTL

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; Main sprite code
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Main:
LDA $9D ;\ If sprites are locked...
BNE Return ;/ Go to "Return".

LDA $13 ;\
LSR A ; |
BCC Return ;/ Only run code every other frame.

STZ $1412 ; Disable Vertical Scroll so that the code can run properly.

LDA $18B9 ;\ If the first extra bit is set...
AND #$40 ; |
BNE Down ;/ Go to "Down".

STZ $55 ; Make sure that new tiles are uploaded from the top of the screen

LDA $1464 ;\
ORA $1465 ; |If the screen position is at the top of the level...
BEQ Return ;/ Go to "Return".

REP #$20 ; 16-bit (accumulator).
DEC $1464 ; Make the screen scroll upwards automatically.
SEP #$20 ; 8-bit (accumulator).

Return:
RTS ;Return from Subroutine.

Down:
LDA #$02 ;\ Make sure that new tiles are uploaded from the bottom of the screen
STA $55 ;/

LDA $1465 ;\ If the screen position is at screen 10 (last screen of a vertical layer 2 level)...
CMP STOP ; |
BEQ Return ;/ Go to "Return".

REP #$20 ; 16-bit (accumulator).
INC $1464 ; Make the screen scroll downwards automatically.
SEP #$20 ; 8-bit (accumulator).

RTS ;Return from Subroutine.