This code will stop the screen from scrolling past a specific
point, useful to make the screen not to show "unessary" empty
areas in a level. Unlike other "scroll stoppers" this one
doesn't trap the screen, rather allow the screen to go back,
It also has a bug fix that if the screen is moving too fast,
it stops the screen at the exact pixel position, so the screen
isn't slightly past the stopping point while its locked in place.
It almost works exactly like smw's area where the screen won't go
past.

You could even make a "one way" screen. Example,
using "scroll left border" on screen boundry $01 while the
player's screen spawns at $00, if the player goes past that
line, the player can't go back to $00.

keep in mind that if the lock animations and sprite flag
($7E:009D) is being set other than by L/R scroll, player
getting hurt, player getting a powerup, yoshi haching from
his egg or any other smw's freeze effect causes the game
to unable to reset the freeze flag back zero again, causing
$9D to be stuck at a non-zero value and freezing the game
entirely. You'll have to use STZ $9D to cancel the freeze
on certin conditions, otherwise motion keeps playing even
when its not suppose to (like things still animate as mario
getting a powerup).

;-----------------------stop screen from scrolling right----------------------;

!stop_right_scroll = $01		;>screen number to stop on.


	REP #$20			;>begin 16 bit value
	LDA $1462			;\so if the screen skips a pixel, still
	CMP #!stop_right_scroll*$100	;|detect and locks the screen
	BCC freescreen			;|
	CMP #!stop_right_scroll*$100+10	;|
	BCS freescreen			;/
	LDA $7E				;\Allow screen to go back even if its
	CMP $142C			;|is locked in place. (can go left)
	BCC unlock_screen		;/
	LDA #!stop_right_scroll*$100	;\not only lock the screen, also if a pixel or
	STA $1462			;/2 are off, set it to be the correct position
	SEP #$20			;>end 16 bit value
	STZ $1411			;>and stop the screen
	LDA $13FE			;\if L/R scroll...
	BNE cancel_freeze		;/then cancel freeze
	BRA done_scroll
cancel_freeze:
	LDA $71
	BNE done_scroll
	STZ $9D				;>found a way to stop permanent freeze by l/r scroll
	STZ $1401			;and remove akward effects, like it keeps scrolling
	STZ $13FD			;right when mario head back after L scroll in border
	STZ $13FE			;>also glitches like fade to map instantly as soon mario dies
	STZ $1400
	BRA done_scroll
freescreen:
	SEP #$20
	BRA done_scroll
unlock_screen:
	SEP #$20
	LDA #$01
	STA $1411
done_scroll:
	RTS
;-------------------------stop screen from scrolling left---------------------;

!stop_left_scroll = $01			;>screen number to stop on.

	REP #$20			;>begin 16 bit value
	LDA $1462			;\so if the screen skips a pixel, still
	CMP #!stop_left_scroll*$100-10	;|detect and locks the screen
	BCC freescreen			;|
	CMP #!stop_left_scroll*$100+1	;|
	BCS freescreen			;/
	LDA $7E				;\Allow screen to go back even if its
	CMP $142E			;|locked in place (can go right)
	BCS unlock_screen		;/
	LDA #!stop_left_scroll*$100	;\and set the locked screen in the
	STA $1462			;/correct pixel-position
	SEP #$20			;>end 16 bit value
	STZ $1411			;>and stop the screen
	LDA $13FE			;\if L/R scroll...
	BNE cancel_freeze		;/then cancel freeze
	BRA done_scroll
cancel_freeze:
	LDA $71
	BNE done_scroll
	STZ $9D				;>found a way to stop permanent freeze by l/r scroll
	STZ $1401			;and remove akward effects, like it keeps scrolling
	STZ $13FD			;right when mario head back after R scroll in border
	STZ $13FE			;>also glitches like fade to map instantly as soon mario dies
	STZ $1400
	BRA done_scroll
freescreen:
	SEP #$20
	BRA done_scroll
unlock_screen:
	SEP #$20
	LDA #$01
	STA $1411
done_scroll:
	RTS
;------------------------stop screen from scrolling down----------------------;

!stop_down_scroll = $01			;>screen number to stop on.
!scroll_up_unlock = $0064		;>if you change $00:F806 (0x07A06), then you
					;should change this also (must be 4 digits).

	REP #$20			;>begin 16 bit value
	LDA $1464			;\so if the screen skips a pixel, still
	CMP #!stop_down_scroll*$100	;|detect and locks the screen
	BCC freescreen			;|
	CMP #!stop_down_scroll*$100+10	;|
	BCS freescreen			;/
	LDA $80				;\Allow screen to go back even if its
	CMP #!scroll_up_unlock		;|lock in place (can go up)
	BCC unlock_screen		;/
	LDA #!stop_down_scroll*$100	;\and set the locked screen in the
	STA $1464			;/correct pixel-position
	SEP #$20			;>end 16 bit value
	STZ $1412			;>and stop the screen
	BRA done_scroll
freescreen:
	SEP #$20
	BRA done_scroll
unlock_screen:
	SEP #$20
	LDA #$01
	STA $1412
done_scroll:
	RTS
;-----------------------stop screen from scrolling up-------------------------;

!stop_up_scroll = $01			;>screen number to stop on.
!scroll_down_unlock = $007D		;>how low mario must be to move the screen down
					;(must be 4 digits).

	REP #$20			;>begin 16 bit value
	LDA $1464			;\so if the screen skips a pixel, still
	CMP #!stop_up_scroll*$100-10	;|detect and locks the screen
	BCC freescreen			;|
	CMP #!stop_up_scroll*$100+1	;|
	BCS freescreen			;/
	LDA $80				;\Allow screen to go back even if its
	CMP #!scroll_down_unlock	;|lock in place (can go down)
	BCS unlock_screen		;/
	LDA #!stop_up_scroll*$100	;\and set the locked screen in the
	STA $1464			;/correct pixel-position
	SEP #$20			;>end 16 bit value
	STZ $1412			;>and stop the screen
	BRA done_scroll
freescreen:
	SEP #$20
	BRA done_scroll
unlock_screen:
	SEP #$20
	LDA $81				;\counts as "more than scroll_down_unlock"
	BMI done_scroll			;/because above screen = go past the line (being negative)
	LDA #$01
	STA $1412
done_scroll:
	RTS