;
; UFRGS - INF - M Johann - 2023 - johann@inf.ufrgs.br
; Small up/down counter that appears as cycle,
; back and forth path on PocketRamses data view

; The trick here is to maintain everything on registers
; so no extra mem access changes memory view
; It would be shorter just with a sequence of 24 writes

; ----- DATA 

		org	h80
screen:		dab	32,32,32,32,32,32,32,32,32,32,32,32

		
; ----- BEGIN MAIN CODE 

		org	0
startup:	LDR	A,#111		; lowercase o
		LDR	B,#32		; lowercase o

initup:		LDR	X,#screen		; position to write
		STR	A,0,X
		STR	B,screen+1
		NOP
		NOP
		NOP
		NOP

loop1:		ADD 	X,#2
		SUB	X,#screen+#12
		JZ	initdown
		ADD	X,#screen+#12
		STR	A,0,X
		SUB	X,#2
		STR	B,0,X
		ADD	X,#2
		JMP	loop1

initdown:	LDR	X,#screen+#11
		STR	A,0,X
		STR	B,screen+10
		NOP
		NOP
		NOP
		NOP

loop2:		SUB 	X,#2
		SUB	X,#screen-#1
		JZ	initup
		ADD	X,#screen-#1
		STR	A,0,X
		ADD	X,#2
		STR	B,0,X
		SUB	X,#2
		JMP	loop2
		
; END OF FILE