###########################################
#
#  Next Test
#
#  Demonstrates the use of the :next
#  operative to refer to the second byte
#  of an instruction. Mainly used for
#  writing self-modifying code.
#
###########################################

: lt  v0 := -1 i := vx save v0 ;
: rt  v0 :=  1 i := vx save v0 ;

: up  v0 := -1 i := vy save v0 ;
: dn  v0 :=  1 i := vy save v0 ;

: main
	va := 2
	vb := 8

	loop
		clear
		i := hex vc
		sprite va vb 5

		:next vx va += 1
		:next vy vb += 1

		if va ==  0 then rt
		if va == 59 then lt
		if vb ==  0 then dn
		if vb == 26 then up
	again