Krakout: Advance the active ball by its velocity

A4F6: Advance the active ball by its velocity

Advances the active ball one frame. Its pool object moves by the pixels its sub-pixel accumulators earn.
step_ball	A4F6	LD A,(active_ball)	Select the active ball state record: this arm takes ball A.
	A4F9	OR A
	A4FA	JR NZ,step_ball_use_b
	A4FC	LD IY,ball_a_x_accum
	A500	JR step_ball_init_passes
step_ball_use_b	A502	LD IY,ball_b_x_accum	This arm takes ball B.
step_ball_init_passes	A506	LD (IY+$07),$00	Reset its pass counter.
step_ball_pass	A50A	XOR A	Each pass: clear the X-step flag.
	A50B	LD (x_stepped),A	Each pass: clear the X-step flag.
	A50E	LD A,(IY+$02)	Does the X axis move? (a zero threshold means no X motion)
	A511	OR A
	A512	JR Z,step_ball_y_axis
	A514	LD B,$05	Each pass adds a fixed sub-pixel increment to the X accumulator.
step_ball_x_accumulate	A516	INC (IY+$00)	It goes on one unit at a time.
	A519	LD A,(IY+$00)	Each unit is weighed against the threshold.
	A51C	LD C,(IY+$02)
	A51F	CP C
	A520	JR NZ,step_ball_x_loop
	A522	XOR A	At that value, the accumulator wraps and flags an X pixel step.
	A523	LD (IY+$00),A
	A526	INC A
	A527	LD (x_stepped),A
step_ball_x_loop	A52A	DJNZ step_ball_x_accumulate	Repeat until the pass's five sub-pixel units are done.
step_ball_y_axis	A52C	XOR A	Same for the Y axis, on its own accumulator and threshold.
	A52D	LD (y_stepped),A
	A530	LD A,(IY+$03)
	A533	OR A
	A534	JR Z,step_ball_locate_object
	A536	LD B,$05
step_ball_y_accumulate	A538	INC (IY+$01)	Add one sub-pixel unit to the Y accumulator and weigh it against the threshold.
	A53B	LD A,(IY+$01)
	A53E	LD C,(IY+$03)
	A541	CP C
	A542	JR NZ,step_ball_y_loop
	A544	XOR A	At the threshold, the accumulator wraps and flags a Y pixel step.
	A545	LD (IY+$01),A
	A548	INC A
	A549	LD (y_stepped),A
step_ball_y_loop	A54C	DJNZ step_ball_y_accumulate	Repeat until the pass's five sub-pixel units are done.
step_ball_locate_object	A54E	LD A,(active_ball)	Point IX at the ball's screen object.
	A551	OR A
	A552	JR NZ,step_ball_object_b
	A554	LD IX,ball_a_object
	A558	JR step_ball_apply_x
step_ball_object_b	A55A	LD IX,obj2_object	Ball B steers its own record instead.
step_ball_apply_x	A55E	LD A,(x_stepped)	Did X step on the current pass?
	A561	OR A
	A562	JR Z,step_ball_apply_y
	A564	LD A,(IY+$04)	If so, a leftward X sign moves the object one pixel left.
	A567	OR A
	A568	JR NZ,step_ball_x_positive
	A56A	DEC (IX+$00)
	A56D	JR step_ball_apply_y
step_ball_x_positive	A56F	INC (IX+$00)	A rightward sign moves it right.
step_ball_apply_y	A572	LD A,(y_stepped)	Did Y step on the current pass?
	A575	OR A
	A576	JR Z,step_ball_next_pass
	A578	LD A,(IY+$05)	If so, an upward Y sign moves it one pixel up.
	A57B	OR A
	A57C	JR NZ,step_ball_y_positive
	A57E	DEC (IX+$01)
	A581	JR step_ball_next_pass
step_ball_y_positive	A583	INC (IX+$01)	A downward sign moves it down.
step_ball_next_pass	A586	INC (IY+$07)	Count the pass and loop until passes are done.
	A589	LD A,(IY+$07)
	A58C	LD C,(IY+$06)
	A58F	CP C
	A590	RET Z
	A591	JP step_ball_pass

Fork me on GitHub

Routines