Krakout: Advance an object one step along its heading

CA54: Advance an object one step along its heading

Moves an object one step along its heading. It also publishes the heading code for that heading in the per-slot heading-code array.

Input
IY	the object's logic record (obj2_record), read for its heading and speed
IX	the object's sprite record (object_table), whose position is advanced
C	the object's pool-slot index, which selects its heading-code cell in bat_heading

Two jobs per frame. First publish the heading code. The object's heading is mapped through the heading-code table to a heading code in the ball-angle code set. The code goes in the object's cell of the heading-code array. For the ball gem that cell is ball B's heading code. Then, unless the move-freeze flag is set, advance the sprite one step along the heading.

The pixel step equals the movement speed for speeds 0 and 1, and is one less for any greater speed.

The heading chooses which axes move. Rightward headings move X right and leftward headings move it left. Downward headings move Y down and upward headings move it up. So the pure left and right headings move horizontally only, and the pure up and down headings vertically only. The diagonal headings move by the same step on both axes.

move_object

CA54

LD A,(IY+$00)

Publish the heading code: map the heading to its heading code and store it in the per-slot array at the object's index C.

CA57

LD E,A

CA58

LD D,$00

CA5A

LD HL,heading_dir_table

CA5D

ADD HL,DE

CA5E

LD A,(HL)

CA5F

LD HL,bat_heading

CA62

LD E,C

CA63

ADD HL,DE

CA64

LD (HL),A

CA65

LD A,(move_freeze)

Skip all movement while the move-freeze flag is set; the heading code above is still published.

CA68

OR A

CA69

RET NZ

CA6A

LD A,(IY+$01)

Take the movement speed as the pixel step, less one above the two slowest speeds.

CA6D

LD (object_step),A

CA70

OR A

CA71

JR Z,move_object_apply_x

CA73

CP $01

CA75

JR Z,move_object_parity

CA77

SUB $01

CA79

LD (object_step),A

CA7C

JR move_object_apply_x

move_object_parity

CA7E

LD A,($0002)

BUG: at the slowest movement speed, the code tries to move only on alternate frames via a parity flag. But the flag address $0002 is ROM, so the store is discarded and the RET Z never fires. The pixel step keeps its value, and the object still moves a pixel every frame.

CA81

XOR $01

CA83

LD ($0002),A

CA86

RET Z

move_object_apply_x

CA87

LD A,(IY+$00)

Move along X by the heading; 5, 6 and 7 run leftward.

CA8A

CP $05

CA8C

JR Z,move_object_x_sub

CA8E

CP $06

CA90

JR Z,move_object_x_sub

CA92

CP $07

CA94

JR NZ,move_object_x_right

move_object_x_sub

CA96

LD A,(object_step)

Those take the step off the position.

CA99

LD E,A

CA9A

LD A,(IX+$00)

CA9D

SUB E

CA9E

LD (IX+$00),A

CAA1

JR move_object_apply_y

move_object_x_right

CAA3

CP $01

Not that heading; the rightward group is the next three.

CAA5

JR Z,move_object_x_add

CAA7

CP $02

CAA9

JR Z,move_object_x_add

CAAB

CP $03

CAAD

JR NZ,move_object_apply_y

move_object_x_add

CAAF

LD A,(object_step)

Those put the step onto the position.

CAB2

ADD A,(IX+$00)

CAB5

LD (IX+$00),A

move_object_apply_y

CAB8

LD A,(IY+$00)

Move along Y by the heading; 7, 0 and 1 run upward.

CABB

CP $07

CABD

JR Z,move_object_y_sub

CABF

CP $00

CAC1

JR Z,move_object_y_sub

CAC3

CP $01

CAC5

JR NZ,move_object_y_down

move_object_y_sub

CAC7

LD A,(object_step)

Those take the step off the position.

CACA

LD E,A

CACB

LD A,(IX+$01)

CACE

SUB E

CACF

LD (IX+$01),A

CAD2

RET

move_object_y_down

CAD3

CP $03

The downward group is the next three.

CAD5

JR Z,move_object_y_add

CAD7

CP $04

CAD9

JR Z,move_object_y_add

CADB

CP $05

CADD

RET NZ

move_object_y_add

CADE

LD A,(object_step)

Those put the step onto the position.

CAE1

ADD A,(IX+$01)

CAE4

LD (IX+$01),A

CAE7

RET

heading_dir_table

CAE8

DEFB $60

The heading code for each of the eight headings

CAE9

DEFB $57

CAEA

DEFB $4E

CAEB

DEFB $8D

CAEC

DEFB $84

CAED

DEFB $7B

CAEE

DEFB $72

CAEF

DEFB $69

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Routines