Routines
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28637: Handler: Move Ship Right
Used by the routine at Handler_ShipMovement.
Moves the player's ship one pixel to the right by shifting the ship graphic and updating the player position. Movement timing is controlled by the movement animation frame counter (counts down from 7 to 0).
Check if the ship is already at the right edge (221) and whether this frame should perform a movement step.
Handler_MoveShip_Right 28637 POP AF Restore the player position from the stack.
28638 CP 221 Compare the player position with 221.
28640 LD A,(26278) Load the movement animation frame counter from MovementAnimationFrameCounter.
28643 JR NZ,MoveShipRight_UpdateCounter Jump to MoveShipRight_UpdateCounter if the ship is not at the right edge.
28645 OR A Jump to Handler_UpdateShipPosition if the movement animation frame counter is zero (skip movement while touching the edge).
28646 JR Z,Handler_UpdateShipPosition
Decrement the movement animation frame counter, wrapping from 0 back to 7. Only when the counter wraps (value 7) is the player position updated.
MoveShipRight_UpdateCounter 28648 OR A Jump to MoveShipRight_ResetCounter if the movement animation frame counter is zero (wrap to 7).
28649 JR Z,MoveShipRight_ResetCounter
28651 DEC A Decrease the movement animation frame counter by one.
28652 JR MoveShipRight_SaveCounter Jump to MoveShipRight_SaveCounter.
MoveShipRight_ResetCounter 28654 LD A,7 Load 7 into the movement animation frame counter.
MoveShipRight_SaveCounter 28656 LD (26278),A Write the updated counter back to *MovementAnimationFrameCounter.
Move the ship only when the counter has just wrapped (value 7).
28659 CP 7 Compare the counter with 7.
28661 LD A,(26349) Load the player position from PlayerAttributeBufferPosition.
28664 JR NZ,MoveShipRight_Setup Jump to MoveShipRight_Setup if the counter is not 7 (no movement on this frame).
28666 INC A Increment the player position by one (move right).
28667 LD (26349),A Write the updated position to *PlayerAttributeBufferPosition.
Prepare the screen position and animate the ship graphic by shifting pixels to the right.
MoveShipRight_Setup 28670 PUSH AF Stash the ship position on the stack.
28671 ADD A,2 Add 2 to A to point to the rightmost ship column.
28673 LD L,A Copy the screen column into L.
28674 LD B,2 Set a height counter in B for 2 rows (ship height).
Process each row of the ship graphic.
MoveShipRight_RowLoop 28676 PUSH BC Stash the row counter and screen position on the stack.
28677 PUSH HL
28678 LD B,8 Set a line counter in B for 8 pixel lines.
Process each pixel line in the current row.
MoveShipRight_LineLoop 28680 PUSH BC Stash the line counter and screen position on the stack.
28681 PUSH HL
28682 LD B,3 Set a width counter in B for 3 bytes (ship width).
Shift each byte of the ship graphic right by one pixel, propagating the carry between adjacent bytes.
MoveShipRight_ByteLoop 28684 SRL (HL) Shift the byte at *HL right.
28686 DEC HL Move to the previous byte in the ship graphic.
28687 BIT 0,(HL) Test bit 0 of the previous byte to see if a bit needs to carry.
28689 RES 0,(HL) Clear bit 0 of that byte ready for the carry.
28691 INC HL Move back to the current byte.
28692 JR Z,MoveShipRight_NextByte Jump to MoveShipRight_NextByte if the width counter has reached zero (no carry to set).
28694 SET 7,(HL) Set bit 7 of the current byte (propagate the carry from the left).
MoveShipRight_NextByte 28696 DEC HL Move to the previous byte again.
28697 DJNZ MoveShipRight_ByteLoop Decrease the width counter by one and loop back to MoveShipRight_ByteLoop until the 3 bytes are shifted.
Clean up after shifting the line.
28699 INC HL Move to the next byte.
28700 RES 7,(HL) Clear bit 7 of that byte (remove any overflow).
28702 POP HL Restore the screen position from the stack.
28703 INC H Move down one pixel line in the screen buffer.
28704 POP BC Restore the line counter from the stack.
28705 DJNZ MoveShipRight_LineLoop Decrease the line counter by one and loop back to MoveShipRight_LineLoop until all 8 lines are processed.
Move to the next row and continue processing.
28707 POP HL Restore the screen position from the stack.
28708 SET 5,L Move to the next row position (set bit 5 of L, add 32).
28710 POP BC Restore the row counter from the stack.
28711 DJNZ MoveShipRight_RowLoop Decrease the row counter by one and loop back to MoveShipRight_RowLoop until both rows are processed.
28713 POP AF Restore the ship position from the stack.
28714 CALL CheckCollision Call the collision detection routine at CheckCollision.
28717 JP Handler_UpdateShipPosition Jump to Handler_UpdateShipPosition.
Continue on to Handler_UpdateShipPosition.
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