Routines
Prev: 36567 Up: Map Next: 36710
36591: Update Velocity
Used by the routine at ControlPlayer.
Changes the velocities of an entity that is controlled by ControlPlayer, and works out how many whole pixels it will move this frame. Each velocity (at offsets 6 and 7) has the change added, and is then limited to the fastest speed in either direction. The top four bits of the velocity are the pixels to move, which is GetWholePixels. While the steps of an automatic walk are being counted down by AutoWalkStep (the bottom four bits of offset 2) the pixels moved are 0 or 2 instead, depending on whether the velocity is below 16 with the sign of the velocity.
Input
DE The change in each velocity (X in E and Y in D)
HL The fastest velocity (X in L and Y in H)
IX The entity
Output
DE The pixels to move (X in E and Y in D)
UpdateVelocity 36591 LD A,(IX+2) Fetch offset 2 of the entity.
36594 AND %00001111 Keep only bits 0-3.
36596 JR NZ,UpdateVelocity_Walk Jump to UpdateVelocity_Walk if an automatic walk is under way.
Change the X velocity, and keep it within the fastest speed.
36598 LD A,E Fetch the change in the X velocity from E.
36599 ADD A,(IX+6) Add the X velocity.
36602 JP M,UpdateVelocity_NegativeX Jump to UpdateVelocity_NegativeX if the result is negative.
36605 CP L Jump to UpdateVelocity_SetX if the X velocity is slower than the fastest X velocity.
36606 JR C,UpdateVelocity_SetX
36608 LD A,L Use the fastest X velocity.
UpdateVelocity_SetX 36609 LD (IX+6),A Update the X velocity.
Change the Y velocity, and keep it within the fastest speed.
36612 LD A,D Fetch the change in the Y velocity from D.
36613 ADD A,(IX+7) Add the Y velocity.
36616 JP M,UpdateVelocity_NegativeY Jump to UpdateVelocity_NegativeY if the result is negative.
36619 CP H Jump to UpdateVelocity_SetY if the Y velocity is slower than the fastest Y velocity.
36620 JR C,UpdateVelocity_SetY
36622 LD A,H Use the fastest Y velocity.
UpdateVelocity_SetY 36623 LD (IX+7),A Update the Y velocity.
Get the whole pixels from the velocities.
36626 LD A,(IX+6) Fetch the X velocity.
36629 CALL GetWholePixels Call GetWholePixels to convert the velocity to whole pixels.
36632 LD E,A Keep the whole pixels of the X velocity in E.
36633 LD A,(IX+7) Fetch the Y velocity.
36636 CALL GetWholePixels Call GetWholePixels to convert the velocity to whole pixels.
36639 LD D,A Keep the whole pixels of the Y velocity in D.
36640 RET Return.
The X velocity is negative, so limit it using its size.
UpdateVelocity_NegativeX 36641 NEG Make the X velocity positive.
36643 CP L Jump to UpdateVelocity_NegateX if the X velocity is slower than the fastest X velocity.
36644 JR C,UpdateVelocity_NegateX
36646 LD A,L Use the fastest X velocity.
UpdateVelocity_NegateX 36647 NEG Make the X velocity negative again.
36649 JR UpdateVelocity_SetX Jump to UpdateVelocity_SetX to set the X velocity.
The Y velocity is negative, so limit it using its size.
UpdateVelocity_NegativeY 36651 NEG Make the Y velocity positive.
36653 CP H Jump to UpdateVelocity_NegateY if the Y velocity is slower than the fastest Y velocity.
36654 JR C,UpdateVelocity_NegateY
36656 LD A,H Use the fastest Y velocity.
UpdateVelocity_NegateY 36657 NEG Make the Y velocity negative again.
36659 JR UpdateVelocity_SetY Jump to UpdateVelocity_SetY to set the Y velocity.
An automatic walk is under way, so move by two pixels on an axis if its velocity is 16 or more, or else not at all.
UpdateVelocity_Walk 36661 LD A,(IX+6) Fetch the X velocity.
36664 AND A Jump to UpdateVelocity_WalkNegativeX if the X velocity is negative.
36665 JP M,UpdateVelocity_WalkNegativeX
36668 AND %11110000 Keep only bits 4-7.
36670 JR Z,UpdateVelocity_WalkY Jump to UpdateVelocity_WalkY if the result is zero.
36672 LD A,2 A=2 (move two pixels right).
UpdateVelocity_WalkY 36674 LD E,A Keep the X move in E.
36675 LD A,(IX+7) Fetch the Y velocity.
36678 AND A Jump to UpdateVelocity_WalkNegativeY if the Y velocity is negative.
36679 JP M,UpdateVelocity_WalkNegativeY
36682 AND %11110000 Keep only bits 4-7.
36684 JR Z,UpdateVelocity_WalkEnd Jump to UpdateVelocity_WalkEnd if the result is zero.
36686 LD A,2 A=2 (move two pixels down).
UpdateVelocity_WalkEnd 36688 LD D,A Keep the Y move in D.
36689 RET Return.
UpdateVelocity_WalkNegativeX 36690 NEG Make the X velocity positive.
36692 AND %11110000 Keep only bits 4-7.
36694 JR Z,UpdateVelocity_WalkY Jump to UpdateVelocity_WalkY if the result is zero.
36696 LD A,254 A=-2 (move two pixels left).
36698 JR UpdateVelocity_WalkY Jump to UpdateVelocity_WalkY to do the walk on the Y axis.
UpdateVelocity_WalkNegativeY 36700 NEG Make the Y velocity positive.
36702 AND %11110000 Keep only bits 4-7.
36704 JR Z,UpdateVelocity_WalkEnd Jump to UpdateVelocity_WalkEnd if the result is zero.
36706 LD A,254 A=-2 (move two pixels up).
36708 JR UpdateVelocity_WalkEnd Jump to UpdateVelocity_WalkEnd to finish the walk.
Prev: 36567 Up: Map Next: 36710