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Routines |
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The handler of the object types 30, A0 and A4, which are creatures that chase the player. Each frame the creature speeds up a little towards the player along each axis, bounces off anything that it hits along x or y, and moves.
The speeds that the creature works out are kept in the stored speeds at 14 to 16 of its record in sixteenths, which gives it finer control than the whole numbers that ApplyGravityAndMove uses. The speeds that are used for moving it, in 09 to 0B, are the stored speeds rounded to whole numbers, which can be from FC to 04.
The lowest two bits of the type of the creature are the number of the picture of it, and are changed each frame to make it move.
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Move the creature and bounce off anything that it hits.
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| RunHomingCreature | CC4B | CALL SetSpriteOffset_Minus12_Minus6 | Call SetSpriteOffset_Minus12_Minus6 to set the offsets of the sprite. | |||||
| CC4E | LD (IX+$04),$0A | Set the half size of the creature along x to 0A. | ||||||
| CC52 | LD (IX+$05),$0A | Set the half size of the creature along y to 0A. | ||||||
| CC56 | CALL ApplyGravityAndMove | Call ApplyGravityAndMove to apply gravity to the creature and move it. | ||||||
| CC59 | LD A,(IX+$0C) | Fetch the collision flags. | ||||||
| CC5C | AND %00000111 | Keep the flags for hitting something along x, along y and landing. | ||||||
| CC5E | JR Z,RunHomingCreature_Steer | Jump to RunHomingCreature_Steer if the creature did not hit anything. | ||||||
| CC60 | AND %00000001 | Keep the flag for hitting something along x. | ||||||
| CC62 | CALL NZ,ReverseSpeedX | Call ReverseSpeedX to turn the creature around along x, if it did. | ||||||
| CC65 | BIT 1,(IX+$0C) | Test the flag for hitting something along y. | ||||||
| CC69 | CALL NZ,ReverseSpeedY | Call ReverseSpeedY to turn the creature around along y, if it did. | ||||||
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Steer towards the player along x, y and z.
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| RunHomingCreature_Steer | CC6C | LD A,($A770) | Fetch the x position of the player. | |||||
| CC6F | SUB (IX+$01) | Subtract the x position of the creature. | ||||||
| CC72 | LD A,(IX+$14) | Fetch the stored speed of the creature along x. | ||||||
| CC75 | JR NC,RunHomingCreature_ForwardsX | Jump to RunHomingCreature_ForwardsX to speed up forwards if the player is further along. | ||||||
| CC77 | CALL SpeedUpBackwards | Call SpeedUpBackwards to speed up backwards. | ||||||
| RunHomingCreature_StoreX | CC7A | LD (IX+$14),A | Store the speed along x. | |||||
| CC7D | LD A,($A771) | Fetch the y position of the player. | ||||||
| CC80 | SUB (IX+$02) | Subtract the y position of the creature. | ||||||
| CC83 | LD A,(IX+$15) | Fetch the stored speed of the creature along y. | ||||||
| CC86 | JR NC,RunHomingCreature_ForwardsY | Jump to RunHomingCreature_ForwardsY to speed up forwards if the player is further along. | ||||||
| CC88 | CALL SpeedUpBackwards | Call SpeedUpBackwards to speed up backwards. | ||||||
| RunHomingCreature_StoreY | CC8B | LD (IX+$15),A | Store the speed along y. | |||||
| CC8E | LD A,($A792) | Fetch the z position of the body of the player. | ||||||
| CC91 | SUB (IX+$03) | Subtract the z position of the creature. | ||||||
| CC94 | LD A,(IX+$16) | Fetch the stored speed of the creature along z. | ||||||
| CC97 | JR NC,RunHomingCreature_ForwardsZ | Jump to RunHomingCreature_ForwardsZ to speed up forwards if the player is higher. | ||||||
| CC99 | CALL SpeedUpBackwards | Call SpeedUpBackwards to speed up backwards. | ||||||
| RunHomingCreature_StoreZ | CC9C | LD (IX+$16),A | Store the speed along z. | |||||
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Round the stored speeds to whole numbers.
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| CC9F | LD B,$03 | Set a counter for the three speeds in B. | ||||||
| RunHomingCreature_RoundSpeeds | CCA1 | INC IX | Move IX on by one byte, so that the stored speed is at 13 and the speed to move at is at 08, for each speed in turn. | |||||
| CCA3 | LD A,(IX+$13) | Fetch the stored speed. | ||||||
| CCA6 | ADD A,$08 | Add half of the sixteen that is to be divided by, which rounds the result. | ||||||
| CCA8 | SRA A | Divide it by sixteen, keeping the sign. | ||||||
| CCAA | SRA A | |||||||
| CCAC | SRA A | |||||||
| CCAE | SRA A | |||||||
| CCB0 | LD (IX+$08),A | Store it as the speed to move at. | ||||||
| CCB3 | DJNZ RunHomingCreature_RoundSpeeds | Jump to RunHomingCreature_RoundSpeeds until the three speeds have been set. | ||||||
| CCB5 | DEC IX | Put the record pointer back. | ||||||
| CCB7 | DEC IX | |||||||
| CCB9 | DEC IX | |||||||
| CCBB | JR RunHomingCreature_Animate | Jump to RunHomingCreature_Animate to change the picture of the creature. | ||||||
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Speed up forwards along the axis that the player is further along.
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| RunHomingCreature_ForwardsX | CCBD | CALL SpeedUpForwards | Call SpeedUpForwards to speed up forwards. | |||||
| CCC0 | JR RunHomingCreature_StoreX | Jump to RunHomingCreature_StoreX to store the speed. | ||||||
| RunHomingCreature_ForwardsY | CCC2 | CALL SpeedUpForwards | Call SpeedUpForwards to speed up forwards. | |||||
| CCC5 | JR RunHomingCreature_StoreY | Jump to RunHomingCreature_StoreY to store the speed. | ||||||
| RunHomingCreature_ForwardsZ | CCC7 | CALL SpeedUpForwards | Call SpeedUpForwards to speed up forwards. | |||||
| CCCA | JR RunHomingCreature_StoreZ | Jump to RunHomingCreature_StoreZ to store the speed. | ||||||
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Cycle the picture of the creature.
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| RunHomingCreature_Animate | CCCC | INC (IX+$10) | Add one to the picture counter of the creature. | |||||
| CCCF | LD A,(IX+$10) | Fetch it. | ||||||
| CCD2 | AND %00000011 | Keep the lowest two bits. | ||||||
| CCD4 | LD B,A | Put them in B. | ||||||
| CCD5 | LD A,(IX+$00) | Fetch the type of the creature. | ||||||
| CCD8 | AND %11111100 | Keep the type without its lowest two bits. | ||||||
| CCDA | ADD A,B | Add the picture number. | ||||||
| CCDB | LD (IX+$00),A | Set it as the type of the creature, which carries on with SetWipeAndDrawFlags. | ||||||
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