Routines
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30018: Prepare Old Sprite
Used by the routine at SetUpPreviousSprite.
Finds the graphic for the previous state of an object, using the copy of its X position and state that StoreEntity saved at Actor. The rest of the routine, from ActorGetSpriteAddress, is shared with ActorMoveSprite.
The sprite is looked up in SpritesTable (for types 1-3) or SpritesTable_Other (for types 4-13). The entry is chosen by the object type (the state with bits 6 and 7 removed), by the X position within a character cell and by bit 6 of the state, which selects the second set of sprites.
Output
DE Address of the sprite
PrepareOldSprite 30018 LD A,(24075) A=the saved X position (from Actor).
30021 AND %00000110 Keep only bits 1-2 (the X position within a character cell, in steps of two pixels).
30023 LD C,A Keep the X position bits in C.
30024 LD A,(24077) A=the saved state (from 24077).
This entry point is used by the routine at ActorMoveSprite.
Find the sprite from the state in A and the X position bits in C.
ActorGetSpriteAddress 30027 BIT 6,A Jump to ActorGetSpriteAddress_Type if bit 6 of the state is clear.
30029 JR Z,ActorGetSpriteAddress_Type
30031 SET 3,C Set bit 3 of C to select the second set of sprites.
ActorGetSpriteAddress_Type 30033 DEC A Decrease the state by one (so that type 1 becomes 0).
30034 CP 3 Jump to ActorGetSpriteAddress_OtherTypes if the result is 3 or more.
30036 JR NC,ActorGetSpriteAddress_OtherTypes
Types 1-3 have eight sprites each (four X positions for each of the two directions).
30038 RLCA Multiply the type by 16 (the length of a type's entries in the table).
30039 RLCA
30040 RLCA
30041 RLCA
30042 AND %11110000 Keep only bits 4-7.
30044 OR C Add in the X position and direction bits from C.
30045 LD HL,30234 HL=SpritesTable.
ActorGetSpriteAddress_Lookup 30048 LD C,A Store the offset in C.
30049 LD B,0 B=0.
30051 ADD HL,BC HL=the table address + the offset.
30052 LD E,(HL) Fetch the sprite address into DE.
30053 INC HL
30054 LD D,(HL)
30055 RET Return.
Types 4 and above have four sprites each, which only change with bit 2 of the X position and with the direction.
ActorGetSpriteAddress_OtherTypes 30056 SUB 3 Subtract 3 from the type (it is already one less than the real type), so that type 4 becomes 0.
30058 RLCA Multiply the type by 8 (the length of a type's entries in the table).
30059 RLCA
30060 RLCA
30061 AND %11111000 Keep only bits 3-7.
30063 LD B,A Store the result in B.
30064 LD A,C A=C.
30065 RRCA Move the X position bits and the direction bit down by one place.
30066 AND %00000110 Keep only bits 1-2.
30068 OR B Add in the type offset.
30069 LD HL,30282 HL=SpritesTable_Other.
30072 JR ActorGetSpriteAddress_Lookup Jump to ActorGetSpriteAddress_Lookup.
View the equivalent code in;
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