Routines
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47417: Prepare Draw 2/ 3 Byte Sprite
Used by the routines at RemoveEntity and DisplayEntity.
Gets the registers ready to draw the sprite of an entity at its old position, which was saved by StoreEntity.
The shift table page is put in B, the screen address of the bottom row is put in HL and the pointer to the pixel data is put in DE. The operand of the jump at DrawOldEntityRow_0 is changed so that the unrolled code that follows it draws the right number of bytes for each row, and the number of rows is stored in Old_Sprite_Row_Count.
Output
B Page of the shift table
C 0
DE Address of the pixel data
HL Screen address of the bottom row
PrepDraw23 47417 CALL GetSpriteA Fetch the address of the sprite data in DE.
47420 LD HL,(38560) Fetch the old X position into L and the old Y position into H from Previous_X_Position.
47423 LD A,L A=L.
47424 DEC A Subtract one.
47425 RLCA Multiply by two (rotate left).
47426 AND %00001110 Keep only bits 1-3.
47428 OR %11110000 Set bits 4-7 to point at the pages 240 to 255.
47430 LD B,A Store the page of the shift table in B.
47431 LD A,(DE) Fetch the width of the sprite in bytes.
47432 INC DE Move on to the height of the sprite.
47433 NEG Negate the width.
47435 AND %00000111 Keep only bits 0-2.
47437 ADD A,A Multiply by four.
47438 ADD A,A
47439 LD C,A Store the result in C.
47440 ADD A,A Multiply by two.
47441 ADD A,C Add the result so that it is multiplied by twelve (the size of the code for one byte of the row).
47442 LD C,A Store the offset in C.
47443 LD A,L A=L.
47444 AND %00000111 Keep only the pixel offset (bits 0-2).
47446 LD A,C Fetch the offset.
47447 JR NZ,PrepDraw23_SetJump Jump to PrepDraw23_SetJump if the sprite does not start on a character boundary.
47449 ADD A,6 Add 6 to the offset to skip the first six bytes of the code for the first byte of the row, as no cell to the left is needed.
PrepDraw23_SetJump 47451 LD (47635),A Change the operand of the jump at DrawOldEntityRow_0.
47454 LD A,(DE) Fetch the height of the sprite.
47455 INC DE Move on to the pixel data.
47456 LD (38563),A Write the number of rows of the old sprite to Old_Sprite_Row_Count.
47459 CALL ScreenAddress Convert the position into the screen address.
47462 LD C,0 C=0.
47464 RET Return.
View the equivalent code in;
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