Routines
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53287: Print Sprite
Used by the routines at PrintSprites, DisplayObjectsCarried and PrintTableSprite.
Draws the sprite of an object into ScreenBuffer, at the pixel position of the object. A sprite has a width byte (the vertical flip in bit 7, the horizontal flip in bit 6 and the width in columns in the low bits), a height byte, and then the data for each line from the top. Each column of a line is a pair of bytes: a mask byte, then a data byte. The mask clears the bits of the background and the data sets bits. A sprite that does not start on a byte boundary has each pair shifted across two bytes of the buffer, with the tables built by BuildLookupTables. The routine copies the sprite with unrolled code, which it enters part of the way along, so that it does as many columns as the sprite has. To do this it changes the code, and it uses the stack pointer to read the data of the sprite.
Input
IX The address of the object
Find the sprite and choose the draw path.
PrintSprite 53287 CALL GetSpriteAddress Call GetSpriteAddress.
53290 LD A,(IX+26) Fetch the pixel x position.
53293 AND %00000111 Keep bits 0 to 2.
53295 JR Z,PrintSprite_Aligned Jump to PrintSprite_Aligned if it is zero.
Choose the page of the shift table.
53297 RLCA Double it.
53298 AND %00001110 Keep bits 1 to 3.
53300 OR %11110000 Set bits 4 to 7.
53302 LD H,A Copy it to H.
53303 LD A,(DE) Fetch the width byte of the sprite.
53304 INC DE Increment DE.
53305 AND %00000111 Keep bits 0 to 2.
53307 INC A Increment A.
53308 LD B,A Copy it to B.
53309 LD (IX+24),A Set the width in bytes of the object.
53312 DEC A Decrement A.
53313 AND %00000111 Keep bits 0 to 2.
53315 ADD A,A Multiply it by sixteen.
53316 ADD A,A
53317 ADD A,A
53318 ADD A,A
53319 NEG Negate it.
53321 ADD A,80 Add 80.
Change the jump and the step.
PrintSprite_Patch 53323 LD (53436),A Write it to 53436.
53326 LD A,B Fetch B.
53327 CPL Invert it.
53328 ADD A,34 Add 34.
53330 LD (53520),A Write it to 53520.
Clip the sprite to the top of the screen.
53333 LD A,(DE) Fetch the height of the sprite.
53334 INC DE Increment DE.
53335 LD (IX+25),A Set the height in lines of the object.
53338 ADD A,(IX+27) Add the pixel y position.
53341 SUB 192 Subtract 192.
53343 JR C,PrintSprite_Draw Jump to PrintSprite_Draw if carry is set.
53345 NEG Negate A.
53347 ADD A,(IX+25) Add the height in lines.
53350 LD (IX+25),A Set the height in lines of the object.
Draw the rows that are on the screen.
PrintSprite_Draw 53353 LD C,(IX+26) Fetch the pixel x position.
53356 LD B,(IX+27) Fetch the pixel y position.
53359 CALL CalculateBufferAddress Call CalculateBufferAddress.
53362 LD (23305),SP Set SavedStackPointer.
53366 EX DE,HL Point the stack at the sprite data.
53367 LD SP,HL
53368 EX DE,HL
53369 LD A,(IX+25) Fetch the number of lines.
53372 JR DrawLines Jump to DrawLines.
The sprite is aligned, so the bytes do not need to be shifted.
PrintSprite_Aligned 53374 LD A,(DE) Fetch the width byte of the sprite.
53375 INC DE Increment DE.
53376 AND %00001111 Keep bits 0 to 3.
53378 LD (IX+24),A Set the width in bytes of the object.
53381 LD B,A Copy it to B.
53382 ADD A,A Multiply it by eight.
53383 ADD A,A
53384 ADD A,A
53385 NEG Negate it.
53387 SUB 6 Subtract 6.
53389 JR PrintSprite_Patch Jump to PrintSprite_Patch.
View the equivalent code in;
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