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Routines |
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Used by the routines at print_lives_gfx, multiple_print_sprite, display_object, display_frame and transfer_sprite_and_print.
A sprite is a header -- width in bytes in bits 0 to 3, the flip state in bits 6 and 7, then the height in rows -- and then a pair of bytes for every byte of every row: a mask and an image. Rows are drawn from the bottom up. Each background byte is cleared where the mask is set and then has the image put in, so a sprite punches its own shape out of whatever was drawn before it and objects can overlap in depth.
The row loop at sprite_row is unrolled for the widest sprite, five bytes, and this routine patches it for the one in hand: the offset of the JR at sprite_row_jump decides how far into the unrolled code a row starts, and the operand of the ADD at sprite_next_row how far BC must then move to reach the next row up. There are two unrolled runs. A sprite whose pixel x is a multiple of 8 uses the plain one; any other uses the shifted one, which reads its bytes through the tables from shift_tbls up and so touches one byte more per row.
The stack pointer is borrowed to read the sprite: SP is pointed at the data and each POP DE fetches a mask byte into E and its image byte into D. The real SP is kept in $5BA9 meanwhile. The listing has no EI anywhere, so the game runs with interrupts off and nothing can push onto the sprite while SP is borrowed.
Bytes $18 and $19 come out as the width and height actually drawn, which wipe_next_object uses next frame to wipe the picture away. Only the top of the screen is clipped; nothing is clipped at the sides.
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| print_sprite | D718 | CALL flip_sprite | DE = the sprite, turned the way byte 7 asks | |||||||
| D71B | LD A,(IX+$1A) | x within its byte; 0 goes to the aligned case at sprite_aligned | ||||||||
| D71E | AND $07 | |||||||||
| D720 | JR Z,sprite_aligned | |||||||||
| D722 | RLCA | H = $F0 + 2 * the shift, the page pair build_lookup_tbls built for it | ||||||||
| D723 | AND $0E | |||||||||
| D725 | OR $F0 | |||||||||
| D727 | LD H,A | |||||||||
| D728 | LD A,(DE) | the width from the header, plus one: the bytes a shifted row touches, into byte $18 | ||||||||
| D729 | INC DE | |||||||||
| D72A | AND $07 | |||||||||
| D72C | INC A | |||||||||
| D72D | LD B,A | |||||||||
| D72E | LD (IX+$18),A | |||||||||
| D731 | DEC A | the JR offset for sprite_row_jump: 16 bytes of code per byte of width, counted back from the end of the shifted run | ||||||||
| D732 | AND $07 | |||||||||
| D734 | ADD A,A | |||||||||
| D735 | ADD A,A | |||||||||
| D736 | ADD A,A | |||||||||
| D737 | ADD A,A | |||||||||
| D738 | NEG | |||||||||
| D73A | ADD A,$50 | |||||||||
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