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Routines |
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Used by the routine at upd_55.
One routine serves both axes by patching the displacement bytes of two IX instructions: the read of the position at shuttle_block_step and the write of the step at shuttle_block_move.
The block follows the frame counter. Folding the counter at bit 4 gives a triangle wave, 0 up to 15 and back down over 32 frames, and the block steps one unit a frame towards that position within a 16-unit span. Blocks in odd-numbered object records add 16 to the counter first, which puts them half a cycle out of step with their even-numbered neighbours: one is at the far end while the next is at the near one.
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| shuttle_block | B6BF | LD A,H | patch the two displacements | |||||||
| B6C0 | LD ($B6E0),A | |||||||||
| B6C3 | LD A,L | |||||||||
| B6C4 | LD ($B6F1),A | |||||||||
| B6C7 | CALL upd_6_7 | pixel offsets | ||||||||
| B6CA | PUSH IX | C = 16 if IX is an odd-numbered record, else 0 | ||||||||
| B6CC | POP BC | |||||||||
| B6CD | LD A,C | |||||||||
| B6CE | RRCA | |||||||||
| B6CF | AND $10 | |||||||||
| B6D1 | LD C,A | |||||||||
| B6D2 | LD A,($5BA2) | frame counter plus C, folded when bit 4 is set | ||||||||
| B6D5 | ADD A,C | |||||||||
| B6D6 | BIT 4,A | |||||||||
| B6D8 | JR Z,shuttle_block_target | |||||||||
| B6DA | CPL | |||||||||
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