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Routines |
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Used by the routine at GameInitialisation.
Builds a lookup table in the top 4096 bytes of memory (61440 to 65535) which the sprite drawing routines use to position a sprite at any pixel.
For every byte value, and for each of the eight possible pixel offsets, the value shifted left by that many bits is stored as a 16-bit number. The low byte is written to the page at 255 - (2 * offset) and the high byte to the page below it, both at the position given by the byte value.
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Work through each byte value in turn, starting with 0.
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| BuildShiftTable | 47362 | LD L,0 | L=0 (the byte value). | |
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Start with no shift (E holds the low byte and D the high byte of the shifted value).
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| BuildShiftTable_Value | 47364 | LD D,0 | D=0. | |
| 47366 | LD E,L | E=L. | ||
| 47367 | LD H,255 | H=255 (the page for the first low byte). | ||
| 47369 | LD B,8 | B=8 (counter, one for each pixel offset). | ||
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Store the low and the high byte, then shift the 16-bit value left by one bit for the next offset.
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| BuildShiftTable_Offset | 47371 | LD (HL),E | Write the low byte. | |
| 47372 | DEC H | Move down a page. | ||
| 47373 | LD (HL),D | Write the high byte. | ||
| 47374 | DEC H | Move down a page. | ||
| 47375 | SLA E | Shift E left one bit (bit 7 goes into the carry flag). | ||
| 47377 | RL D | Rotate D left one bit, bringing in the carry flag. | ||
| 47379 | DJNZ BuildShiftTable_Offset | Decrease counter by one and loop back to BuildShiftTable_Offset until zero. | ||
| 47381 | INC L | Move onto the next byte value. | ||
| 47382 | JR NZ,BuildShiftTable_Value | Jump back to BuildShiftTable_Value until every byte value has been done. | ||
| 47384 | RET | Return. | ||
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View the equivalent code in;
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