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Routines |
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Used by the routine at GameInitialisation.
Builds a lookup table in the top 1000 bytes of memory (F000 to FFFF) 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 FF - (02 * 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 00.
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| BuildShiftTable | B902 | LD L,$00 | L=00 (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 | B904 | LD D,$00 | D=00. | |
| B906 | LD E,L | E=L. | ||
| B907 | LD H,$FF | H=FF (the page for the first low byte). | ||
| B909 | LD B,$08 | B=08 (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 | B90B | LD (HL),E | Write the low byte. | |
| B90C | DEC H | Move down a page. | ||
| B90D | LD (HL),D | Write the high byte. | ||
| B90E | DEC H | Move down a page. | ||
| B90F | SLA E | Shift E left one bit (bit 7 goes into the carry flag). | ||
| B911 | RL D | Rotate D left one bit, bringing in the carry flag. | ||
| B913 | DJNZ BuildShiftTable_Offset | Decrease counter by one and loop back to BuildShiftTable_Offset until zero. | ||
| B915 | INC L | Move onto the next byte value. | ||
| B916 | JR NZ,BuildShiftTable_Value | Jump back to BuildShiftTable_Value until every byte value has been done. | ||
| B918 | RET | Return. | ||
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View the equivalent code in;
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