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Routines |
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Used by the routine at 63561. Returns with the column of the play area that was at the far left of the screen before this routine was called in A.
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63512 | LD B,0 | BC will be used as the counter for the LDIR/LDDR instruction at 63526; initialise the MSB to 0 | ||
63514 | EXX | |||
63515 | LD C,21 | There are 21 rows to scroll (the bottom 3 rows of the screen are fixed) | ||
63517 | LD B,8 | 8 pixel lines per row of the screen | ||
63519 | EXX | |||
63520 | LD H,D | Copy the display file address from DE to HL | ||
63521 | LD L,E | |||
63522 | INC L | This instruction is set to INC L if we're scrolling to the left, or DEC L if we're scrolling to the right (see 63561) | ||
63523 | LD A,E | Save the LSB of the display file address in A briefly | ||
63524 | LD C,31 | 31 bytes to shift per pixel line | ||
63526 | LDIR | This instruction is set to LDIR if we're shifting to the left, or LDDR if we're shifting to the right (see 63561) | ||
63528 | LD E,A | Restore the LSB of the display file address to E | ||
63529 | INC D | Point DE at the first (or last) byte of the next pixel line in this row | ||
63530 | EXX | |||
63531 | DJNZ 63519 | Shift the remaining pixel lines in this row | ||
63533 | EXX | |||
63534 | LD A,E | Set E to the LSB of the display file address for the first (or last) byte of the first pixel line in the next row down | ||
63535 | ADD A,32 | |||
63537 | LD E,A | |||
63538 | JR C,63544 | Jump if we just finished shifting the eighth or sixteenth row | ||
63540 | LD A,D | Adjust D appropriately in the case where we haven't finished with the current 8-row third of the screen | ||
63541 | SUB 8 | |||
63543 | LD D,A | |||
63544 | EXX | |||
63545 | DEC C | Next row | ||
63546 | JR NZ,63517 | Jump back until all 21 rows have been shifted | ||
63548 | EXX | |||
63549 | LD A,(32767) | A=column of the play area (that was) at the far left of the screen | ||
63552 | RET |
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