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Routines |
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DE | 16384 or 16415 |
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 on screen to scroll (the bottom 3 rows of the screen are fixed) |
63517 | LD B,8 | 8 pixel lines per row of screen |
63519 | EXX | |
63520 | LD H,D | Copy the display file address from DE to HL |
63521 | LD L,E | |
63522 | INC/DEC L | This will be INC L if we're scrolling to the left, DEC L otherwise |
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/LDDR | Shift to the left or the right |
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 play area (that was) at the far left of screen |
63552 | RET |
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