Routines
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B4E8: Draw Shifted Rows
Draws the rows of a sprite that is shifted by one to three pixels, using the stack pointer to read the sprite data. The shifted bytes are found in the tables that start at the page in H, and one more byte than the width is drawn. There are four entry points, which are for widths of four, three, two and one bytes. The address in the table at RowRoutineTable is the entry point for the width.
Input
DE The address in the screen buffer
H The page of the table for the shift
B The number of rows
SP The sprite data
DrawShiftedRows B4E8 POP BC Draw one byte of the shifted sprite.
B4E9 LD L,C
B4EA AND (HL)
B4EB LD L,B
B4EC XOR (HL)
B4ED CPL
B4EE LD (DE),A
B4EF INC DE
B4F0 INC H
B4F1 LD L,C
B4F2 LD A,(DE)
B4F3 AND (HL)
B4F4 LD L,B
B4F5 XOR (HL)
B4F6 CPL
B4F7 DEC H
DrawShiftedRows_Three B4F8 POP BC Draw one byte of the shifted sprite.
B4F9 LD L,C
B4FA AND (HL)
B4FB LD L,B
B4FC XOR (HL)
B4FD CPL
B4FE LD (DE),A
B4FF INC DE
B500 INC H
B501 LD L,C
B502 LD A,(DE)
B503 AND (HL)
B504 LD L,B
B505 XOR (HL)
B506 CPL
B507 DEC H
This entry point is used by the routine at DrawBlockBytes.
DrawShiftedRows_Two B508 POP BC Draw one byte of the shifted sprite.
B509 LD L,C
B50A AND (HL)
B50B LD L,B
B50C XOR (HL)
B50D CPL
B50E LD (DE),A
B50F INC DE
B510 INC H
B511 LD L,C
B512 LD A,(DE)
B513 AND (HL)
B514 LD L,B
B515 XOR (HL)
B516 CPL
B517 DEC H
This entry point is used by the routine at DrawNeedleTip.
DrawShiftedRows_One B518 POP BC Draw one byte of the shifted sprite.
B519 LD L,C
B51A AND (HL)
B51B LD L,B
B51C XOR (HL)
B51D CPL
B51E LD (DE),A
B51F INC DE
B520 INC H
B521 LD L,C
B522 LD A,(DE)
B523 AND (HL)
B524 LD L,B
B525 XOR (HL)
B526 CPL
B527 DEC H
The row count is held in the alternate accumulator. The width subtracted below, and the two jumps, are patched before the routine is entered.
DrawShiftedRows_Store B528 LD (DE),A Write the prepared byte.
B529 EX AF,AF' Count a row, and finish when none remain.
B52A DEC A
B52B JR Z,DrawShiftedRows_Return
B52D EX AF,AF' Bring the sprite byte back.
B52E LD A,E Step back by the patched width.
B52F SUB $00
B531 LD E,A
B532 LD A,(DE) Read the first byte of the next row.
DrawShiftedRows_Step B533 JP NC,DrawShiftedRows_Step Stay on this page if the step did not borrow.
B536 DEC D Move to the previous page and read its byte.
B537 LD A,(DE)
DrawShiftedRows_Patched B538 JP DrawShiftedRows_Patched Draw the next row. The target is patched.
DrawShiftedRows_Return B53B LD SP,($5CDA) Restore the stack pointer.
B53F RET Return.
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