Routines
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99AB: Clear Play Area
Used by the routine at Draw_Room.
Clears the pixels of the B0 lines of the play area and then sets the attributes of its 16 rows to white. The entry at Clear_Play_Area_Attributes only sets the attributes.
Before After Clear_Play_Area
clear-play-area-before clear-play-area-after
Clear_Play_Area 99AB LD HL,$BF00 Set the x position to 00 and the y position to BF (BF00).
99AE CALL ScreenAddress Convert the position into a screen address.
99B1 LD C,$B0 Set the number of lines to clear to B0.
Clear_Play_Area_Line_Loop 99B3 PUSH HL Stash the screen address on the stack.
99B4 LD B,$20 Set the number of bytes in a line to 20.
Clear_Play_Area_Byte_Loop 99B6 LD (HL),$00 Clear a byte of the line.
99B8 INC L Move on to the next byte.
99B9 DJNZ Clear_Play_Area_Byte_Loop Decrease the byte counter and loop back until the line is clear.
99BB POP HL Restore the screen address from the stack.
99BC CALL ScreenPosition1PixelAbove Move up one pixel line.
99BF DEC C Decrease the line counter and loop back until all of the lines are clear.
99C0 JR NZ,Clear_Play_Area_Line_Loop
This entry point is used by the routine at Clear_Screen.
Clear_Play_Area_Attributes 99C2 LD HL,$5840 Point at the second row of the attribute buffer (5840).
99C5 LD C,$16 Set the number of rows to 16.
Clear_Play_Area_Row_Loop 99C7 LD B,$20 Set the number of cells in a row to 20.
Clear_Play_Area_Cell_Loop 99C9 LD (HL),$47 Set the attribute of the cell to white ink on black paper (bright).
99CB INC HL Move on to the next cell.
99CC DJNZ Clear_Play_Area_Cell_Loop Decrease the cell counter and loop back until the row is done.
99CE DEC C Decrease the row counter and loop back until all of the rows are done.
99CF JR NZ,Clear_Play_Area_Row_Loop
99D1 RET Return.
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