Routines
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AE71: Screen Address One Pixel Above
Used by the routines at Clear_Play_Area, Draw_Line and Draw_Line_Step.
Moves a display file address up by one pixel line.
Input
HL Display file address
Output
HL Display file address of the line above
ScreenPosition1PixelAbove AE71 DEC H Move up a pixel line.
AE72 LD A,H Copy H to A.
AE73 CPL Flip all of the bits.
AE74 AND $07 Keep only the line number within the character.
AE76 RET NZ Return if a character boundary has not been crossed.
AE77 LD A,L Copy L to A.
AE78 SUB $20 Move up a character row.
AE7A LD L,A Store the result in L.
AE7B RET C Return if the subtraction borrowed.
AE7C LD A,H Copy H to A.
AE7D ADD A,$08 Add 08.
AE7F LD H,A Store the result in H.
AE80 RET Return.
View the equivalent code in;
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