Atic Atac: Routine at 9F03

9F03: Screen Address One Pixel Above

Used by the routines at DrawGraphic_Upright, DrawGraphic_Mirrored, DrawGraphic_RotatedLeft, DrawGraphic_Transposed, DrawGraphic_Flipped, DrawGraphic_RotatedHalf, DrawGraphic_TransposedFlipped, DrawGraphic_RotatedRight, DrawOldSpriteRow_Aligned, DrawActorSpriteRow_Aligned and ClearSprite.

Moves a screen address up by one pixel row, allowing for the layout of the screen. The XOR sprite row routines continue here so that they finish with the screen address of the row above the one just drawn.

Input
HL	The screen address

Output
HL	The screen address one pixel row above

ScreenPosition1PixelAbove

9F03

DEC H

Decrease H by one to move up one pixel row within the character cell.

9F04

LD A,H

A=H.

9F05

CPL

Flip the bits.

9F06

AND %00000111

Keep only bits 0-2.

9F08

RET NZ

Return if still inside the same character cell.

9F09

LD A,L

A=L.

9F0A

SUB $20

Subtract 20 to move up a character row.

9F0C

LD L,A

L=A.

9F0D

RET C

Return if that moved into the previous third of the screen (the address is then already correct).

9F0E

LD A,H

A=H.

9F0F

ADD A,$08

Add 08.

9F11

LD H,A

H=A.

9F12

RET

Return.

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