Lunar Jetman: Routine at 34294

34294: Screen Address One Pixel Above

Moves a screen address up one pixel, wrapping through the Spectrum display layout.

Input
HL	Current position

Output
HL	Address for new position

Move the screen address up one pixel, wrapping the row.

ScreenPosition1PixelAbove

34294

DEC H

Move up one pixel.

34295

LD A,H

Invert the new line byte.

34296

CPL

34297

AND %00000111

Keep only bits 0-2.

34299

RET NZ

Return if a character-row boundary was not crossed.

34300

LD A,L

Subtract 32 from the column address.

34301

SUB 32

34303

LD L,A

34304

RET C

Return if that wrapped into the previous third.

34305

LD A,H

Add 8 to the line byte.

34306

ADD A,8

34308

LD H,A

34309

RET

Return.

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