Underwurlde: Routine at 9C2B

9C2B: Screen Address One Pixel Below

Moves a screen address down by one pixel line.

Input
HL	Screen address

Output
HL	Screen address of the next line down

ScreenPosition1PixelBelow

9C2B

INC H

Move down a line within the character.

9C2C

LD A,H

Fetch the high byte of the screen address.

9C2D

AND $07

Keep only the line number.

9C2F

RET NZ

Return unless the character boundary has been crossed.

9C30

LD A,L

Fetch the low byte of the screen address.

9C31

ADD A,$20

Move on to the next character row.

9C33

LD L,A

Store the low byte of the screen address.

9C34

AND $E0

Keep only the row bits.

9C36

RET Z

Return if the third of the screen has not been crossed.

9C37

LD A,H

Copy H to A.

9C38

SUB $08

Go back up to the first line of the character.

9C3A

LD H,A

Store the high byte of the screen address.

9C3B

RET

Return.

View the equivalent code in;