Atic Atac: Routine at 9BC1

9BC1: Screen Address One Pixel Below

Moves a screen buffer address down by one pixel line.

Input
HL	The screen buffer address

Output
HL	The screen buffer address of the line below

ScreenPosition1PixelBelow

9BC1

INC H

Move down a pixel line.

9BC2

LD A,H

Fetch the high byte of the address.

9BC3

AND %00000111

Keep only bits 0-2.

9BC5

RET NZ

Return if a character boundary has not been crossed.

9BC6

LD A,L

Fetch the low byte of the address.

9BC7

ADD A,$20

Move down a character row.

9BC9

LD L,A

Keep the new low byte in L.

9BCA

AND %11100000

Keep only bits 5-7.

9BCC

RET Z

Return if the next third of the screen has been reached.

9BCD

LD A,H

Fetch the high byte of the address.

9BCE

SUB $08

Subtract 08.

9BD0

LD H,A

Keep the new high byte in H.

9BD1

RET

Return.

View the equivalent code in;