Underwurlde: Routine at 44186

44186: Plot Pixel

Used by the routines at Draw_Line and Draw_Line_Step.

Plots a pixel in the display file by combining it with the byte that is already there using XOR, so plotting the same pixel again erases it. Nothing is plotted when the address is not in the display file (when the high byte is 88 or more).

The pixel within the byte is bit 7 less the lowest three bits of the x position.

Input
HL	Display file address
C	x position (only the lowest three bits are used)

Plot_Pixel

44186

LD A,H

Fetch the high byte of the address.

44187

CP 88

Return if it is not in the display file.

44189

RET NC

44190

LD A,C

Fetch the x position.

44191

AND %00000111

Keep only the lowest three bits.

44193

INC A

Add one to make the number of rotations.

44194

LD B,A

Store the number in B.

44195

XOR A

Start with an empty byte.

44196

SCF

Set the carry flag, which is the pixel.

Plot_Pixel_Rotate_Loop

44197

RRA

Rotate the pixel along the byte.

44198

DJNZ Plot_Pixel_Rotate_Loop

Decrease the rotation counter and loop back until the pixel is in the right place.

44200

XOR (HL)

Combine the pixel with the display file byte using XOR.

44201

LD (HL),A

Write the result back to the display file.

44202

RET

Return.