Underwurlde: Routine at AC9A

AC9A: 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 58 or more).

The pixel within the byte is bit 07 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

AC9A

LD A,H

Fetch the high byte of the address.

AC9B

CP $58

Return if it is not in the display file.

AC9D

RET NC

AC9E

LD A,C

Fetch the x position.

AC9F

AND %00000111

Keep only the lowest three bits.

ACA1

INC A

Add one to make the number of rotations.

ACA2

LD B,A

Store the number in B.

ACA3

XOR A

Start with an empty byte.

ACA4

SCF

Set the carry flag, which is the pixel.

Plot_Pixel_Rotate_Loop

ACA5

RRA

Rotate the pixel along the byte.

ACA6

DJNZ Plot_Pixel_Rotate_Loop

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

ACA8

XOR (HL)

Combine the pixel with the display file byte using XOR.

ACA9

LD (HL),A

Write the result back to the display file.

ACAA

RET

Return.