Underwurlde: Routine at ADB0

ADB0: Draw Line Step

Used by the routine at Draw_Line.

Draws one step of a line. It plots a pixel with Plot_Pixel, adds the slope in DE and the fraction in Shadow_Line_Fraction to the x position, and moves the display file address across by the new x position and up by one pixel line.

Input
HL	Display file address
C	x position
DE	Slope

Output
HL	New display file address
C	New x position

Draw_Line_Step

ADB0

PUSH BC

Stash the steps and the direction on the stack.

ADB1

CALL Plot_Pixel

Plot a pixel.

ADB4

LD A,H

Stash the high byte of the display file address in AF'.

ADB5

EX AF,AF'

ADB6

LD A,($AAA1)

Fetch the fraction of the line.

ADB9

LD H,C

Copy the x position to H.

ADBA

LD B,L

Stash the low byte of the display file address in B.

ADBB

LD L,A

Copy the fraction to L.

ADBC

ADD HL,DE

Add the slope to the x position and the fraction.

ADBD

LD A,L

Copy the new fraction to A.

ADBE

LD ($AAA1),A

Store it as the fraction of the line.

ADC1

LD C,H

Copy the new x position to C.

ADC2

LD A,B

Restore the low byte of the display file address.

ADC3

AND %11100000

Keep only the row bits of the low byte.

ADC5

LD B,A

Store them in B.

ADC6

LD A,C

Copy the new x position to A.

ADC7

RRA

Divide the x position by 08.

ADC8

RRA

ADC9

RRA

ADCA

AND %00011111

Keep only five bits to make the column.

ADCC

OR B

Combine the row bits with the column.

ADCD

LD L,A

Store the new low byte of the display file address in L.

ADCE

LD A,C

Copy the new x position to A.

ADCF

POP BC

Restore the steps and the direction from the stack.

ADD0

LD C,A

Store the new x position in C.

ADD1

EX AF,AF'

Restore the high byte of the display file address and store it in H.

ADD2

LD H,A

ADD3

JP ScreenPosition1PixelAbove

Jump to ScreenPosition1PixelAbove to move up a pixel line.