Martianoids: Routine at AD87

AD87: Clear Icon Area

Used by the routine at QueueProgram.

Clears an area of the screen that is four characters wide and thirty-two lines high, working upwards from the address that is given.

Input
HL	Screen address of the bottom left

ClearIconArea

AD87

PUSH BC

Save the registers.

AD88

PUSH DE

AD89

PUSH HL

AD8A

LD C,$20

There are 20 lines to clear.

ClearIconArea_Line

AD8C

LD B,$04

There are four bytes in a line.

AD8E

XOR A

Clear a byte...

ClearIconArea_Byte

AD8F

LD (HL),A

...on the screen.

AD90

INC L

Move on to the next byte.

AD91

DJNZ ClearIconArea_Byte

Loop back until the line has been cleared.

AD93

DEC L

Go back to the start of the line.

AD94

DEC L

AD95

DEC L

AD96

DEC L

AD97

LD A,H

Fetch the high byte of the address...

AD98

DEC H

...and move up one line.

AD99

AND $07

Jump if that has not moved into the character above.

AD9B

JR NZ,ClearIconArea_Next

AD9D

LD A,H

Otherwise fetch the high byte again, as it has moved up a row of characters...

AD9E

ADD A,$08

...add eight to it...

ADA0

LD H,A

...and store it.

ADA1

LD A,L

Fetch the low byte...

ADA2

SUB $20

...take one row of characters off it...

ADA4

LD L,A

...and store it.

ClearIconArea_Next

ADA5

DEC C

Loop back until all of the lines have been cleared.

ADA6

JR NZ,ClearIconArea_Line

ADA8

POP HL

Restore the registers.

ADA9

POP DE

ADAA

POP BC

ADAB

RET

Return.