Avalon: Routine at 56533

56533: Draw/erase graphics - graphic partly out of bounds - LEFT border of play area

Used by the routine at 56309.

Input
HL	Graphic data address pointer
DE	Screen display address
A	Screen column (0-31)
C	Number of remaining bytes in the tile to draw
B	Number of (right) pixel shifts needed for graphic byte

This routine incorporates slightly tweaked versions of the:

Bit shifting routine at 56362
Erase graphic byte routine at 56385
Draw graphic byte routine at 56396
Re-calculate screen display address 1 pixel lower routine at 56403

The opposite routine (graphic partially obscured by right-hand screen border) is at 56457.

56533

LD A,(HL)

Get graphics byte

56534

INC HL

...and move graphic address pointer along, ready for next one

56535

CP 0

Byte empty?

56537

JR Z,56577

...If so, skip this byte, don't bother shifting or drawing

56539

PUSH HL

Store address pointer

Shifts graphic so that the right-shifted graphics byte is contained in 2 bytes, H and L, ready to draw on screen

56540

LD H,A

H contains the unshifted graphic byte

56541

LD L,0

Clear L register ready to receive shifted bits

56543

LD A,B

Temp store for the 'shift counter' (containing number of right-shifts needed)

56544

CP 0

Any more shifts needed?

56546

JR Z,56555

...If not, skip out of the shift loop

56548

RR H

Rotate byte in H register into L register

56550

RR L

56552

DJNZ 56548

Repeat for number of shifts needed (counter)

56554

LD B,A

Reset the shift counter (from A register)

56555

EX DE,HL

Swap registers so that D & E contain the byte values to draw, and HL contains the screen display address to draw at

56556

LD A,(60111)

Check draw/erase flag

56559

CP 1

0 = Erase, 1 = Draw

56561

JR Z,56570

If flag = 1, jump to DRAW routine. Otherwise ERASE (next routine)

ERASE SCREEN BYTE

56563

LD A,E

Don't need to erase the first graphic byte (stored in D register) as it's on the screen border, instead get the second byte (stored in E)

56564

INC HL

Move display address pointer right one character square to character column 2, just inside the left border

56565

CPL

Invert the graphics byte

56566

AND (HL)

AND with screen byte (i.e. keeps existing screen byte, clears sprite byte)

56567

LD (HL),A

...and put back on screen.

56568

JR 56574

DRAW SCREEN BYTE

56570

LD A,E

Don't need to draw the first graphic byte (stored in D register) as it's on the screen border, instead get the second byte (stored in E)

56571

INC HL

Move display address pointer right one character square

56572

OR (HL)

OR with existing screen byte, preserve any background graphics

56573

LD (HL),A

...and put back on screen.

Graphic byte now drawn/erased

56574

DEC HL

Move back left one character space/screen display address

56575

EX DE,HL

56576

POP HL

HL now back to the graphic display address pointer, DE contains the screen display address

Recalculate screen address after drawing/erasing the graphic byte (also jump in to here from 56537 if byte to draw is empty)

56577

LD A,D

Get high byte of screen address

56578

INC A

...and increment (move down 1 pixel row)

56579

AND 7

Are we crossing into a new character row (8th pixel row)?

56581

JR Z,56586

56583

INC D

If not, increment screen display address pointer (move down 1 pixel row)

56584

JR 56600

56586

LD A,D

Otherwise, adjust high byte to the correct screen address

56587

AND 248

Reset the low 3 bits (pixel row number) to zero

56589

LD D,A

56590

LD A,E

Get the low byte of the screen address

56591

ADD A,32

Increment column by 32 to move down to next screen row

56593

LD E,A

56594

JR NC,56600

Crossing a 1/3 screen boundary? ...If not, skip next 3 instructions

56596

LD A,D

...If so, adjust high byte of screen address accordingly

56597

ADD A,8

56599

LD D,A

Store correct screen address high byte back in D register

Check if encroaching out of bounds into bottom third of screen, where the scroll status panel is

56600

LD A,D

56601

CP 80

Are we into the last 3rd of the screen?

56603

DEC C

(Decrement number of tile bytes to draw)

56604

JP NC,56442

Jump to 56442 if into last 3rd of screen (out of bounds)

56607

JR NZ,56533

If bytes left to draw in the tile - continue to draw next tile byte

56609

JP 56446