Routines
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56457: Draw/erase graphics - graphic partly out of bounds - RIGHT 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
The graphics byte is pixel-shifted into two screen bytes, but the second byte is now in the right border area.
graphic partly on right border of screen
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 left-hand screen border) is at 56533.
56457 LD A,(HL) Get graphics byte
56458 INC HL ...and move graphic address pointer along, ready for next one
56459 CP 0 Byte empty?
56461 JR Z,56498 ...If so, skip this byte, don't bother shifting or drawing
56463 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:
56464 LD H,A H contains the unshifted graphic byte
56465 LD L,0 Clear L register ready to receive shifted bits
56467 LD A,B Temp store for the 'shift counter' (containing number of right-shifts needed)
56468 CP 0 Any more shifts needed?
56470 JR Z,56479 ...If not, skip out of the shift loop
56472 RR H Rotate byte in H register into L register
56474 RR L
56476 DJNZ 56472 Repeat for number of shifts needed (counter)
56478 LD B,A Reset the shift counter (from A register)
56479 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
56480 LD A,(60111) Check draw/erase flag
56483 CP 1 0 = Erase, 1 = Draw
56485 JR Z,56493 If flag = 1, jump to DRAW routine. Otherwise ERASE (next routine)
ERASE SCREEN BYTE
Don't need to erase the second graphic byte (stored in E register) as it's on the screen border.
56487 LD A,D Get first graphic byte
56488 CPL Invert it
56489 AND (HL) AND with screen byte (i.e. keeps existing screen byte, clears sprite byte)
56490 LD (HL),A ...and put back on screen.
56491 JR 56496
DRAW SCREEN BYTE
No need to draw the second graphic byte (stored in E register) as it's on the screen border.
56493 LD A,D Get first graphic byte
56494 OR (HL) OR with existing screen byte, preserve any background graphics
56495 LD (HL),A ...and draw back on screen.
56496 EX DE,HL
56497 POP HL HL now back to the graphic display address pointer, DE contains the screen display address
Recalculate screen address 1 pixel line down after drawing/erasing the graphic byte (also jump in to here from 56461 if byte empty)
56498 LD A,D Get high byte of screen address
56499 INC A ...and increment (move down 1 pixel row)
56500 AND 7 Are we crossing into a new character row (8th pixel row)?
56502 JR Z,56507
56504 INC D If not, increment screen display address pointer (move down 1 pixel row)
56505 JR 56521
56507 LD A,D Otherwise, adjust high byte to the correct screen address
56508 AND 248 Reset the low 3 bits (pixel row number) to zero
56510 LD D,A
56511 LD A,E Get the low byte of the screen address
56512 ADD A,32 Increment column by 32 to move down to next screen row
56514 LD E,A
56515 JR NC,56521 Crossing a 1/3 screen boundary? ...If not, skip next 3 instructions
56517 LD A,D ...If so, adjust high byte of screen address accordingly
56518 ADD A,8
56520 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
56521 LD A,D
56522 CP 80 Are we into the last 3rd of the screen?
56524 DEC C (Decrement number of tile bytes to draw)
56525 JP NC,56442 Jump to 56442 if into last 3rd of screen (out of bounds)
56528 JR NZ,56457 If bytes left to draw in the tile - continue to draw next tile byte
56530 JP 56446
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