FadeToBlackZ80

; Fade Screen to Black
; Improvement would be to handle bright and flash attributes as well

; BASIC Test code
; 1 bright 1: flash 1: ink 6: paper 3: cls: print "a"


ORG 0x8000
; Start of attribute memory
SCREEN = 0x4000
; 24 rows, 32 columns
; 8 x 8 characters
; 24 * 8 = bytes per row = 192
; 192 * 32 = 614
; 24 * 32 = 768 attributes
ATTRIBUTES_COUNT = 0x300
;ATTRIBUTES = (SCREEN + ATTRIBUTES_COUNT)
ATTRIBUTES = 0x5800
INK = %00000111
PAPER = %00111000

LD D, 7; Maximum of 7 subtractions to get to black
MAIN_LOOP
LD HL, ATTRIBUTES
LD BC, ATTRIBUTES_COUNT

; Decrement attribute colours
ATTR_LOOP

LD A,0x80
PAUSE
DEC A
JR NZ, PAUSE

LD A, (HL)

EXX
LD D, A
LD E, A

AND INK
JR Z, IS_ZERO_INK
DEC A
IS_ZERO_INK
LD E, A

LD A, D
AND PAPER
JR Z, IS_ZERO_PAPER
DEC A
IS_ZERO_PAPER
AND PAPER

OR E
EXX

LD (HL), A
INC HL
; End Decrement attribute colours

; If BC != 0 loop
DEC BC
XOR A
OR B
JR NZ, ATTR_LOOP
XOR A
OR C
JR NZ, ATTR_LOOP

; If D == 0 return
DEC D
XOR A
OR D
JR NZ, MAIN_LOOP
RET