GRADUAL DECLINE Disasm

;GRADUAL DECLINE Disassembly
;Assemble with RGBDS
;This is a very quick disasm of a Gameboy ROM called GRADUAL DECLINE, a ROM which writes random data to IO registers, VRAM, etc.
;Stay posted for better disasm's of this ROM

INCLUDE "gbhw.inc"

SECTION "start", ROM0[$0100]
    nop
    jp  Start

db   $CE,$ED,$66,$66,$CC,$0D,$00,$0B,$03,$73,$00,$83,$00,$0C,$00,$0D
db   $00,$08,$11,$1F,$88,$89,$00,$0E,$DC,$CC,$6E,$E6,$DD,$DD,$D9,$99
db   $BB,$BB,$67,$63,$6E,$0E,$EC,$CC,$DD,$DC,$99,$9F,$BB,$B9,$33,$3E
db   "GRADUAL DECLINE"
db   $00
db   $D3,$00
db   $00
db   $00
db   $00
db   $00
db   $00
db   $00
db   $00
db   $00
db   $00,$00

Start:
    ld   c,$80
    ld   hl,$0181
Sub0:
    ldi  a,[hl]
    ld   [$ff00+c],a
    inc  c
    ld   a,c
    cp   a,$D1
    jr   nz,Sub0
    ld   hl,$C000
Sub1:
    ld   a,e
    xor  [hl]
    inc  l
    xor  [hl]
    inc  l
    xor  [hl]
    inc  l
    xor  [hl]
    inc  l
    ld   e,a
    ld   a,d
    xor  [hl]
    inc  l
    xor  [hl]
    inc  l
    xor  [hl]
    inc  l
    xor  [hl]
    inc  hl
    ld   d,a
    ld   a,h
    cp   a,$E0
    jr   nz,Sub1
    ld   l,e
    ld   h,d
    ld   de,$8000
    jp   $FF80
    ld   b,$08
Sub3:
    add  hl,hl
    bit  6,h
    jr   nz,Sub6
    set  0,l
Sub6:
    ld   a,l
    bit  7,h
    jr   z,Sub2
    xor  a,$01
Sub2:
    ld   l,a
    ld   [$C000],a
    dec  b
    jr   nz,Sub3
    ld   [de],a
    ld   a,[$FF00+04]
    xor  h
    ld   e,a
    inc  d
    res  5,d
    ld   a,[$FF00+04]
    add  c
    add  h
    ld   c,a
    res  7,c
    ld   a,c
    cp   a,$40
    jr   z,Sub4
    cp   a,$26
    jr   z,Sub4
    jr   Sub5
Sub4:
    ld   a,[$FF00+c]
    xor  l
    set  7,a
    jr   Sub8
Sub5:
    ld   a,[$FF00+c]
    xor  l
Sub8:
    ld   [$FF00+c],a
    ld   a,[$FF00+$FE]
    ld   b,a
    ld   a,[$FF00+00]
    ld   [$FF00+$FE],a
    xor  b
    and  a,$0F
    xor  l
    ld   l,a
    xor  a
    ld   [$FF00+$0F],a
    dec  a
    ld   [$FF00+$FF],a
    halt
    nop
    jr   Sub3
    pop  hl
    pop  af
    reti
    ld   a,[de]
    ld   h,a
    inc  e
    ld   a,[de]
    ld   l,a
    add  hl,hl
    jr   nc,Sub9
    set  0,l
Sub9:
    ld   a,l
    bit  7,h
    jr   z,Sub7
    xor  a,$01
Sub7:
    ld   [de],a
    dec  e
    ld   a,h
    ld   [de],a
    ret
    adc  e