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;; DECLARE SOME VARIABLES HERE
.enum $0000	;;start variables at ram location 0

scroll     .dsb 1  ; horizontal scroll count
nametable  .dsb 1  ; which nametable to use, 0 or 1
columnLow  .dsb 1  ; low byte of new column address
columnHigh .dsb 1 ; high byte of new column address
sourceLow  .dsb 1  ; source for column data
sourceHigh .dsb 1
columnNumber .dsb 1  ; which column of level data to draw
.ende

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        .ORG $7ff0
Header:                         ;16 byte .NES header (iNES)
	.db "NES", $1a		;NES followed by MS-DOS end-of-file
	.db $02			;size of PRG ROM in 16kb units
	.db $01			;size of CHR ROM in 8kb units
	.db #%00000001		;flags 6, set to: mapper 4,
	.db #%00000000		;flags 7, set to: mapper 4
        .db $00                 ;size of PRG RAM in 8kb RAM
        .db $00                 ;flags 9, set to 0
        .db $00                 ;flags 10, set to 0
        .db $00                 ;11 - the rest are zeroed out
        .db $00                 ;12
        .db $00                 ;13
        .db $00                 ;14
        .db $00                 ;15

        .org $8000
RESET:
  SEI          ; disable IRQs
  CLD          ; disable decimal mode
  LDX #$40
  STX $4017    ; disable APU frame IRQ
  LDX #$FF
  TXS          ; Set up stack
  INX          ; now X = 0
  STX $2000    ; disable NMI
  STX $2001    ; disable rendering
  STX $4010    ; disable DMC IRQs

vblankwait1:       ; First wait for vblank to make sure PPU is ready
  BIT $2002
  BPL vblankwait1

clrmem:
  LDA #$00
  STA $0000, x
  STA $0100, x
  STA $0300, x
  STA $0400, x
  STA $0500, x
  STA $0600, x
  STA $0700, x
  LDA #$FE
  STA $0200, x
  INX
  BNE clrmem

vblankwait2:      ; Second wait for vblank, PPU is ready after this
  BIT $2002
  BPL vblankwait2


LoadPalettes:
  LDA $2002             ; read PPU status to reset the high/low latch
  LDA #$3F
  STA $2006             ; write the high byte of $3F00 address
  LDA #$00
  STA $2006             ; write the low byte of $3F00 address
  LDX #$00              ; start out at 0
LoadPalettesLoop:
  LDA palette, x        ; load data from address (palette + the value in x)
  STA $2007             ; write to PPU
  INX                   ; X = X + 1
  CPX #$20              ; Compare X to hex $10, decimal 16 - copying 16 bytes = 4 sprites
  BNE LoadPalettesLoop  ; Branch to LoadPalettesLoop if compare was Not Equal to zero
                        ; if compare was equal to 32, keep going down



LoadSprites:
  LDX #$00              ; start at 0
LoadSpritesLoop:
  LDA sprites, x        ; load data from address (sprites +  x)
  STA $0200, x          ; store into RAM address ($0200 + x)
  INX                   ; X = X + 1
  CPX #$10              ; Compare X to hex $20, decimal 16
  BNE LoadSpritesLoop   ; Branch to LoadSpritesLoop if compare was Not Equal to zero
                        ; if compare was equal to 16, keep going down


InitializeNametables:
  LDA #$01
  STA nametable
  LDA #$00
  STA scroll
  STA columnNumber
InitializeNametablesLoop:
  JSR DrawNewColumn     ; draw bg column
  LDA scroll            ; go to next column
  CLC
  ADC #$08
  STA scroll
  INC columnNumber
  LDA columnNumber      ; repeat for first nametable
  CMP #$20
  BNE InitializeNametablesLoop

  LDA #$00
  STA nametable
  LDA #$00
  STA scroll
  JSR DrawNewColumn     ; draw first column of second nametable
  INC columnNumber

  LDA #$00              ; set back to increment +1 mode
  STA $2000
InitializeNametablesDone:


InitializeAttributes:
  LDA #$01
  STA nametable
  LDA #$00
  STA scroll
  STA columnNumber
InitializeAttributesLoop:
  JSR DrawNewAttributes     ; draw attribs
  LDA scroll                ; go to next column
  CLC
  ADC #$20
  STA scroll

  LDA columnNumber      ; repeat for first nametable
  CLC
  ADC #$04
  STA columnNumber
  CMP #$20
  BNE InitializeAttributesLoop

  LDA #$00
  STA nametable
  LDA #$00
  STA scroll
  JSR DrawNewAttributes     ; draw first column of second nametable
InitializeAttributesDone:

  LDA #$21
  STA columnNumber


  LDA #%10010000   ; enable NMI, sprites from Pattern Table 0, background from Pattern Table 1
  STA $2000

  LDA #%00011110   ; enable sprites, enable background, no clipping on left side
  STA $2001

Forever:
  JMP Forever     ;jump back to Forever, infinite loop



NMI:
  INC scroll            ; add one to our scroll variable each frame


NTSwapCheck:
  LDA scroll            ; check if the scroll just wrapped from 255 to 0
  BNE NTSwapCheckDone
NTSwap:
  LDA nametable         ; load current nametable number (0 or 1)
  EOR #$01              ; exclusive OR of bit 0 will flip that bit
  STA nametable         ; so if nametable was 0, now 1
                        ;    if nametable was 1, now 0
NTSwapCheckDone:


NewAttribCheck:
  LDA scroll
  AND #%00011111            ; check for multiple of 32
  BNE NewAttribCheckDone    ; if low 5 bits = 0, time to write new attribute bytes
  jsr DrawNewAttributes
NewAttribCheckDone:


NewColumnCheck:
  LDA scroll
  AND #%00000111            ; throw away higher bits to check for multiple of 8
  BNE NewColumnCheckDone    ; done if lower bits != 0
  JSR DrawNewColumn         ; if lower bits = 0, time for new column

  lda columnNumber
  clc
  adc #$01             ; go to next column
  and #%01111111       ; only 128 columns of data, throw away top bit to wrap
  sta columnNumber
NewColumnCheckDone:


  LDA #$00
  STA $2003
  LDA #$02
  STA $4014       ; sprite DMA from $0200

  ; run other game graphics updating code here

  LDA #$00
  STA $2006        ; clean up PPU address registers
  STA $2006

  LDA scroll
  STA $2005        ; write the horizontal scroll count register

  LDA #$00         ; no vertical scrolling
  STA $2005

  ;;This is the PPU clean up section, so rendering the next frame starts properly.
  LDA #%10010000   ; enable NMI, sprites from Pattern Table 0, background from Pattern Table 1
  ORA nametable    ; select correct nametable for bit 0
  STA $2000

  LDA #%00011110   ; enable sprites, enable background, no clipping on left side
  STA $2001

  ; run normal game engine code here
  ; reading from controllers, etc

  RTI              ; return from interrupt





DrawNewColumn:
  LDA scroll       ; calculate new column address using scroll register
  LSR A
  LSR A
  LSR A            ; shift right 3 times = divide by 8
  STA columnLow    ; $00 to $1F, screen is 32 tiles wide

  LDA nametable     ; calculate new column address using current nametable
  EOR #$01          ; invert low bit, A = $00 or $01
  ASL A             ; shift up, A = $00 or $02
  ASL A             ; $00 or $04
  CLC
  ADC #$20          ; add high byte of nametable base address ($2000)
  STA columnHigh    ; now address = $20 or $24 for nametable 0 or 1

  LDA columnNumber  ; column number * 32 = column data offset
  ASL A
  ASL A
  ASL A
  ASL A
  ASL A
  STA sourceLow
  LDA columnNumber
  LSR A
  LSR A
  LSR A
  STA sourceHigh

  LDA sourceLow       ; column data start + offset = address to load column data from
  CLC
  ADC #<columnData
  STA sourceLow
  LDA sourceHigh
  ADC #>columnData
  STA sourceHigh

DrawColumn:
  LDA #%00000100        ; set to increment +32 mode
  STA $2000

  LDA $2002             ; read PPU status to reset the high/low latch
  LDA columnHigh
  STA $2006             ; write the high byte of column address
  LDA columnLow
  STA $2006             ; write the low byte of column address
  LDX #$1E              ; copy 30 bytes
  LDY #$00
DrawColumnLoop:
  LDA sourceLow, y
  STA $2007
  INY
  DEX
  BNE DrawColumnLoop

  RTS



DrawNewAttributes:
  LDA nametable
  EOR #$01          ; invert low bit, A = $00 or $01
  ASL A             ; shift up, A = $00 or $02
  ASL A             ; $00 or $04
  CLC
  ADC #$23          ; add high byte of attribute base address ($23C0)
  STA columnHigh    ; now address = $23 or $27 for nametable 0 or 1

  LDA scroll
  LSR A
  LSR A
  LSR A
  LSR A
  LSR A
  CLC
  ADC #$C0
  STA columnLow     ; attribute base + scroll / 32

  LDA columnNumber  ; (column number / 4) * 8 = column data offset
  AND #%11111100
  ASL A
  STA sourceLow
  LDA columnNumber
  LSR A
  LSR A
  LSR A
  LSR A
  LSR A
  LSR A
  LSR A
  STA sourceHigh

  LDA sourceLow       ; column data start + offset = address to load column data from
  CLC
  ADC #<attribData
  STA sourceLow
  LDA sourceHigh
  ADC #>attribData
  STA sourceHigh

  LDY #$00
  LDA $2002             ; read PPU status to reset the high/low latch
DrawNewAttributesLoop
  LDA columnHigh
  STA $2006             ; write the high byte of column address
  LDA columnLow
  STA $2006             ; write the low byte of column address
  LDA sourceLow, y    ; copy new attribute byte
  STA $2007

  INY
  CPY #$08              ; copy 8 attribute bytes
  BEQ DrawNewAttributesLoopDone

  LDA columnLow         ; next attribute byte is at address + 8
  CLC
  ADC #$08
  STA columnLow
  JMP DrawNewAttributesLoop
DrawNewAttributesLoopDone:

  rts
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palette:
  .db $22,$29,$1A,$0F,  $22,$36,$17,$0F,  $22,$30,$21,$0F,  $22,$27,$17,$0F   ;;background palette
  .db $22,$16,$27,$18,  $22,$1A,$30,$27,  $22,$16,$30,$27,  $22,$0F,$36,$17   ;;sprite palette

sprites:
     ;vert tile attr horiz
  .db $80, $32, $00, $80   ;sprite 0
  .db $80, $33, $00, $88   ;sprite 1
  .db $88, $34, $00, $80   ;sprite 2
  .db $88, $35, $00, $88   ;sprite 3


columnData:
  .incbin "SMBlevel.bin"

attribData:
  .incbin "SMBattrib.bin"

  .incbin "mario.chr"   ;includes 8KB graphics file from SMB1

  .org $FFFA     ;first of the three vectors starts here
  .dw NMI        ;when an NMI happens (once per frame if enabled) the
                   ;processor will jump to the label NMI:
  .dw RESET      ;when the processor first turns on or is reset, it will jump
                   ;to the label RESET:
  .dw 0          ;external interrupt IRQ is not used in this tutorial


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