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PostPosted: Tue May 02, 2017 4:04 am 
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Joined: Wed Apr 26, 2017 8:02 am
Posts: 4
Hi again guys!
I'm trying to modify Advanced Nerdy Nights #3 scroll code in order to scroll only when you press right and sprite moves right. It kinda works but looks like every X iterations nametable number switches for a moment and wrong nametable background is displayed for a fraction of time. You can see it downloading the rom following this link. Im using NESASM and FCEUXD emulator.

http://s000.tinyupload.com/?file_id=132 ... 2808043245

I've checked places in code where nametable number is switched but I can't find whats wrong, The code is as follows

Code:
  .inesprg 1   ; 1x 16KB PRG code
  .ineschr 1   ; 1x  8KB CHR data
  .inesmap 0   ; mapper 0 = NROM, no bank swapping
  .inesmir 1   ; VERT mirroring for HORIZ scrolling
 

;;;;;;;;;;;;;;;

;; DECLARE SOME VARIABLES HERE
  .rsset $0000  ;;start variables at ram location 0
 
scroll     .rs 1  ; horizontal scroll count
nametable  .rs 1  ; which nametable to use, 0 or 1
columnLow  .rs 1  ; low byte of new column address
columnHigh .rs 1  ; high byte of new column address
sourceLow  .rs 1  ; source for column data
sourceHigh .rs 1
columnNumber .rs 1  ; which column of level data to draw
 
;;;;;;;;;;;;
   
  .bank 0
  .org $C000
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:
 


FillAttrib0:
  LDA $2002             ; read PPU status to reset the high/low latch
  LDA #$23
  STA $2006             ; write the high byte of $23C0 address (nametable 0 attributes)
  LDA #$C0
  STA $2006             ; write the low byte of $23C0 address
  LDX #$40              ; fill 64 bytes
  LDA #$00
FillAttrib0Loop:
  STA $2007
  DEX
  BNE FillAttrib0Loop


FillAttrib1:
  LDA $2002             ; read PPU status to reset the high/low latch
  LDA #$27
  STA $2006             ; write the high byte of $27C0 address (nametable 1 attributes)
  LDA #$C0
  STA $2006             ; write the low byte of $27C0 address
  LDX #$40              ; fill 64 bytes
  LDA #$FF
FillAttrib1Loop:
  STA $2007
  DEX
  BNE FillAttrib1Loop

         
             
  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:

  ;JMP AutoScroll

  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


LatchController:
  LDA #$01
  STA $4016
  LDA #$00
  STA $4016       ; tell both the controllers to latch buttons
   


  LDA $4016
  LDA $4016
  LDA $4016
  LDA $4016
  LDA $4016
  LDA $4016
  LDA $4016
  LDA $4016       ; player 1 - right
  AND #%00000001  ; only look at bit 0
  BEQ ReadRightDone   ; branch to ReadADone if button is NOT pressed (0)
                  ; add instructions here to do something when button IS pressed (1)

  LDY #$00
 

LoopR:
 
  LDA $0203, y
  CLC
  ADC #$01
  STA $0203,y

  INY
  INY
  INY
  INY
  CPY #$10
  BNE LoopR

  INC scroll

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:


NewColumnCheck:
  LDA scroll
  AND #%00000111           ; throw away higher bits
  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:

ReadRightDone:

 
  RTI              ; return from interrupt
 
 

AutoScroll:


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
  AND #%11111000
  LSR A
  LSR A
  LSR A
  STA sourceHigh
 
  LDA sourceLow       ; column data start + offset = address to load column data from
  CLC
  ADC #LOW(columnData)
  STA sourceLow
  LDA sourceHigh
  ADC #HIGH(columnData)
  STA sourceHigh

DrawColumn:
  LDA #%10010100        ; 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
;;;;;;;;;;;;;; 
 
 
 

 
  .bank 1
  .org $E000
palette:
  .db $22,$29,$1A,$0F,  $22,$36,$17,$0F,  $22,$30,$21,$0F,  $22,$27,$17,$0F   ;;background palette
  .db $22,$1C,$15,$14,  $22,$02,$38,$3C,  $22,$1C,$15,$14,  $22,$02,$38,$3C   ;;sprite palette

sprites:
     ;vert tile attr horiz
  .db $BF, $32, $00, $00   ;sprite 0
  .db $BF, $33, $00, $08   ;sprite 1
  .db $C7, $34, $00, $00   ;sprite 2
  .db $C7, $35, $00, $08   ;sprite 3


columnData:
  .incbin "SMBlevel.bin"


  .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
 
 
;;;;;;;;;;;;;; 
 
 
  .bank 2
  .org $0000
  .incbin "mario.chr"   ;includes 8KB graphics file from SMB1


Thanks for your help!


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PostPosted: Tue May 02, 2017 4:39 am 
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Joined: Fri May 08, 2015 7:17 pm
Posts: 1686
Location: DIGDUG
Anytime you see a misaligned screen, you can be sure that you are writing to the PPU registers during rendering (and not inside v-blank).

V-blank is very very very short. You don't have time to do logic before you write to the background.

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PostPosted: Tue May 02, 2017 4:44 am 
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Joined: Thu Sep 15, 2016 6:29 am
Posts: 248
Location: Denmark (PAL)
Without going indepth into the code, what's the chance that this happens every time you load new data into your nametables, and the code that writes to the PPU exceeds the vblank time? That would explain why it only happens for one frame around every row of tiles.

edit: dougeff beat me to it :(


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PostPosted: Tue May 02, 2017 6:08 am 
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Joined: Wed Apr 26, 2017 8:02 am
Posts: 4
You were damn right. I've changed the code so I check the controller pad after drawing and it works perfect now. I read about vblank but I didn't think it was this short.

Thanks for your help!


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