Background not loading correctly

[instantaphex]

So I've seen a couple of other posts about background loading issues and clarification, but I'm not able to get my background working correctly. Maybe someone can clear things up for me. I'm trying to load a full nametable with attributes once on reset and I'm essentially just getting garbage. For whatever reason, my code is not actually loading any of the data. The screen is filled with mostly zeros (I'm using mario.chr from SMB1). I've removed everything unrelated to drawing the background to keep this code dump to a minimum but it's still a code dump. I'm not able to see what it is that I'm doing wrong here. Can anyone help me out?

Code: Select all

  .inesprg 1   ; 1x 16KB PRG code
  .ineschr 1   ; 1x  8KB CHR data
  .inesmap 0   ; mapper 0 = NROM, no bank swapping
  .inesmir 1   ; background mirroring

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; Zero Page Pointers
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
	.rsset $0000
bg_ptr_hi .rs 1 ; bg pointer high byte
bg_ptr_low .rs 1 ; bg pointer low byte

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; Bank 0
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
  .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    ;move all sprites off screen
  INX
  BNE clrmem
   
vblankwait2:      ; Second wait for vblank, PPU is ready after this
  BIT $2002
  BPL vblankwait2

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; Load Assets
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
LoadBackground:
  LDA $2002       ; reset high/low latch
  LDA #$20
  STA $2006       ; write high byte
  LDA #$00
  STA $2006       ; write low byte

  ; set up pointer to bg
  LDA #LOW(background) ; #$00
  STA bg_ptr_low  ; put low byte of bg into pointer
  LDA #HIGH(background)
  STA bg_ptr_hi   ; put high byte into pointer

  LDX #$04
  LDY #$00
LoadBackgroundLoop:
  LDA [bg_ptr_low], y ; one byte from address + y
  STA $2007
  INY                 ; increment inner loop counter
  BNE LoadBackgroundLoop 
  INC bg_ptr_hi
  DEX 
  BNE LoadBackgroundLoop

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
LoadPalettesLoop:
  LDA palette, x        ;load palette byte
  STA $2007             ;write to PPU
  INX                   ;set index to next byte
  CPX #$20            
  BNE LoadPalettesLoop  ;if x = $20, 32 bytes copied, all done


;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; Housekeeping
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
  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

	; tell the PPU we aren't doing any scrolling at the end of NMI
  LDA #$00
  STA $2005
  STA $2005

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

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; VBlank Code
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
NMI:
  RTI

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; Bank 1
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
  .bank 1
  .org $E000
background:
  .db $24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24  ;;row 1
  .db $24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24  ;;all sky

  .db $24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24  ;;row 2
  .db $24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24  ;;all sky

  .db $24,$24,$24,$24,$45,$45,$24,$24,$45,$45,$45,$45,$45,$45,$24,$24  ;;row 3
  .db $24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$53,$54,$24,$24  ;;some brick tops

  .db $24,$24,$24,$24,$47,$47,$24,$24,$47,$47,$47,$47,$47,$47,$24,$24  ;;row 4
  .db $24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$55,$56,$24,$24  ;;brick bottoms

  .db $24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24  ;;row 5
  .db $24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24  ;;all sky

  .db $24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24  ;;row 6
  .db $24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24  ;;all sky

  .db $24,$24,$24,$24,$45,$45,$24,$24,$45,$45,$45,$45,$45,$45,$24,$24  ;;row 7
  .db $24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$53,$54,$24,$24  ;;some brick tops

  .db $24,$24,$24,$24,$47,$47,$24,$24,$47,$47,$47,$47,$47,$47,$24,$24  ;;row 8
  .db $24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$55,$56,$24,$24  ;;brick bottoms

  .db $24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24  ;;row 9
  .db $24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24  ;;all sky

  .db $24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24  ;;row 10
  .db $24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24  ;;all sky

  .db $24,$24,$24,$24,$45,$45,$24,$24,$45,$45,$45,$45,$45,$45,$24,$24  ;;row 11
  .db $24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$53,$54,$24,$24  ;;some brick tops

  .db $24,$24,$24,$24,$47,$47,$24,$24,$47,$47,$47,$47,$47,$47,$24,$24  ;;row 12
  .db $24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$55,$56,$24,$24  ;;brick bottoms

  .db $24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24  ;;row 13
  .db $24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24  ;;all sky

  .db $24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24  ;;row 14
  .db $24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24  ;;all sky

  .db $24,$24,$24,$24,$45,$45,$24,$24,$45,$45,$45,$45,$45,$45,$24,$24  ;;row 15
  .db $24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$53,$54,$24,$24  ;;some brick tops

  .db $24,$24,$24,$24,$47,$47,$24,$24,$47,$47,$47,$47,$47,$47,$24,$24  ;;row 16
  .db $24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$55,$56,$24,$24  ;;brick bottoms

  .db $24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24  ;;row 17
  .db $24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24  ;;all sky

  .db $24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24  ;;row 18
  .db $24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24  ;;all sky

  .db $24,$24,$24,$24,$45,$45,$24,$24,$45,$45,$45,$45,$45,$45,$24,$24  ;;row 19
  .db $24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$53,$54,$24,$24  ;;some brick tops

  .db $24,$24,$24,$24,$47,$47,$24,$24,$47,$47,$47,$47,$47,$47,$24,$24  ;;row 20
  .db $24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$55,$56,$24,$24  ;;brick bottoms

  .db $24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24  ;;row 21
  .db $24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24  ;;all sky

  .db $24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24  ;;row 22
  .db $24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24  ;;all sky

  .db $24,$24,$24,$24,$45,$45,$24,$24,$45,$45,$45,$45,$45,$45,$24,$24  ;;row 23
  .db $24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$53,$54,$24,$24  ;;some brick tops

  .db $24,$24,$24,$24,$47,$47,$24,$24,$47,$47,$47,$47,$47,$47,$24,$24  ;;row 24
  .db $24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$55,$56,$24,$24  ;;brick bottoms

  .db $24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24  ;;row 25
  .db $24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24  ;;all sky

  .db $24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24  ;;row 26
  .db $24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24  ;;all sky

  .db $24,$24,$24,$24,$45,$45,$24,$24,$45,$45,$45,$45,$45,$45,$24,$24  ;;row 27
  .db $24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$53,$54,$24,$24  ;;some brick tops

  .db $24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24  ;;row 28
  .db $24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24  ;;all sky

  .db $24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24  ;;row 29
  .db $24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24  ;;all sky

  .db $24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24  ;;row 30
  .db $24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24,$24  ;;all sky

attributes:  ;8 x 8 = 64 bytes
  .db %00000000, %00010000, %01010000, %00010000, %00000000, %00000000, %00000000, %00110000
  .db %00000000, %00010000, %01010000, %00010000, %00000000, %00000000, %00000000, %00110000
  .db %00000000, %00010000, %01010000, %00010000, %00000000, %00000000, %00000000, %00110000
  .db %00000000, %00010000, %01010000, %00010000, %00000000, %00000000, %00000000, %00110000
  .db %00000000, %00010000, %01010000, %00010000, %00000000, %00000000, %00000000, %00110000
  .db %00000000, %00010000, %01010000, %00010000, %00000000, %00000000, %00000000, %00110000
  .db %00000000, %00010000, %01010000, %00010000, %00000000, %00000000, %00000000, %00110000
  .db %00000000, %00010000, %01010000, %00010000, %00000000, %00000000, %00000000, %00110000

  .db $24,$24,$24,$24, $47,$47,$24,$24 
  .db $47,$47,$47,$47, $47,$47,$24,$24 
  .db $24,$24,$24,$24 ,$24,$24,$24,$24
  .db $24,$24,$24,$24, $55,$56,$24,$24  ;;brick bottoms
  .db $47,$47,$47,$47, $47,$47,$24,$24 
  .db $24,$24,$24,$24 ,$24,$24,$24,$24
  .db $24,$24,$24,$24, $55,$56,$24,$24 

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

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

I've tried reworking my background loading loop several times to no avail. I'm following the nerdy nights tutorial, which is probably obvious to anyone looking at my code. I've tried comparing my code to the example from that tutorial but I can't see where I'm going wrong. Does anything stick out to anyone else?

[Quietust]

Most of the code looks okay, though I see at least two things that could cause problems:

1. You should wait until VBLANK before enabling rendering in your "Housekeeping" section, otherwise you'll get a glitched-out half frame.
2. You should set your scroll ($2000/$2005/$2005) before enabling rendering ($2001), though this should only be an issue if you were mid-frame above.

[dougeff]

The positions of pointer low and pointer high need to be reversed. 6502 is little endian, with the low byte coming first in RAM addresses.

Simple fix...

.rsset $0000
bg_ptr_low .rs 1 ; bg pointer low byte
bg_ptr_hi .rs 1 ; bg pointer high byte


with no other changes. might work.

[instantaphex]

The positions of pointer low and pointer high need to be reversed. 6502 is little endian, with the low byte coming first in RAM addresses.

Awesome, that fixed it! Thanks a lot!

[Quietust]

The positions of pointer low and pointer high need to be reversed.

Funny how you can miss the simplest mistakes - I usually allocate pointers as 2-byte sequences (and use macros to select the upper and lower bytes), so I didn't think to look at the order in which the two bytes had been declared.

[instantaphex]

The positions of pointer low and pointer high need to be reversed.

Funny how you can miss the simplest mistakes - I usually allocate pointers as 2-byte sequences (and use macros to select the upper and lower bytes), so I didn't think to look at the order in which the two bytes had been declared.

That's a good idea. I'm still trying to wrap my mind around doing this stuff in such a primitive feeling way. It's fun but it's taking me quite a lot of time to bang out just a few lines of code. I might have to try using some macros once I get a bit more comfortable.

[tokumaru]

I always allocate the total number of bytes of multi-byte variables, and refer to the individual bytes by adding their indices to the base address (i.e. VariableName+0 is the first byte, VariableName+1 is the second byte, and so on). Adding 0 to the base address may look silly, but makes it clear that this is indeed the lowest byte of a multi-byte variable.