hello,

I have been putting together my first game project, and have run into a slight graphical glitch. From what I read, all writes to VRAM should be executed during vblank, and in most emulators the code I have works.. I tried the game in nestopia. When I perform a hard reset (power on/off, or on load), occasionally one or two garbage tiles appear on-screen. Also, in PAL mode (or any mode where the top 8 scanlines are visible), the second entry in pattern table appears 100% of the time in the top left corner (bg is supposed to be disabled). The first entry in PT is a blank, and there are three entries total. My startup code looks something like this (compiler is nesasm):

main:

cld ; clear decimal flag
sei ; disable interrupts
ldx #$FF ; ensure stack is pointing to top
txs
inx

jsr waitvb
stx $2000
stx $2001 ; this SHOULD disable the PPU during startup

; clear $0000-$07FF in RAM.
ldy #$07
sty <$01
lda #$00
sta <$00

clr_ram:
sta [$00],y ; clear $100 bytes
dey
bne clr_ram

dec <$01
bpl clr_ram

;Initialize the Palette

lda #$3f
sta $2006
lda #$00
sta $2006
tay
do_pal:
lda palette,y
sta $2007
iny
cpy #$20
bne do_pal

;clear out $0000-$1FFF in VRAM
lda #0
sta $2006
sta $2006
ldy #$20
cleartiles1:
ldx #0
cleartiles2:
sta $2007
inx
bne cleartiles2
dey
bne cleartiles1

tiles: ; setup the pattern table
lda #0
sta $2006
sta $2006

ldy #0
tiles1:
lda tileset, y
sta $2007
iny
cpy #$30
bne tiles1

inits:
ldy #0
inits1:
lda initdata, y
sta P1.PX, y
lda initsprites, y
sta P1A.Y, y
iny
cpy #$10
bne inits1

lda #7
sta $4014

jsr waitvb
;!Initialize the PPU!
lda #%10000000 ;NMI on Vblank on!
sta $2000
lda #%00010100 ;sprites on, no clipping
sta $2001
cli

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Other than the glitch on startup, the game otherwise functions normally.