i've written a small nes demo witch doesnt do much but should do alot more. My code is very simple and basic, but i cant seems to understand how to display something on the screen properly. I write to NT1 values but tiles never shows at the right place. For example, if i write to vram $2000, i see nothing but if i write to $2120, i see my stuff at the top of the screen. Here's a sample of my code:

start:
sei
cld

lda $2002
bpl start

lda #$00
sta $2005
sta $2005

lda #%00000000
sta $2000
lda #%00011110
sta $2001

lda #$20
sta $2006
lda #$00
sta $2006

lda #$01
sta $2007

Should this display my tiles at the top of the screen? I tested it out with fceultra but it doenst work.

thanks

You need to set the scroll data through $2005 as your *last* step, not your first. The PPU renders based on the last values in its scroll registers, which overlap somewhat with what $2006 writes do.

Also, ensure that you've actually enabled backgrounds and selected the appropriate nametable and such by writing to $2000 and $2001.

Also, have $2006 point to $0000 (or $2000) before you turn the screen on.

it doesnt seems to help. my code now look like this:

start:
sei
cld

;wait for vblank
lda $2002
bpl start

;activate background, set the name table ... etc
lda #%00000000
sta $2000
lda #%00011110
sta $2001

;set vram address to the first NT
lda #$20
sta $2006
lda #$00
sta $2006

;write data to NT1
lda #$01
sta $2007
lda #$02
sta $2007
....

lda #$00
sta $2006
sta $2006
sta $2005
sta $2005
loop:
jmp loop

and it doesnt show anything with fceultra. If i write to the second line of NT1 ($2020), i began to see some stuff at the top of the screen.

Any idea?

Oh, I see the problem. Set $2001 to zero before you do any writing to VRAM. Then turn the screen on after you've cleared $2005 and $2006. That should fix it.

The program is modifying the VRAM address ($2006) at the same time the PPU is using it. That's why VRAM can only be updated during vblank, or when the sprites+bg are disabled.

You should also put some colors in the palette ram ($3F00), tiles is not visible when they have the same color as the background.

hummm. I set the PAL option in fceultra and now i am seeing my tiles. Could this be that putting stuff to $2000-$201f (first line) only shows on pal machine? And how an emulator can auto detect PAL/NTSC?

Hmm, that's something else I didn't consider. On NTSC the top 8 lines (and bottom 8 I think) are drawn but aren't visable on an NTSC TV. I think FCEUltra has an option to show them in NTSC mode.

The emulators can't autodetect PAL/NTSC, at least not with the iNES format. Unless it does a CRC check on the ROM and it has an internal database.

As far as I can tell, you're correct about the top row of tiles not being displayed. Since I don't know what hardware or emulators would display them (but I'm sure it's out there), you're probably best off filling in the name table as if they were visible.