In our example we first set the scroll registers to 0 for the static status bar. The nametable is also set to 0. That makes sure that the background and sprite 0 will overlap in the correct place.

NMI:

  ; all graphics updating code goes here

  LDA #$00
  STA $2006        ; clean up PPU address registers
  STA $2006
  
  LDA #$00         ; start with no scroll for status bar
  STA $2005
  STA $2005
  
  LDA #%10010000   ; enable NMI, sprites from Pattern Table 0, background from Pattern Table 1
  STA $2000        ; start with nametable = 0 for status bar

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

Next we make sure the sprite 0 hit flag is clear, to avoid it being tripped from the previous frame. The flag is cleared at the end of vblank, so once it equals 0 you know the next frame has started.

WaitNotSprite0:
  lda $2002
  and #%01000000
  bne WaitNotSprite0   ; wait until sprite 0 not hit

Now we wait until the sprite 0 is hit. How long this takes depends on how far down the screen your sprite 0 is placed.

WaitSprite0:
  lda $2002
  and #%01000000
  beq WaitSprite0      ; wait until sprite 0 is hit

When that loop finishes, the PPU is drawing the first pixels of sprite 0 that overlap pixels on the background. We add a small wait loop so the rest of the status bar is drawn, and then change the scroll registers. The rest of the screen down is drawn using those settings.

  ldx #$10
WaitScanline:
  dex
  bne WaitScanline
  
  LDA scroll
  STA $2005        ; write the horizontal scroll count register
  LDA #$00         ; no vertical scrolling
  STA $2005
    
  LDA #%10010000   ; enable NMI, sprites from Pattern Table 0, background from Pattern Table 1
  ORA nametable    ; select correct nametable for bit 0
  STA $2000

So the order is:

  1 - set scroll to 0 for status bar
  2 - wait for sprite 0 hit = 0
  3 - wait for sprite 0 hit = 1
  4 - delay so scanline finishes drawing
  5 - set scroll for level background

The only other change is to make sure your graphics updating code does not draw over the status bar. The previous DrawNewColumn function handles the graphics updates so it has a few small differences. The starting address is increased by $80 to skip the first 4 rows of background. Then the source address is increased by $04 for the same reason.



Putting It All Together
Download and unzip the sprite0.zip sample files. sprite0.asm is the same as the previous scrolling5.asm file plus the changes covered here. This is another good one to watch in an emulator.