raydempsey wrote:
Code:
[end of NMI]
lda #0 ;<-------------------- A
sta $2005
lda #0 ;<-------------------- B
sta $2005
rts
[... in IRQ for raster split]
lda #0 ;<--------------- C
sta $2006
lda #0 ;<--------------- D
sta $2005
[... a small delay ...]
lda #0 ;<--------------- E
sta $2005
lda #0 ;<--------------- F
sta $2006
I have been trying unsuccessfully to create a banner. I don't know what the values in A thru F mean. I tried experimenting and I still don't get it. [...] What do A thru F mean and what should I put where?
A and B:
as long as your NMI is short enough, A and B are the lower 8 bits of the X and Y coordinates of the initial (i.e. top of screen) scroll.
C,D,E,F: This is a messy thing that is an artifact of how scrolling works on the NES.
wiki:PPU scrolling#Split X & Y scroll is an exactingly detailed explanation...
The easy ones:
C specifies the nametable, or equivalently the 256s bit of X scroll and the 240s bit of Y scroll
E and D (notice reversed order) are the same as A and B, but for the scroll values desired after the split.
F needs to be a bit-shifted and combined copy of your X and Y scroll values.
Quote:
What should I do to make it so I have a banner that starts 32 scanlines down where the top has no vert or horiz scroll while the last of the scanlines look shifted to the right n pixels.
To the best of my knowledge, this thing you already have:
Code:
lda #31
sta $5203
lda #$80
sta $5204
should be enough to make
something happen 32 scanlines down.
Quote:
What am I doing wrong?
It would help if you told us what you saw instead...
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