How many clock cycles fit in one vblank?

djcouchycouch · Post by **djcouchycouch** » Fri Jun 17, 2011 1:43 pm

I have no idea how much code I can put into vblank without causing trouble. Are there any rules of thumb or guidelines that would give me an idea?

cpow · Post by **cpow** » Fri Jun 17, 2011 2:11 pm

shawnleblanc wrote:I have no idea how much code I can put into vblank without causing trouble. Are there any rules of thumb or guidelines that would give me an idea?

About 2273 CPU cycles. Depending on your instruction mix that's either a chunk of code (1136 2-cycle opcodes) or a little bit of code (324 7-cycle opcodes). So, I guess that isn't very helpful. You'll be somewhere between 324 and 1136, roughly.

Dwedit · Post by **Dwedit** » Fri Jun 17, 2011 2:24 pm

262 total scanlines, 240 are visible.
1 scanline is the pre-vblank scanline and you can't use it unless you have IRQs or other timed code to know when that scanline is coming.
1 scanline is the pre-render scanline, you can use it as extra vblank time if you turn off the screen, then do a complete set of scroll writes after turning the screen back on.

If you just use the normal vblank lines, that's 21 scanlines at 341 PPU cycles (113.666 CPU cycles) each, for 2387 total CPU cycles. Of course, there's also the NMI interrupt triggering, and all your other code to get ready to start spitting out graphics, so there's less time than that actually available for your graphics display loop.

qbradq · Post by **qbradq** » Fri Jun 17, 2011 4:26 pm

One thing I do is to set a breakpoint at the end of my VBlank routine in Nintendulator. It has this nice feature in e debugger that shows you what scan line and pixel you are on.

tokumaru · Post by **tokumaru** » Fri Jun 17, 2011 5:06 pm

qbradq wrote:One thing I do is to set a breakpoint at the end of my VBlank routine in Nintendulator. It has this nice feature in e debugger that shows you what scan line and pixel you are on.

I'd like to second this suggestion.

cpow · Post by **cpow** » Fri Jun 17, 2011 7:48 pm

tokumaru wrote:
qbradq wrote:One thing I do is to set a breakpoint at the end of my VBlank routine in Nintendulator. It has this nice feature in e debugger that shows you what scan line and pixel you are on.
I'd like to second this suggestion.

I'll put an alternative out there:

I set up the red execution marker in the nesyar source over its entire NMI routine, and the green marker over the jsr to the sound routine. The Execution Visualizer shows me where each marked code region is executing in the PPU frame and how many CPU cycles it took last time it ran. It's a bit hard to see in the image but the area to the right of the quotile shield is HBLANK (the red/green stretches into it. The image is 341x262 (or 341x312) pixels big, so you can see it covers the entire PPU clock domain and shows where the CPU is doing whatever you marked to see. I can add eight different marked regions and see where they are in relation to each other or how long they are taking. Plan is to add min/max/avg also to cycle counts. In the image, the top left (0,0) is the screen pixel (0,0), so VBLANK time is at the bottom (where it's all red). Obviously nesyar eats into the screen time a bit in NMI but only to do sound updates.

thefox · Post by **thefox** » Sat Jun 18, 2011 1:21 am

Yet another option is to use NintendulatorDX with code like this:

Code: Select all

nmi:
  sta $4020 ; Start the timer.

  ; TODO: Do the vblank updates here.

  sta $4030 ; Stop the timer.

  rti

Then you can see the number of cycles taken by the vblank updates in the debugging window. As long as the "max" value is less than ~2270, you're in the good.

Post by **tepples** » Sat Jun 18, 2011 8:38 am

cpow wrote:Plan is to add min/max/avg also to cycle counts.

With sparklines?

qbradq · Post by **qbradq** » Sat Jun 18, 2011 11:23 am

Dang CPOW! That's awesome! Is that something you wrote? Where can I get it?

koitsu · Post by **koitsu** » Sat Jun 18, 2011 11:36 am

Aww cpow, reminding me of my Apple IIGS days where you'd change the system border colour to something of your choice at the start of your routine, then back to whatever it was previously at the end, effectively using the border as a way to tell how much CPU time something took. :-) So happy.

Bregalad · Post by **Bregalad** » Sat Jun 18, 2011 1:07 pm

koitsu wrote:Aww cpow, reminding me of my Apple IIGS days where you'd change the system border colour to something of your choice at the start of your routine, then back to whatever it was previously at the end, effectively using the border as a way to tell how much CPU time something took. So happy.

You can do that on the NES with the $2001 register.

cpow · Post by **cpow** » Sat Jun 18, 2011 2:26 pm

qbradq wrote:Dang CPOW! That's awesome! Is that something you wrote? Where can I get it?

It's part of NESICIDE, which is something I wrote, yes.

cpow · Post by **cpow** » Sat Jun 18, 2011 2:32 pm

tepples wrote:
cpow wrote:Plan is to add min/max/avg also to cycle counts.
With sparklines?

Funny you should mention that tepples! I attended Edward Tufte's "Information Design" seminar last year. As part of that I got signed copies of all four of his books and a poster. I am a strong believer in techniques for visualization of information. Sparklines are so obviously useful, concise, and unencumbered with need for prior knowledge of the visualization technique.

I could probably whip up a sparkline control in Qt pretty quick...so now that you've planted the idea, expect to see it.

cpow · Post by **cpow** » Sat Jun 18, 2011 3:23 pm

qbradq wrote:Dang CPOW! That's awesome! Is that something you wrote? Where can I get it?

It just struck me that you might have been referring to the game visual, not the tool. No, I did not write nesyar. That was authored by clueless. Haven't seen him around in a while though.

qbradq · Post by **qbradq** » Sat Jun 18, 2011 5:24 pm

I was referring to the visualization. Can I run any ROM with timer writes in that or does it have to be a NESICIDE project?

How many clock cycles fit in one vblank?

How many clock cycles fit in one vblank?

Re: How many clock cycles fit in one vblank?