VWait

[bloodline]

Odd question, but does anyone know how VWait works?

If I use it in AMIGA mode, then it works perfectly fine.
If I use it in BLITZ mode, it never returns and the machine remains in a wait state.

-edit- for clarification, this is not a problem with Blitz Basic. It is a problem with my machine, I'm wondering if Blitz uses some kind of copper list hack to wait for the VBL rather installing an IRQ handler or something.

[bloodline]

it seems the offending code is:

E ce8e0c: 0839 0000 00df f005 : btst    #$0, $dff005.l
E ce8e14: 6700 fff6                  : beq     ce8e0c

Now to figure out how to handle it

-edit- actually quite simple, 0xDFF005 is the low byte of vposr, so Blitz is waiting for the vpos to reset, not entirely sure why it does that.

[earok]

With SetInt 5 you can define your own custom VBlank handler, I'm assuming Blitz doesn't use an Interrupt handler for VWait since it may interfere with whatever you've put in SetInt 5.

It's also possible to do this

SetInt 5
 Interrupted = true
Endif

While Interrupted=false
Wend

[bloodline]

Originally Posted by earok

With SetInt 5 you can define your own custom VBlank handler, I'm assuming Blitz doesn't use an Interrupt handler for VWait since it may interfere with whatever you've put in SetInt 5.

It's also possible to do this

SetInt 5
 Interrupted = true
Endif

While Interrupted=false
Wend

Hey there! Thanks for replying.

This is a pretty nice idea, but I needed to know what the command was doing as it was highlighting a bug in the Amiga Emulator I’m writing.

As you can see, I found that Vwait actually busy waits for the vposr register to clear the lowest bit. I’m not 100% sure why it does that (I can think of a few reasons, but none compelling), but upon fixing my Emulator to ensure the bit is cleared properly with each new frame, vwait works as it should.

[Daedalus]

I wonder if it uses different code for Blitz mode and Amiga mode maybe? The manual states that VWait should be used differently between the two modes. If so, busy waiting in Blitz mode is probably the simplest implementation, though I don't know why an interrupt-driven solution wouldn't be used for both.

[bloodline]

Originally Posted by Daedalus

I wonder if it uses different code for Blitz mode and Amiga mode maybe? The manual states that VWait should be used differently between the two modes. If so, busy waiting in Blitz mode is probably the simplest implementation, though I don't know why an interrupt-driven solution wouldn't be used for both.

At a guess, Amiga Mode uses graphics.library waitTOF() to ensure multitasking friendliness

[Daedalus]

Yeah, probably. In Blitz mode, there's no multitasking to care about so busy-looping is "acceptable".