Turn on/off blitter priority during blit

[LeCaravage]

Hi folks,

I wonder as I meet the case, but is it safe to turn on or off the blitter priority over cpu while the blitter is running ?

As it seems to work on unexpanded Amiga 500, what happens with A1200/A3000/A4000 etc... ?

[Thomas Richter]

Generally not advisable. The problem is that the CPU then gets no slot whatsoever, in particular neither for processing interrupts, which may - depending on the nature of the interrupt - cause quite some disturbance in the system.

[chb]

Originally Posted by Thomas Richter

The problem is that the CPU then gets no slot whatsoever, in particular neither for processing interrupts, which may - depending on the nature of the interrupt - cause quite some disturbance in the system.

OT: That's only true for blits without idle cycles. A simple D clear blit e.g. will leave free cycles for the CPU even with blit priority on.

[phx]

Thomas and chb are right.

It is safe to change Blitter priority while the Blitter is running, but when the Blitter leaves no free cycles you have to know what you're doing.

For example in my games setting the Blitter priority is part of the WAITBLIT macro, because the CPU definitely has nothing else to do when it reaches WAITBLIT. I have only VERTB and CIAB-timer interrupts enabled. The VERTB is no problem, because my game engine runs within the interrupt. The timer interrupt is for the music, and it usually doesn't matter when it is delayed until a blit finishes.

It's a design decision you have to make.

[a/b]

Depends on context...
OS-killer? Do whatever you want.
OS friendly with multitasking disabled? Do whatever you want.
OS friendly with multitasking enabled? Sure, if you know what you are doing. But as Thomas hinted, there might be unwated side effects, some of them are depending on hardware config (eg. yes/no fast ram?).

[LeCaravage]

Sorry I forgot to give context.

Amiga 500 KS1.3 (512/512).
I tun off system and restablish it at exit.

I use the blitter priority over cpu with only one Destination and zero source.

As I can understand by reading your answers, it's safe to use it in this context, but is it the same with faster Amigas ? A1200,A4000... ?

[roondar]

Originally Posted by LeCaravage

Sorry I forgot to give context.

Amiga 500 KS1.3 (512/512).
I tun off system and restablish it at exit.

I use the blitter priority over cpu with only one Destination and zero source.

As I can understand by reading your answers, it's safe to use it in this context, but is it the same with faster Amigas ? A1200,A4000... ?

It shouldn't function any differently on A1200/A4000.

The only thing to keep in mind is that you should always include a Blitter wait routine/macro even if you run with the Blitter set to have priority, as faster systems might still have the CPU running (either from the cache or from Fast Memory) even if the Blitter is set up this way and this can potentially end up setting up Blitter registers for new blits while the old one is still running.

[LeCaravage]

Originally Posted by roondar

It shouldn't function any differently on A1200/A4000.

The only thing to keep in mind is that you should always include a Blitter wait routine/macro even if you run with the Blitter set to have priority, as faster systems might still have the CPU running (either from the cache or from Fast Memory) even if the Blitter is set up this way and this can potentially end up setting up Blitter registers for new blits while the old one is still running.

It is hard to resist temptation of removing waitblit() ^^

[paraj]

Originally Posted by LeCaravage

It is hard to resist temptation of removing waitblit() ^^

Resist this temptation. If you're making some record breaking demo for A500 I'll give a pass, otherwise it's just not worth it. You will regret it.

Instead as mentioned upthread doing D=0 only uses half of blitter cycles, better to use them with the CPU

[LeCaravage]

Originally Posted by phx

Thomas and chb are right.

It is safe to change Blitter priority while the Blitter is running, but when the Blitter leaves no free cycles you have to know what you're doing.

For example in my games setting the Blitter priority is part of the WAITBLIT macro, because the CPU definitely has nothing else to do when it reaches WAITBLIT. I have only VERTB and CIAB-timer interrupts enabled. The VERTB is no problem, because my game engine runs within the interrupt. The timer interrupt is for the music, and it usually doesn't matter when it is delayed until a blit finishes.

It's a design decision you have to make.

It's the first time I hear a engine like yours. You mean the blit operatios are within the IRQ ? What happens if the frame drops ?

[phx]

It's not so uncommon.

Originally Posted by LeCaravage

You mean the blit operatios are within the IRQ ?

Yes. Everything.

What happens if the frame drops ?

Shouldn't happen. These games are designed to run at 50 fps on all Amigas.

Advantage of this approach (for me) is that the main program can still do some low priority tasks in parallel, like loading and decoding data from disk, while the rendering is done at high priority during the interrupt.