How does Paula handle pending interrupts?

[bloodline]

I think I've found a proble with my code.

In this thought experiment assume all ints are enabled.

If a VBL interrupt bit is set, the CPU interrupt level is set to 3, if during the processing of that exception, a timer interrupt bit is set, does Paula wait for the int_ack from the CPU before she sets the lower pri level?

[Toni Wilen]

Paula does not wait anything, IPL signals change instantly (there is one or two clock delay, probably to guarantee no spurious interrupts but you don't need to care about that).

CPU starts new interrupt exception if new IPL is higher than current interrupt mask in SR (=interrupts can stack).

[bloodline]

Originally Posted by Toni Wilen

Paula does not wait anything, IPL signals change instantly (there is one or two clock delay, probably to guarantee no spurious interrupts but you don't need to care about that).

CPU starts new interrupt exception if new IPL is higher than current interrupt mask in SR (=interrupts can stack).

Many thanks for your help again, so if low priority int is asserted at the same time as a higher priority one, they will both be executed in priority order?

Assuming the above to be true, my Emulator now just gets to a loop where is appears to be testing bit 6 in the the DMACONR reg... am I going to need to emulate the Blitter? I was only ever planning to implement the very minimum to get the OS booting

[Toni Wilen]

There is no "interrupt state" in CPU, CPU does nothing more than check between each instruction if external interrupt level (IPL pins from Paula) are higher than SR register interrupt mask. If it is higher, new interrupt exception starts and mask is updated to current level. (Level 7 works differently but it is not needed, Amiga can't generate it without extra hardware)

You can even write to SR and lower interrupt mask in interrupt routine, if current IPL is higher than new mask (=enabled and active in Paula), new interrupt starts immediately.

Yes, you need to emulate blitter because all graphics (and possibly some non-graphics too) stuff use it, including KS insert disk screen.

[bloodline]

Originally Posted by Toni Wilen

There is no "interrupt state" in CPU, CPU does nothing more than check between each instruction if external interrupt level (IPL pins from Paula) are higher than SR register interrupt mask. If it is higher, new interrupt exception starts and mask is updated to current level. (Level 7 works differently but it is not needed, Amiga can't generate it without extra hardware)

You can even write to SR and lower interrupt mask in interrupt routine, if current IPL is higher than new mask (=enabled and active in Paula), new interrupt starts immediately.

Now I understand, so Paula just holds the IPL pins at the highest enabled Interrupt level. That makes sense and is easy to implement.

Yes, you need to emulate blitter because all graphics (and possibly some non-graphics too) stuff use it, including KS insert disk screen.

I naïvely assumed that Kick1.3 would just copy the Kickstart image to chipram and then set up the blp pointers to display it... (since there is no animation).

The Blitter is going to take quite a bit of work... and I suspect I will need to make my timings far more accurate, with some proper DMA emulation too... I strongly suspect it would have been easier just to use UAE sources but that would have been much less fun for me.

I'll have to publish the sources as an example of how NOT to write an Amiga emulator.

[Toni Wilen]

Blitter timing is 100% unimportant in your use case. You can emulate whole blit in a single pass when program writes to BLTSIZE. (Same as UAE "immediate blitter").

Or just set blitter finished bit when program writes to BLTSIZE and check how far you can get without side-effects

[bloodline]

Originally Posted by Toni Wilen

Blitter timing is 100% unimportant in your use case. You can emulate whole blit in a single pass when program writes to BLTSIZE. (Same as UAE "immediate blitter").

That's a relief

Or just set blitter finished bit when program writes to BLTSIZE and check how far you can get without side-effects

Thanks for the steer, I'll try it and see what happens

Again I appreciate the help with this.