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I now have a working tape-version (.WAV) of the assembly game which I ported from MSX to Sega. Game loads nicely into my real Sega SC-3000 from my laptop.

I use PSGlib for sounds which needs a steady interrupt. This was easy to do with the ROM-version of the game. But with tape-version which is executed from BASIC with a CALL, I cannot figure out how to trap (hook) an interrupt for PSGlib for the sounds to work.

I searched through the reserved RAM-area ($8000-$97ff) for a possible address where one could place the interrupt hook but couldn't find one. How should the hooking be done?

Thanks in advance
/helmha

It depends how (or if) Basic allows you to hook it. Which version are you targeting?

Maxim wrote

It depends how (or if) Basic allows you to hook it. Which version are you targeting?

I have the BASIC LEVEL III B-cartridge.

I looked pretty hard into the Basic IIIB source a few years back. From memory there is no way to hook the VDP interrupt.

So instead you have to write a polling loop that keeps polling the VDP status register until you see that bit 7 is set (which happens just before the interrupt). You then execute your screen / keyboard / sound handling, and go back into the polling loop.

I think you want something very similar to this:

di ; prevent the Basic IIIB interrupt handler from running

...

; VDP polling loop
VDPPollingLoop:
ld c, $BF ; vdp status register is $BF
in a, (c) ; read vdp status register
and $80 ; check if the interrupt has triggered - ie. bit 7 is set
jr z, VDPPollingLoop ; if not triggered, do polling loop again
ret

...

DoSomething:
; do whatever VRAM updates and keyboard / joystick scans you need or PSGlib handling

...

call VDPPollingLoop ; now call the polling loop and wait until next interrupt is ready
jp DoSomething ; and go do your interrupt handling again

My apologies if that isn't quite right. It should be pretty close though :)

Some more info from Sega Computer Magazine August 1988

"Bit 7 is set at the end of each raster scan pending an interrupt. It must be read every interrupt in order to clear the interrupt and receive the new interrupt for the next frame. This is why we must disable interrupts before writing or reading to VRAM. At the end of every interrupt the status register is read and the address register is cleared, resulting in some strange effects if not disabled."

If you enable interrupts then that may cause more trouble. You don’t really want to run the built in interrupt handler, and it may well cause the polling loop to miss frames.

Maxim wrote

If you enable interrupts then that may cause more trouble. You don’t really want to run the built in interrupt handler, and it may well cause the polling loop to miss frames.

Yes, agreed. All the 100% machine code tape games I have seen on the SC-3000 do di right at the start and never enable them again - so you only use the polling loop and the Basic IIIB VDP handler doesn't trigger again.

Will edit the above example to put di at the start

Thank you very much! It seems to work alright :)

No worries. Looking forward to seeing the ported game :)

Cheers

Author	Message
helmha Joined: 11 Dec 2019 Posts: 30 Location: Finland	Real SC3000, BASIC-cartridge and interrupts Posted: Sat Feb 15, 2020 11:30 am
helmha Joined: 11 Dec 2019 Posts: 30 Location: Finland	I now have a working tape-version (.WAV) of the assembly game which I ported from MSX to Sega. Game loads nicely into my real Sega SC-3000 from my laptop. I use PSGlib for sounds which needs a steady interrupt. This was easy to do with the ROM-version of the game. But with tape-version which is executed from BASIC with a CALL, I cannot figure out how to trap (hook) an interrupt for PSGlib for the sounds to work. I searched through the reserved RAM-area ($8000-$97ff) for a possible address where one could place the interrupt hook but couldn't find one. How should the hooking be done? Thanks in advance /helmha

Maxim Site Admin Joined: 19 Oct 1999 Posts: 14744 Location: London	Posted: Sat Feb 15, 2020 12:19 pm
	It depends how (or if) Basic allows you to hook it. Which version are you targeting?

helmha Joined: 11 Dec 2019 Posts: 30 Location: Finland	Posted: Sat Feb 15, 2020 1:51 pm
helmha Joined: 11 Dec 2019 Posts: 30 Location: Finland	Maxim wrote It depends how (or if) Basic allows you to hook it. Which version are you targeting? I have the BASIC LEVEL III B-cartridge.

honestbob Joined: 03 Oct 2011 Posts: 188 Location: New Zealand	Posted: Sun Feb 16, 2020 11:53 pm Last edited by honestbob on Mon Feb 17, 2020 9:27 pm; edited 2 times in total
	I looked pretty hard into the Basic IIIB source a few years back. From memory there is no way to hook the VDP interrupt. So instead you have to write a polling loop that keeps polling the VDP status register until you see that bit 7 is set (which happens just before the interrupt). You then execute your screen / keyboard / sound handling, and go back into the polling loop. I think you want something very similar to this: di ; prevent the Basic IIIB interrupt handler from running ... ; VDP polling loop VDPPollingLoop: ld c, $BF ; vdp status register is $BF in a, (c) ; read vdp status register and $80 ; check if the interrupt has triggered - ie. bit 7 is set jr z, VDPPollingLoop ; if not triggered, do polling loop again ret ... DoSomething: ; do whatever VRAM updates and keyboard / joystick scans you need or PSGlib handling ... call VDPPollingLoop ; now call the polling loop and wait until next interrupt is ready jp DoSomething ; and go do your interrupt handling again My apologies if that isn't quite right. It should be pretty close though :) Some more info from Sega Computer Magazine August 1988 "Bit 7 is set at the end of each raster scan pending an interrupt. It must be read every interrupt in order to clear the interrupt and receive the new interrupt for the next frame. This is why we must disable interrupts before writing or reading to VRAM. At the end of every interrupt the status register is read and the address register is cleared, resulting in some strange effects if not disabled."

Maxim Site Admin Joined: 19 Oct 1999 Posts: 14744 Location: London	Posted: Mon Feb 17, 2020 12:46 am
	If you enable interrupts then that may cause more trouble. You don’t really want to run the built in interrupt handler, and it may well cause the polling loop to miss frames.

honestbob Joined: 03 Oct 2011 Posts: 188 Location: New Zealand	Posted: Mon Feb 17, 2020 1:07 am
	Maxim wrote If you enable interrupts then that may cause more trouble. You don’t really want to run the built in interrupt handler, and it may well cause the polling loop to miss frames. Yes, agreed. All the 100% machine code tape games I have seen on the SC-3000 do di right at the start and never enable them again - so you only use the polling loop and the Basic IIIB VDP handler doesn't trigger again. Will edit the above example to put di at the start

helmha Joined: 11 Dec 2019 Posts: 30 Location: Finland	Posted: Tue Feb 18, 2020 1:17 pm
helmha Joined: 11 Dec 2019 Posts: 30 Location: Finland	Thank you very much! It seems to work alright :)

honestbob Joined: 03 Oct 2011 Posts: 188 Location: New Zealand	Posted: Tue Feb 18, 2020 11:41 pm
	No worries. Looking forward to seeing the ported game :) Cheers

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