LD A,$C3
LD ($FD9F),A
LD HL,STARS
LD ($FDA0),HL
RET

I figured out that this makes sets the computer to jump to label STARS on every interrupt.
But which interrupt is this?
And how can I make the computer jump to STARS at the start of VBLANK period?

And...... what does the LD A,$C3 imply to?
or the addresses ($FD9F-$FDA0)?

$FD9F is the VDP interrupt hook, which actually *is* jumped to at the start of the VBLANK. (unless you turned off IE0 in the VDP)

$C3 implies a JP instruction.

Well, first off, always be sure you do this with interrupts DISABLED. If not, you can get a real mess... Those addresses, called hooks, are CALLED every (vdp)interrupt by the MSX BIOS. There's two, one on $FD9A, one on $FD9F. Now, I can never remember which is which, but I *think* $FD9A is called on EVERY interrupt (be it from the VDP or any other source) and $FD9F is only called on the VDP's VBLANK. It could be the other way around tho...

Anyhoo, since the BIOS calls these hooks, you can put some code there to be executed whenever those hooks are called. By writing a $C3 to $FD9F and the address of 'STARS' to $FDA0-$FDA1 you're putting the code JP STARS on that hook. So basically you're telling the BIOS to jump to 'STARS' on every VDP vblank interrupt.

SetHook:
	LD	A,$C3
	LD	HL,MyInterrupt
	DI
	LD	($FD9F),A
	LD	($FDA0),HL
	EI
	RET

Mind you, you're not saving the old hook in this case. It's best to also save the previous code (using an LDIR or something) and jump to it at the end of your new interrupt routine...

)^_^) "Hi edwin!"

cool !!!
Now I know what a 'hook' is !!

Ouch !!! My brain is too small for all of this

another question
(you must be thinkin somethin like: AAAAAAAAAA no more questions !!!!)

If I put a sprite on screen and keep it at a fixed position (y=y+r#23)
in screen (in the centre),
while r#23 scrolls the screen (slowly) by constantly adding +1 to r#23

what happens is:
You see the sprite standing in the middle of the screen and you see a white block scrolling
from the bottom of the screen to the top, and this repeats itself.

When this white block touches the sprite (meaning the sprite has an y coordinate of 211)
the sprite will blink on that line.
If I scroll r#23 with 2 pixels at a time, the sprite will not blink, because it will never have an
y coordinate of 211.

Is there a sollution for this problem, or does this mean that vertical
scrolling allways has to occur with 2,4,6,8 (etc.) pixels at a time??

Quote:

By writing a $C3 to $FD9F and the address of 'STARS' to $FDA0-$FDA1 you're putting the code JP STARS on that hook. So basically you're telling the BIOS to jump to 'STARS' on every VDP vblank interrupt.

Actually, you're telling the Z80 to jump to 'STARS' when apparently the vblank caused the interrupt. The BIOS isn't going anywhere. It'll stay exactly where it is, since it's in ROM.

norakomi: the VDP manual states that a sprite on line 216 won't be displayed, and sprites with a lower priority than that one either. In MSX1 screenmodes that's Y coordinate 208. Perhaps that is what's buggering you

And there's nothing you can do about it, really. Just try to fix a workaround. I myself had problems with some BASIC race game (*sigh*, there we go again :\), which put the sprite on Y=215 whenever the Y coordinate would be 216.

Most games with hardware vertical scroll (if not all) simply jump over Y=216. You might want to use that too.

norakomi, can you read any dutch? then I might have the perfect articles for you lying around somewhere...

some specs on ints:
- Interrupts are called from the VDP times (50 times per second on PAL monitor and 60 time per second on NTSC monitor. you can select pal/ntsc mode with vpd reg 9. bit 1: 0=ntsc, 1=pal.. or was it the other way around?)
- There can also be other interrupt sources like the MSX AUDIO / MoonSound for example. But we'll stick the the default; VPD in these examples.
- There are 2 kinda VDP interrupts. The VBLANK which happens any 1/50(1/60) of a second at the BOTTOM of the screen. This one is kinda default. In BIOS mode (page 0,0,0 <- set to standard BIOS) and will call the "hook" 0xFD9F. The other interrupt is a bit more complicated; the Line Interrupt (LNI for short) You can set more than one LNI per screenbuild up. more on that later when you have some experience. let's concentrate on the Vblank for now
- Every time an (VDP) interrupt occurs the addres 0x38 in the BIOS is called. Take a look with your disassembler at that routine. 0x38 does various things; "call" the FD9F HOOK, handle keyboard input, handel some BASIC stuff (like the PLAY command), check drive status, control some timers and much more stuff.
- the HOOK is where you can "hook" on your own routine. check Sonic's remarks above... FD9F is the VBLANK hook while FD9A is the LNI hook

Hope you get an idea of how all this (complicated) stuff works...
cheers.

FD9A is the hook which is called every interrupt, regardless if it's a VDP line interrupt. This is done to have other than VDP interrupts handled, like MSX-AUDIO, GFX9000, Moonsound, RS232 interfaces and other hardware capable of sending an interrupt signal...

For example... The latest Moonblaster Wave replayer uses the OPL4 timer to handle the music playing on.

This in addition to ro's quite complete explaination...

Quote:

Most games with hardware vertical scroll (if not all) simply jump over Y=216. You might want to use that too.

Mm... don't think that will fix it. It's about to avoid sprites getting Y=216. Since sprite coordinates differ over time (that's what sprites are for, keep 'em moving), it's hard to tell at what R#23 value the sprite flicker will occur. Better to avoid it in the sprite tables.

I don't think you understand... It's not the value of R#23 that causes the sprite flicker, it's the Y coordinate of the sprite you need to update while doing a R#23 scroll to keep the sprite in the same place on the screen during the scroll.

Quote:

norakomi: the VDP manual states that a sprite on line 216 won't be displayed, and sprites with a lower priority than that one either. In MSX1 screenmodes that's Y coordinate 208. Perhaps that is what's buggering you And there's nothing you can do about it, really. Just try to fix a workaround. I myself had problems with some BASIC race game (*sigh*, there we go again :\), which put the sprite on Y=215 whenever the Y coordinate would be 216.

Quoting myself feels a bit awkward ^^

[edit]Extra explanation: I figured you meant R#23 = 216 when you said 'most games using hardware scroll skip that line'. Ha well...[/edit]

Quote:

Quote: By writing a $C3 to $FD9F and the address of 'STARS' to $FDA0-$FDA1 you're putting the code JP STARS on that hook. So basically you're telling the BIOS to jump to 'STARS' on every VDP vblank interrupt. Actually, you're telling the Z80 to jump to 'STARS' when apparently the vblank caused the interrupt. The BIOS isn't going anywhere. It'll stay exactly where it is, since it's in ROM.

Ofcourse if I were equally anal I would ask you how high the Z80 jumps