Spectrum Computing

The bit ranges in the address should be A13-A7 and A6-A0, because the RAM chips have only seven address lines. A15-A14 are not used for memory chips, but for selection logic. I don't know if this actually means that only bits 6.0 from register R affect the snow... I'm not really sure about it. Mayb...

what about this 2 cases? https://i.postimg.cc/0215Pwy3/2cda1e92ef66a6bdf103bbb3fa57cca5.png 3) For the remaining variants of overlapping the operation code fetching cycle with the ULA screen drawing cycle, the ULA works normal, without snow and duplicate. why there is no any glitches? Just because)

а атрибуты от пикселей так отделить низя? No. Use English please. Or you can be banned here also) 7-й бит R влият как нибудь? хотя наверно нет It affects as a 7th bit of address from where value is read.(upd. I'm not sure about it) но в описании то надо бы про это что то написать что память 7х7 итд...

And recommendation for game writers - to avoid the snow effect you need to make sure that the interrupt vector does not get into the slow memory. It is safer and easiest to place it in addresses $8000 ... $BFFF (register I within $80 ... $BF) - on machines with snow it's always fast memory page .

@TheMartian , thanks! Alas, I am far from hardware details, so your explanation perfectly explains and complements my results.

An obvious addition - there is no snow/double effects on Amstrad's black machines (+2A/+2B/+3/...) and on any ZX Spectrum clones except maybe those that based on original ULA.

And the table with a bit more info:

So: 1) If the 4th cycle of the operation code fetching cycle coincides with the 3th cycle of the 8-tacts output cycle of 16 pixels, this leads to snow effect - in the pixels2/attributes2 addresses the low byte of address is replaced with the current contents of the R register (it should already be ...

NEO SPECTRUMAN wrote: ↑Wed Oct 19, 2022 9:15 pm so WUT about hardware/snow scroll?

Only for even columns, alas)

Also, video by @balford :

At the beginning of this year I promised to publish details of the exact snow effect emulation after publication of a new version of my emulator. Well, I changed my mind, the publication of new version of the emulator is planned later, and the details of the exact snow effect emulation I publish now...

New update, the same place to download. Some fixes.

Patrik Rak wrote: ↑Mon Jan 03, 2022 5:04 pm

Code: Select all

byte b = B ;
if ( CF ) {
    b += ( NF ? -1 : 1 ) ;
    HC = ( b ^ B ) ;
}
PV ^= ( b & 7 ) ;

where HF is bit 4 of HC and PF is parity of the value in PV.

It seems like you missed something because PV value is not defined before ^= .
Maybe like PV = (PF ^1)<<3;
Thanks.

[mention]ZjoyKiLer[/mention], I suppose this behavior can be actual too for non-automatic block instructions when the interrupt executes after them (LDI,LDD,CPI,CPD,INI,IND,OUTI, OUTD). Did the crew try to verify their behavior too? (Also it's desirable to check what about non-mascable interrupts fo...

ZjoyKiLer wrote: ↑Sun Jan 02, 2022 8:06 pm
Woody wrote a game that only works OK if this thing is correctly emulated, it's called Super HALT Invaders. The score counter only works if HALT is OK.

Yes, [mention]azesmbog[/mention] shared this game some time ago on zx-prk.ru and I solved its puzzle on my own) Thank you again, anyway.

Well, good luck with maintaining the list of how each particular version of each emulator handled blocked interrupts after each instruction. In my opinion, this is complete nonsense. Feels like you are making an argument just for the sake of it, no matter how impractical it is. Well, there are not ...

I think azemsbog also shared with you the HALT stuff, as I remember he showed us screenshots of Spectramine passing the latest version of Woody's HALT2INT test. But, try this to be sure, and this . Also, we are thinking about testing the ccf and scf instructions again, since it seems that the behav...

Thank you very much for sharing the technical info. It's much appreciated. I don't deserve it yet. Although I think azesmbog have informed you already, I was about to post the new test of Peter Helcmanovsky. It's here: https://spectrumcomputing.co.uk/forums/viewtopic.php?f=23&t=6102 Thank you. ...

I understand your reasoning and, by the way, I congratulate you for the very accurate implementation of the snow effect of Spectramine, but I have to inform you that currently your Z80 implementation is not accurate enough. There are other emulators that are more accurate. Thanks) I was informed ab...

That is quite misguided opinion. Of course the emulation of the particular code has to be identical if you are to get the same outcome. But it doesn't mean it has to be perfect. One of the aims of RZX design was that neither the recording nor the replaying emulator has to have perfect timing. That'...

Ralf wrote: ↑Sun Jan 02, 2022 3:06 pm Anyway, I'm recoding mostly with Spectaculator so I don't hope much for any changes made to it ever.

So you need advanced emulator, not advanced RZX format)

Ralf wrote: ↑Sun Jan 02, 2022 11:21 am I'd like a possibility to insert a full memory snapshot somewhere into rzx file.

But you can insert as much file snapshots as you need in your RZX already. For example, emulators insert snapshots in its RZXs for each rollback point.

As far as I understand, this is only possible by executing the chain of EI commands and/or #DD/#FD prefixes. In this case, the problem is solved simply - after performing a specified number of command fetches of the RZX frame, we check the interrupt blocking flag, and if interrupts are blocked, the...

3) The interrupts are enabled but blocked and the interrupt doesn't happen. Under normal circumstances, the interrupts are not blocked for the entire duration of the interrupt pending period, so the emulator will be able to end the frame a bit later using the cases 1) or 2). But in case the interru...

New update, the same place to download. 3D Math improvement, "simplified game" option.