So what actually IS the story with the Pentagon and Scorpion?

[Alone Coder]

Russian machines (not only Pentagon and Scorpion, but all disk models) use Beta 128 disk interface. All of them have different timings, incompatible with any of original timings. Scorpion has at least 4 different timings (all supported in 8 color editor). Pentagon has 4 others (used in PSG Wins part 1, PSG Wins part 2, Condommed, and one INT test - I had Pentagons with all these timings except the last one), one of which became a "standard" by chance (in fact, this "standard" came from one Scorpion board re-wired for Pentagon compatibility, it differs from the closest Pentagon variant by a half-pixel).
The list of software for "standard" Pentagon: https://zxaaa.net/view_demos.php?t=%2F& ... itle=Title You can't see the correct picture in them on any of Western Speccies. In some cases, these programs want 71680 t-states per frame and don't work otherwise.

Scorpion has extended memory 256K to 2048K using port #1FFD.
Pentagon can have extended memory up to 1024K using the upper bits of port #7FFD. The extra memory is switched off by port #EFF7 bit 2. Pentagon also has ZX Lprint III ROM or a RAM chip instead of it (named "cache") available by in a,(#fb) (disabled by in a,(#7b)).
Profi (more widespread than Scorpion, if we count the known users) has extended memory up to 1024K using the lowest bits of port #DFFD. The other bits switch some other things, including the contents of #0000 and #4000.
ATM-Turbo 2 (the most supported extended memory model, if we count the known titles) has 4 paging ports for every 16K of CPU window: #3FF7, #7FF7, #BFF7, #FFF7 in DOS mode. The lowest bits in these ports select one page of 64, bit 6 selects RAM/ROM, bit 7 is to use data from port #7FFD.

[SamC]

As Alone Coder said, a crucial issue is the timig.

The very first - in english language written - explanation, is here: https://worldofspectrum.net/features/rusfaq/index.html
There was a partial solution in nineties, in order to run new demos (converted to TAPs from TRDs),
to purchase a +2A or +3 model, which is more close to east-european timings...
Also see a contended memory description: https://worldofspectrum.org/faq/referen ... erence.htm

[introspec]

I have never known this and I should like to. What are the differences between the Russian machines and a real Speccy? Which games can and cannot run on all the machines? More precisely, can a Spectrum 128/+2/+3 run anything that a Pentagon 128 can, and if not then what can't it run and why?

There are many clones, with varying degrees of compatibility (ULA was not available to clone-makers, so they "emulated" its behaviour with discrete logic). Many clones were 48K, there were many 128K clones too, some of the clones designed in 1990s have even more memory/features.

I think the key difference from your perspective is to do with what people assume as a "default configuration". While 48K remained more-or-less standard in the UK, the "standard" Russian machines have at least 128K with AY chip and, typically, support Beta Disc interface. As the result, much of the software coming from Russia would assume at least 128K of memory. In addition, many popular clones are contention-free, and some (esp. Pentagon) have slightly slower frame rate than classic models, which means they have about 1% more CPU ticks per each frame. The combination of contention-free memory and these extra ticks means that they can have software running up to, maybe, 10-15% faster than classic models can. Consequently, some of the software coming from the East may start skipping frames on classic models - this can be fixed only by patching the software to somehow get it to try to do less per frame (by optimizing it or by removing some less important features). There also are some programming practices common in the East that do not work on classic models due to bugs in original hardware, but this is truly not a big deal.

Summary: 128K classic models of ZX Spectrum are very similar to the common clones from the East. However, they are a little bit slower, due to the memory contention, so some software may stop working as intended and slow down or crash. In addition, the main medium for software delivery in the East is a floppy, not a tape, so you have to deal with .trd and .scl images instead of more familiar .taps.

[Ralf]

As people say timings are a bit different but it's most important in demos which often use multicolour tricks. If you don't care about demoscene that much than 95% of games should work both on original machines and Russian clones.

Scorpion has more memory but very few programs use that, I could mention less than 10.

The biggest practical difference is a data storage standard. Pentagons may read data from standard tape but
they also have a built in disk system called Tr-Dos. It was actually invented in UK but was further developed and became a standard
only in Russia.

So unfortunately you won't read a Tr-Dos disk on any classic Spectrum without any extra hardware.
If you wondered, Spectrum +3 works with disks too but it uses a totally different standard called CP/M which is imcompatible with
Tr-Dos.

[megaribi]

As people say timings are a bit different but it's most important in demos which often use multicolour tricks. If you don't care about demoscene that much than 95% of games should work both on original machines and Russian clones.

If someone owns Pentagon or Scorpion, I am curious about behaviour of timing critical games how they look on Pentagon and Scorpion:
- Aquaplane (is the horizon well aligned)
- 4D Terror Dactyl start screen
- Vectron, both sound and screen snow
- Thomas the Tank Engine, does the train sprite flicker?
- Incredible Shrinking Fireman: does it work correctly?

[Luzie]

If someone owns Pentagon or Scorpion, I am curious about behaviour of timing critical games how they look on Pentagon and Scorpion:
- Aquaplane (is the horizon well aligned)
- 4D Terror Dactyl start screen
- Vectron, both sound and screen snow
- Thomas the Tank Engine, does the train sprite flicker?
- Incredible Shrinking Fireman: does it work correctly?

Why not try it under the various Emulators which more or less good emulate Pentagon/Scorpion?

[Ralf]

If someone owns Pentagon or Scorpion, I am curious about behaviour of timing critical games how they look on Pentagon and Scorpion:

Without checking, I would say they won't look good and time critical effects like border on Aquaplane will be destroyed.

However Pentagon users mostly dealt with that and made corrected versions of popular games which didn't work well on their systems. Such versions are called "Pentagon fix". You must remember that games in Russia weren't officially sold by their original publishers but were rather distributed by hackers who adapted them to Pentagon before releasing.

[Joefish]

Yes, I've seen several distributions of people claiming to have 'Pentagon Fixed' Buzzsaw+, despite the game being able to detect and adapt to Pentagon timing all on its own.

[Alcoholics Anonymous]

Yes, I've seen several distributions of people claiming to have 'Pentagon Fixed' Buzzsaw+, despite the game being able to detect and adapt to Pentagon timing all on its own.

It seems to work on all timings 48K/128K/+3/Pentagon? It's listed as 48K in the archive. I guess it has to be listed as something; I'd assumed it was timed for 48K only and never bothered to try another.

[megaribi]

If someone owns Pentagon or Scorpion, I am curious about behaviour of timing critical games how they look on Pentagon and Scorpion:
- Aquaplane (is the horizon well aligned)
- 4D Terror Dactyl start screen
- Vectron, both sound and screen snow
- Thomas the Tank Engine, does the train sprite flicker?
- Incredible Shrinking Fireman: does it work correctly?

Why not try it under the various Emulators which more or less good emulate Pentagon/Scorpion?

Well, I am not sure about accuracy of timing in Fuse (and timing is the most difficult part of every emulator) but there is a result.
I have tried the mentioned programs in Spectrum 48K mode, Spectrum +3 48K mode, Spectrum 128 48K mode and Pentagon 128 in 48K mode.

• Aquaplane: On Spectrum 48 horizon is perfectly aligned. On Spectrum 128 border part of the horizon is misaligned by 2 pixels, on +3 by 5 pixels, on Pentagon by 40 pixels.

• 4D Terror Dactyl : almost the same results as Aquaplane, of course with more border stripes

• Vectron: There is no problem with sound on all machines. Fuse does not emulate snow effect. Startup screen border is well aligned on 48K . Other models have bad aligned border but by two pixels only. +3 has flickering.

• Thomas the Tank Engine, no sprite flickering. However, on Pentagon sometimes the train disappears in a new screen

• Incredible Shrinking Fireman works on all models except +3 (does not react on keyboard)

[Joefish]

Yes, I've seen several distributions of people claiming to have 'Pentagon Fixed' Buzzsaw+, despite the game being able to detect and adapt to Pentagon timing all on its own.

It seems to work on all timings 48K/128K/+3/Pentagon? It's listed as 48K in the archive. I guess it has to be listed as something; I'd assumed it was timed for 48K only and never bothered to try another.

I prefer it listed as 48K as that's the machine I had and so that's what it was designed to run on - it has beeper sound and doesn't use any 128K facilities. I did give it adaptive timing so it wouldn't reject other setups but as I say, it's not designed as a '128K game', and it's sometimes frustrating to see it criticised for not using the facilities of what is quite a different machine, capability-wise.
(If you press 'C' on the menu screen right after it's loaded, you can manually switch through the timing options).

I'll get round to writing something specifically for a 128K machine, one day...