Looking for MODE80.COM file?

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Enlighted (4647)

NYYRIKKI's picture

13-09-2017, 14:21

Hard to imagine any mind blowing effects, but who knows... Maybe you can do smooth 80-column scroll or something. Smile Keep us up to date.

By o.geerdink

Champion (499)

o.geerdink's picture

13-09-2017, 22:36

Maybe I'm getting it wrong but isn't it possible to use it for graphic screens as on the amstrad cpc or does that require a gate array??

Even if it's just character output, it could be used as a debug console on a 2nd monitor Big smile maybe I don't understand the amstrad cpc graphics. Wiki says:

"Underlying a CPC's video output is the unusual pairing of a CRTC (Motorola 6845 or compatible) with a custom-designed gate array to generate a pixel display output"

So without the 'gate array' I can't get pixel display output??

By Grauw

Enlighted (6029)

Grauw's picture

13-09-2017, 23:20

Nice PSG music Big smile.

I don’t know much about the CPC’s VDP, however from what I understand one of its main strengths comes from the CPU having direct access to the VRAM (like the ADVRAM mod for MSX). I don’t know whether this cart offers such a thing (is it possible without BUSRQ signal?), nor do I know if without that the CPC VDP has some other interesting features over the V9938…


Champion (260)

MOA's picture

14-09-2017, 01:26

That demo looks pretty spectacular! Curious about how CPC's VDP works now Smile Direct VRAM access is a nice thing to have, but hard to tell how they did things in this demo unless I know more about their VDP. Spectrum also has direct VRAM access... so would be interesting to compare Spectrum VDP & demos.

Is the ADVRAM mod emulated in OpenMSX and were there any nice programs written with it?


Enlighted (4647)

NYYRIKKI's picture

14-09-2017, 06:57

If you are wishing for color or graphics / characters outside of font ROM, I think you are pushing your wishes a bit too high... but no: I don't know how it was build. How ever I think instead of Amstrad CPC it would be more realistic to compare it to some other 6845 based display controller... Like IBM MDA:


By sd_snatcher

Prophet (2533)

sd_snatcher's picture

14-09-2017, 18:53

There's no magic here. Just like in the ZX-Spectrum, the CPC VRAM is organized in a way that makes it more efficient/cheap be draw the screen (cheap 80s designs, remember?). But for the software, its a complete mess. It's not linear like the MSX2 bitmap video modes. So for normal games you'll spend extra cycles calculating the addresses of the pixels in VRAM.

Not to mention that the software have to be careful to what addresses it will access in the CPC VRAM, otherwise the refresh will fail and the VRAM contents will begin to show corruption.

What you see in demos is some very clever programming. Unlike games, the logic is fixed because you don't need to consider the user input. So all VRAM addresses can be pre-calculated or hardcoded. Another aspect is that they use a lot of raster effects. Those very same raster effects could be easily done on an MSX2.

Last, but not least, contrary to the MSX, the CPC was a proprietary closed standard. This means that all machines are just identical clones of one another. Relying on that, programmers write code loose timing coupling because they know note of those timings will ever change. This is normally considered a programming anti-pattern because it contains a lot of race condition bugs, since the timings won't ever change, the bugs aren't triggered.

If someone tries that on an MSX, the software will run only on the programmers model and the very few that are really similar, sometimes even failing to run on variations of the same model for different regions (HX-10 JP or HX-10 PAL, i.e.)

Such practices obviously condemned the CPC to have the same CPU speed and video timings for the rest of its life. If they changed the CPU speed for as little as 10%, or tried to increase the VRAM bandwidth (as happened from the MSX1 to the MSX2), most of these software won't run anymore and it would have been very hard to fix them.

By gdx

Paragon (1532)

gdx's picture

15-09-2017, 00:40

You are right sd_snatcher. These are the timings and the hardware that does not vary which is the most important to make this kind of demo. "Direct access" to VRAM does not generally help. It is rather the opposite except perhaps for demo.

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