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General Category => Amstrad CPC hardware => Topic started by: arnoldemu on 11:27, 23 January 13

Title: the first mode...
Post by: arnoldemu on 11:27, 23 January 13
After fixing a bug in my emu, this lead me to this question:

What is the first mode that the cpc displays following reset?

We know that the mode and rom configuration register is reset to 0, to enable lower rom, upper rom and set mode 0.
We also know that the mode is set after 2 cycles of hsync is seen by the gate-array from the crtc.

But what is the mode immediately after reset and before the first hsync?

I am assuming it's mode 0,  so that a reset forces the mode and the pixel shifters are working in mode 0... but I can't guarantee that.
So what is it?


there is a way to test this ;)

so, you need to set hsync (R3 lower) to be at a position greater than R0. Make the screen large (R1>R0 and R6>R4), and clear the screen to any pen from 8 to 15. Then set the colour of that pen, then measure what colour is sent out of the cpc by the ga.



Title: Re: the first mode...
Post by: Gryzor on 16:49, 23 January 13
Is there any practical consequence in this? :)

Title: Re: the first mode...
Post by: Bryce on 16:59, 23 January 13
It's a bit like the question: How long was the Universe a singularity before the big bang happened?

If no screen is being drawn, then it's not in any mode.

That said, If you edit the firmware and remove all mode commands, what would appear on the screen when you turned it on?

Bryce.
Title: Re: the first mode...
Post by: Gryzor on 14:58, 24 January 13
Quote from: Bryce
It's a bit like the question: How long was the Universe a singularity before the big bang happened?
Depends. Counting from inside the singularity or from an observer's PoV?
Title: Re: the first mode...
Post by: Bryce on 15:12, 24 January 13
It shouldn't make a difference in this case. Besides, if it's a singularity, it couldn't be observed from outside :D

Bryce.
Title: Re: the first mode...
Post by: Gryzor on 15:15, 24 January 13
It's a theoretical model. You can do all you like with it.

Plus, even in astrophysics terms, I think a singularity doesn't preclude an exterior environment, like with black holes?

But I'm pretty certain it does make a difference in any case!
Title: Re: the first mode...
Post by: Bryce on 15:29, 24 January 13
Too far off topic (even in astronimcal terms :D) we can continue this discussion in a PM if you like...

Bryce.
Title: Re: the first mode...
Post by: Gryzor on 15:30, 24 January 13
Cool, send me one :D

Title: Re: the first mode...
Post by: arnoldemu on 15:47, 24 January 13
Is there any practical consequence in this? :)
not really, but the question is still valid and it's testable and can be answered.

for exact emulation, where this can be reproduced, ok, it's a little detail that is probably insignificant, but I just wondered it.


Title: Re: the first mode...
Post by: Grim on 22:24, 24 January 13
I am interested to know that detail too. Maybe someone with a LowerROM board could do the following tests with the attached ROM file (source included):
- Setup the gatest.rom as lower ROM
- Switch OFF/ON your CPC/Plus and report what you see on the screen.
- In case you see nothing (eg. black screen), press ESC to setup the palette and then report what you see.

At some point, you should see vertical stripes displayed on the screen. If you see the stripes before pressing ESC (ie. right after switching your CPC ON), then I'm interested to know what colors they are. In any case, I would like to know how many colors are displayed (so we can find out the video mode).

It would be great if that could be tested on a CPC and a Plus too.
Title: Re: the first mode...
Post by: Bryce on 23:06, 24 January 13
I'll give that a go tomorrow if I get time and take some photos.

Bryce.
Title: Re: the first mode...
Post by: TFM on 23:51, 24 January 13
That said, If you edit the firmware and remove all mode commands, what would appear on the screen when you turned it on?

Bryce.

Haha, I was just reading the first post and had the same idea. Another great application for the LowerROM Box, now we are abouve to solve one of the last riddles of the magic CPC big bang.