Does jrvideo cartridge add EGA?

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Does jrvideo cartridge add EGA?

Postby garfield » Wed Jun 03, 2020 6:17 am

Long time ago, I bought a jrvideo cartridge.

In the documentation, it said mode 9 will be selected if mode D is requested and mode A will be selected if mode E is requested.

Does is suggest jrvideo add ega graphic?

I run into the eagle's nest with jrvideo cartridge is inserted.

The game is always cga instead.

So, what does jrvideo add when ega routines are using?
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Re: Does jrvideo cartridge add EGA?

Postby Brutman » Wed Jun 03, 2020 7:41 am

It doesn't add EGA, but the cartridge is using a cute little "mapping" trick that might work for some programs.

The PCjr extended modes are:

  • 0x8 - 160x200 16 color
  • 0x9 - 320x200 16 color
  • 0xA - 640x200 16 color

On an EGA card these are marked as "reserved", as in don't use them. But an EGA card can do the equivalent of 0x9 and 0xA, so the cartridge substitutes the PCjr modes when it sees the equivalent EGA modes. (This trick only works if the BIOS is used for everything, including the screen writes.)

Substituting mode 2 for mode 7 is a little sketchy; if something thinks it's going to underline characters it that is not going to happen. (You will get something different on a Jr, like blink or something.)
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Re: Does jrvideo cartridge add EGA?

Postby garfield » Wed Jun 03, 2020 9:17 am

Wow!

PCjr could use/have 640x200 with 16 color with jrvideo cartridge?

Awesome.

I wish jrvideo documentation could add more details.

Thanks
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Re: Does jrvideo cartridge add EGA?

Postby Brutman » Wed Jun 03, 2020 10:00 am

Sorry - let me correct that. Video mode 0xA is just 640x200 in four colors on the PCjr. The video mode that the cartridge is mapping that to on EGA is 640x200 in 16 colors.

(I'm not sure how that's going to work ... it might do something, but it's not going to resemble what is expected.)


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