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Author Topic: Cart hardware questions  (Read 3648 times)
GroovyBee
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Busy bee!


« on: August 31, 2009, 06:59:57 pm »

I've been looking at the jag documentation and I can see that the ROM interface can be up to 32 bits and is set using the MEMCON1 register. When you plug a cart into the system how does it detect the bus width of the cart? Does the jag's BIOS look for some magic values in the ROM image? Does it try and detect high order bytes/words of the bus floating? Or is it as simple as some links on the cart connector itself?

What games use the cart's EEPROM interface?

I can see that the ROM is allocated 6MB of space in the memory map. What is the maximum ROM cart image size for the commercially released ROMs? Do any ROM carts use bank switching? Do any of the commercial ROMs run with a 16bit wide bus?

Sorry for all the noob questions.
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Zerosquare
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« Reply #1 on: August 31, 2009, 07:55:58 pm »

I've been looking at the jag documentation and I can see that the ROM interface can be up to 32 bits and is set using the MEMCON1 register. When you plug a cart into the system how does it detect the bus width of the cart?
There's a special location in the ROM where the bus width is written, in such a way that it gives a meaningful value regardless of the bus width selected when you read it. I don't recall the exact address, but it's explained somewhere in the docs.

What games use the cart's EEPROM interface?
Almost all of them, to store the hi-score table and settings, at a minimum.

I can see that the ROM is allocated 6MB of space in the memory map. What is the maximum ROM cart image size for the commercially released ROMs?
4 MB.

Do any ROM carts use bank switching?
No.

Do any of the commercial ROMs run with a 16bit wide bus?
I don't think so, with the possible exception of the JagCD Memory Track cart (I recall vaguely that it was 8 or 16-bit wide instead of 32, but I'm not 100% sure).
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GroovyBee
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« Reply #2 on: August 31, 2009, 09:06:30 pm »

@Zerosquare: Thanks for the information.

I've had another look through the tech docs and can't find the cart width information. I think the cart width must be in the encryption block but I haven't investigated that yet.

Has anybody investigated the inner workings of jag's CD unit with a view to replacing it? By that, I mean replacing it with some solid state media. I remember reading that jag CDs have errors that the firmware expects to find (as an anti-piracy measure). Does any of the jag software communicate directly with the CD hardware or does it all happen through the CD's BIOS (as recommended in the tech docs)?
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Zerosquare
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« Reply #3 on: August 31, 2009, 10:01:22 pm »

I've had another look through the tech docs and can't find the cart width information. I think the cart width must be in the encryption block but I haven't investigated that yet.
My mistake, sorry. I asked SCPCD and he told me the info wasn't actually in the docs, but found through disassembly.

Anyways, here it is, according to him :
The bus-width settings are located at addresses $800400 - $800403 in the ROM (4 identical bytes).
You can find a piece of code from the BOOT ROM here which show how it is used : http://www.atariage.com/forums/index.php?showtopic=13946&view=findpost&p=139726.

Does any of the jag software communicate directly with the CD hardware or does it all happen through the CD's BIOS (as recommended in the tech docs)?
This is very unlikely, since the main chip in the JagCD (Butch) is an ASIC whose specifications have never been released. So a JagCD BIOS replacement should in theory work fine with existing CD games. Keep in mind that you'd have to support I2S audio streaming too, since some CD games use audio tracks.
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