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  • Joined: 03 Jan 2022
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  • Location: Albuquerque, NM, USA
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Game Gear hardware oddities
Post Posted: Mon Jan 17, 2022 2:45 pm
A while back, I got a non-working GG off of eBay and recapped it. It works great, but there are a few strange things it does and I was wondering if the community knows anything about them.

First off, why is it possible to turn on the console and then insert a cartridge? It seems to me like the Z80, without a cartridge inserted, would read floating data lines and increment the program counter. Presumably, then, inserting a cartridge would make execution start at whatever the current value of PC is. My theory is that there's something in the GG that triggers a Z80 reset upon cartridge insertion, like a cartridge interrupt accessible by the hardware but not the Z80.

Secondly, on my unit, if a cartridge is seated poorly, the console will turn on and then shut itself off a second later (like a unit in need of recap). I believe this is caused by the self-protection mode of the power regulator, but I'm not sure. Is this behavior unique to me, or has it happened to anyone else?

Since getting the system, I've done quite a bit of coding for it (emulators only...I don't want a flashcart at this point) and it's a lot of fun. That's why I'm curious about these things it does.

PS: I made an account a couple weeks ago and tried to post, but it never showed up. I don't know if that's correct or not, so hopefully this doesn't cause a double-post or something.
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Post Posted: Mon Jan 17, 2022 3:33 pm
You have also to consider that some Game Gears have BIOSes so when you turn them on without any cartridges, the Z80 still executes code.

In case there's no BIOS instead, the Z80 might just keep on reading $FF from the ROM that isn't there and that's a RST $38 command which means PC=$0038. So the execution is stuck at that point and when you insert a cartridge it might continue. How well it would work then depends on the game...
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  • Joined: 03 Jan 2022
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Post Posted: Mon Jan 17, 2022 4:36 pm
I hadn't thought of a BIOS...though my unit doesn't display the TMSS screen, so it probably doesn't have a BIOS. If it had one, the checksum would fail and the GG would lock up.

How would the Z80 always read $FF---are the data lines pulled up inside the GG or something?

A good example of a game starting up is Sonic 1. If I turn on the console and then insert the cart, the game starts at the Sega screen the way it would normally. The same happens for Super Columns. If the Z80 is repeatedly executing at $0038, then maybe the interrupt handler in the game has some logic to check if the system hasn't been initialized (mapper configuration or something) and can act accordingly. Just a theory, though, and it seems rather unlikely because every developer would have included code like that.
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Post Posted: Tue Jan 18, 2022 11:53 am
I don't think any checksum is performed by the GG BIOS but surely it is checking the SEGA signature string, and that would be missing if no cartridge is present.

So it may be that connecting the cartridge closes some open circuit that would trigger a reset on status toggle? Because I also believe the theory about the software taking care of this condition is very unlikely to be the case...
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Post Posted: Tue Jan 18, 2022 12:16 pm
I suspect the action may be enough to trigger the hardware to reset due to power spikes.
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Post Posted: Tue Jan 18, 2022 11:45 pm
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I suspect the action may be enough to trigger the hardware to reset due to power spikes.

Yeah, that's gotta be it. I was just overthinking.

Still, it does make you wonder what would happen if inserting a cart didn't cause a power cycle. Maybe if one could power a cart from a separate 5V supply and then plug it in to the console...
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