In 2017, with the release of Android Oreo, Google made the biggest change to Android's underlying foundation since Android was first released in 2008: Project Treble. Not only did it make updates easier to roll out for OEMs, but we also saw the birth of GSIs, or generic system images, as an alternative to regular custom firmware.
You've probably come across this concept a couple of times in the context of custom ROMs. The question remains, though: what are GSIs, and how are they different from regular custom ROMs?
What Are Generic System Images?
Typically, the way most conventional custom ROMs like LineageOS are made is that they're compiled from source code with one specific device in mind.
This means that the ROM not only includes a system image, but also includes a kernel and all specific device blobs and libraries, and other hardware-specific code, needed for it to not only replace a phone's original firmware but actually have most components and features work.
This has advantages, but also a lot of downsides. With custom ROMs being specifically made and compiled for one specific phone, a developer or maintainer can take time to fix any device-specific bugs that may arise, like malfunctioning Wi-Fi or cameras.
But this kind of ROM needs a developer—or often a team of developers—to actually take the time to manually do the legwork and get a custom ROM running on a new phone. This requires meticulous reading, testing, and a lot of trial and error. If there isn't anyone up for the task, you might not be able to install a custom ROM on your phone at all.
Related: 12 Reasons to Install a Custom Android ROM
2017 saw a big breakthrough in this regard. With Android Oreo, Google announced Project Treble, which essentially modularizes and separates lower-level, hardware-specific code from the Android system itself.
The system image essentially acts as a layer that gets applied over the lower-level code, meaning that you can swap the operating system out without the need of touching that lower layer. This gave birth to the concept of GSIs, or generic system images, that can be used on more than one phone.
This change was primarily meant to address issues with OEMs being slow and inefficient to roll out Android updates. But it also ended up being a game-changer for the modding community.
While device-specific custom ROMs were and are still a thing, developers also make custom ROMs in the form of GSIs, which you can install on any Android smartphone. The only requirement is that it needs to have an unlockable bootloader.
Will There Be Differences if I Use a GSI?
There might be, or there might not be—it'll all depend on your specific device. The thing with device-specific ROMs is that a developer would devote themselves and give special attention to that specific device they're maintaining.
If an issue does end up arising on those kinds of ROMs, the developer can devote attention towards fixing it properly.
GSIs, though, don't get that same kind of attention since they're made to be used on any Android device. Issues often arise, and to the developer community's credit, titanic efforts are devoted to fixing device-specific bugs and issues on those GSIs so they run better on the largest number of phones possible.
But it's impossible to fix everything for everyone, and unless your issue is happening to several others, it's probably going to be very low on a developer's bucket list.
Related: CopperheadOS: The Secure, Private, Google-Free Android ROM
That's not to say that GSIs are bad. They're the only way a lot of phones can get custom ROMs at all, and for the most part, they're completely fine for daily use on the vast majority of smartphones. But if a device-specific custom ROM is available for your phone, that's probably a better option.
Otherwise, you can give a GSI a shot. Try to check if other people using the same phone as you are using GSIs, and if they're using them, try to take note of the issues they're having, if they have any. And if you do come across something, make sure to report it.
Custom ROMs for Everyone
Device-specific ROMs are always the best option if you're going to delve into the Android modding world. But, if there are none available, a GSI might be your best bet.
It'll allow you to run an alternate Android experience, no matter what phone you have, as long as your phone is Project Treble-compatible. Hopefully, now you know the difference.
