In November 2012, Cemetech discovered the first information about the TI-84 Plus C Silver Edition. The calculator was officially released in 2013; Cemetech received one of the first devices manufactured courtesy of TI for a hands-on review. Thirteen months later, the calculator continues to be extremely popular, with programs and games for the color calculator stuck at the top of ticalc.org's weekly list of most-downloaded programs. Unfortunately, as with any brand-new piece of software, the first OS for the TI-84 Plus C Silver Edition contained a number of mostly minor bugs. We carefully tracked and tabulated a "Big Bad Bundle" of bugs, and we found TI very receptive to what we found. Unfortunately, as of T^3 2013, it was not clear what sort of success the TI-84 Plus C Silver Edition would become, so it was unsurprising that no OS upgrade was immediately forthcoming.

TI-OS 4.2 for the TI-84 Plus C Silver Edition has now been released, and can be downloaded at the link below. On the plus side, it appears at least as stable at OS 4.0, and in testing performed by Cemetech administrators, all existing programs, utilities, shells, and games continue to work. TI-BASIC and z80 Assembly programming features remain unhindered. On the other side of the coin, it appears none of the bugs we have collected thus far has been fixed, so we did a bit of digging to determine what this new upgrade offers. Based on information posted on TI-Planet and at Omnimaga, as well as digging from the inimitable Brandon Wilson, we believe that this new OS version works hand-in-hand with the new version of TestGuard linked below to further guarantee that TestGuarded calculators are running authentic TI-released operating systems. While we suspect the changes are intended to detect modified OSes, time will tell whether this is in fact the case. We look forward to a long future for the TI-84 Plus C Silver Edition as a tool for math and programming, and we hope to see more TI-OS 4.x versions in the future with a steady stream of new features and bugfixes.

Download
TI-84 Plus C Silver Edition OS 4.2
TI-Testguard 4.2 for the TI-84 Plus C Silver Edition
This is the largest OS improvement in TI history! Shock

On a more serious note, though, I wonder if this OS update and TestGuard are strong enough to actually prevent installing a third-party or counterfeit OS on the calculator?
Kerm's news release is typically positive and supportive and it is heartening to see that TI continues to support their flagship calculator.
However, reading between the lines it is clear that TI have a clear agenda and is disapointing that the bugs reported by this community appear largely ignored.
Why is TI working on more TestGuard restrictions instead of fixing well-known, documented, and tested bugs that have very simple fixes. Have enough people bypassed TestGuard through OS modification to justify a new OS?
ti83head wrote:
.However, reading between the lines it is clear that TI have a clear agenda and is disapointing that the bugs reported by this community appear largely ignored.
I did my best to be positive, as I am happy that at least it looks like they're interested in continuing to maintain the OS and expand the calculator's user base. But yes, to be honest, I am a little disappointed that the bugs, most of which would be a very quick fix from looking at the code, remain. Doing my own reading between the lines, I suspect more programmer effort has gone into the TI-Nspire line at the moment. I suspect that may not change in the near future.
The last TI-Nspire OS, 3.6 doesn't even fix the bugs either. It just locks down Ndless even more. OS 3.2 improved Lua image speed, though.
CVSoft wrote:
Why is TI working on more TestGuard restrictions instead of fixing well-known, documented, and tested bugs that have very simple fixes. Have enough people bypassed TestGuard through OS modification to justify a new OS?
I know of no OS modifications made by the community for the TI-84 Plus C Silver Edition, but this change could be out of an abundance of caution rather than a response to something specific.
After staring at OS 4.0 and 4.2 differences all night, I've come to the conclusion that they didn't really change or add anything...at most, they may have fixed a TestGuard bug, but I haven't been able to find it.

As far as TestGuard 4.x, they are in fact now validating the OS remotely. The TestGuard application itself has hard-coded MD5 hashes of every known OS and boot code combination. This seems very wasteful to me, but it does provide us with an exhaustive list of OS versions, as well as MD5 hashes we can use to validate any we may not have yet.

It's worth noting that the MD5 hashes are NOT the same ones from the 8XU/8CU signature -- they don't hash 00:0056h as though it were 0xFF (as the OS receiving routines in the boot code do).

It's also worth noting that they also hash the boot code on models that once allowed modifying it.

I'm sure implementing this "validation" feature helped to put a teacher somewhere at ease, but it's very easily bypassable, which I plan to demonstrate soon.
1: Hack the TestGuard App to remove OS validations or add you own hashes.
2: Say "Teacher, your calculator has an update, may I install it for you?"
3: Install your hacked TestGuard App and OS 4.2.
4: Enjoy custom OSes on your calc.

And IIRC, you used to have to be an instructer to get the TestGuard App.
the link to the OS leads to a 403 error page at TI.com
MrLarkins wrote:
the link to the OS leads to a 403 error page at TI.com

Yes, TI removed it. You have to email them.
  
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