The TI-85 is IMHO the ultimate hacker's calculator. It can run machine code programs though it was never designed to!
When the TI-85 was released, someone decided it would be really cool to run their own software on it. Certainly, there was a mangled version of BASIC in calculator ROM that could be used to write simple and slow programs, but it would have been so much cooler to write in assembly and send compiled machine code to the calculator via the TI's link cable. After all, the TI-85 was based on the ZiLog Z80 processor, which was very well documented.
The hard part was running the program. The calculator OS didn't let anyone run programs from RAM, and neither did BASIC. This presented a problem until a unique solution was discovered.
The TI-85 has a feature called the "custom menu". This is a feature whereby commonly used functions are loaded from the catalog of functions into a special menu for fast access. Somebody discovered that the entries in the custom menu were pointers into ROM where the functions lived. All they needed to do was to hijack an entry in the custom menu and make it point to a program in RAM. This program would be in charge of running other user's assembly language programs, essentially functioning as a second OS.
Thus it began. With the release of ZShell, the first TI-85 calculator shell, users were now able to build assembly code on their PCs, compile it, and send the results via the link cable to the calculator. Other shells came later, the most popular being Usgard. They all functioned the same way. Turn the calculator on, push "custom" to bring up the custom menu, and then push "F1" to execute the shell.
With the shell in place, the TI-85 became more than just a calulator; now it was a portable computer capable of running many sorts of software. The most common programs were games. High school math teachers around the globe suddenly had to deal with kids playing Tetris on their calculators instead of listening to their lectures.
Subsequent TI calculators had the ability to run machine code from their OSs, but none of their implementations were as beautiful as the reverse engineered, hacked and cracked marvel, the TI-85. | 
