Note: This is not for newbies or novices.
I have an advantage in that I've used one other IDE with a built in chip-pinout-tool and system code generator like KDS w/Proccesor Expert. Eclipse is the editor/project manager for KDS.
Well, I downloaded KDS and dived in.
At the download site, I did see mention of a Mac version compatible with XCode.
Windows 7
800MB download (glad I have 35Mbps ISP)
Virus check OK.
Install (simple wizard and answer the questions). About 2 minutes.
Click on desktop icon KDS
I read slowly. Don't jump in and get lost. 15 minutes.
A bit of confusion: The Eclipse first-program demo is for an x86 target. That didn't compile as I'm missing stuff it wants. Freescale should have edited their own file for this so it's ARM focused. Sloppy.
So, I should have overruled the first-use instructions and chosen ARM as the target for the helloWorld program.
I then ran Processor Expert - the elaborate tool to auto-generate code and pick up library drivers and auto-generate linker scripts for the chosen ARM CPU. Since I've used the equivalent for a different ARM M4, I understood the concept.
I managed to choose the K20 256K CPU (not a board based project) and go through the barebones steps. 5 minutes.
Found and clicked "Generate Code". That went OK without error. Now I have a project for Eclipse/ARM GNU with all the startup code and I/O init and drivers for the few K20 peripherals I chose. And all the trap ISRs, the vector table, etc.
Auto-generated the linker script for the default arrangement of memory sectors, etc.
OK. Bravely I told Eclipse to build. It did, in about 5 seconds. Now I have the elf file and I see a debugger choice to download to the ARM target.
The other IDEs have an option to convert the elf file to a .HEX file using the GNU ARM utilities - and I assume it'll work here too. So you can use the .hex file with Teensy 3's loader.
Eclipse is bundled so it just worked. Mercifully.
The compile went fast and no errors. (I admit: I have a newly purchased $150 SSD, 500GB. Makes all the difference in everything I do on this computer. I store downloads and photos on a small NAS, not on the computer.)
Next will be to learn how to select, say, the UART and interrupt handlers using Processor Expert. I saw too a click here for printf() and scanf(). Hope that's easy. No ready-to-use USB/Serial as in Teensy 3. That can come later. Use UART for now.
First look, Processor Expert (PE) has all the mechanisms I had used in the other ARM vendor's tools, though PE seems much less GUI based.
From the other vendor ARM work I've done, I have emulators for a few Arduino-ish things like pinMode(), digitalRead/write(), attachInterrupt, delay(), microseconds() - these use HAL calls to do the same. And the I/O calls like digitalRead() codify in two nibbles which of 16 I/O ports to use and which of 16 I/O bits to use. So the app. code just needs constants changed from pin # to port and bit. Personally, I don't like the board-specific pin # concept. But that's just an opinion.
This experiment looks promising. And I have only an hour invested so far.
That's where I left it for now.