ARM Programming
M-Short, Ell C 
What is an ARM?
Let's start with what an ARM processor is. They are used in everything from the Redboard Turbo to the Raspberry Pi to most cellphones, but that's a large range of performance. ARM is actually a unique business model. Arm Holdings does the design work for the cores and holds the patents/copyright/other legal things and then licenses the design out. The cores are then put into CPUs, microcontrollers, SOCs (System on Chip), etc. A company might decide they want to build a camera that uses the ARM core. They can license the core, maximize power efficiency, add some silicone for the camera sensor interface, and build the entire system onto a chip.
If you look around you'll actually see quite a few naming conventions. The v7 architectures lists 3 different profiles:
- Cortex-A: the Application profile
- Cortex-R: the Real-time profile
- Cortex-M: the Microcontroller profile
We are going to be looking at Cortex-Ms. The Cortex M0/M0+ and M1 are actually from the v6 architecture and can be considered a subset for the v7 profile. All that to say that we are going to be looking at programming the SamD21 on our Redboard Turbo (and other boards) as well as the SamD51 on the Thing Plus. The SAMD21 is an ARM Cortex-M0, where the SAMD51 is an ARM Cortex-M4F.