Cortex-M

ARM doesn't make chips; it sells the blueprint. The Cortex-M3 arrived in 2004, the M0 in 2009 as the smallest 32-bit core anyone could reasonably build, the M4 in 2010 with DSP instructions and a floating-point unit, and the M33 in 2016 with TrustZone to wall off secure code. Licensees — ST, NXP, Nordic, Microchip, Raspberry Pi and hundreds more — bolt on memory and peripherals and sell the result for well under a dollar.

What sets the M profile apart from ARM's application cores is predictability: no MMU, Thumb instructions only, and an integrated interrupt controller that stacks registers in hardware, so an interrupt handler can be an ordinary C function. The upshot was the slow eviction of the 8-bit world — AVR, PIC, 8051 — by a core where the same toolchain, debugger and CMSIS headers work regardless of vendor.