Cortex-R
Arm's real-time core family: more muscle than Cortex-M, without the virtual memory that makes Cortex-A unpredictable.
The line began with the Cortex-R4 in 2006 and lives wherever an answer has to arrive on time rather than quickly on average — hard-drive and SSD controllers, cellular baseband modems, brake and airbag controllers. You almost never see one; it hides inside some other chip.
What separates it from a scaled-up Cortex-M is the reliability plumbing: tightly coupled memory with fixed access times, ECC on memories and caches, and the option to run two cores in lockstep, comparing results cycle by cycle. Memory protection comes from an MPU rather than an MMU, because a page-table walk is hard to put an upper time bound on. The 2020 Cortex-R82 was the first 64-bit member.