Do low-end embedded system have process isolation?

Question

I am studying memory management. In particular, I am studying MMU and the mapping between the process logical space pages and the RAM frames. My question is: what about low-end embedded systems? If I'm correct, MMU can't be used in this systems due to their smaller memory. So how computers with less memory available can avoid the problem of shared memory between processes?

Answer 1

For embedded systems, the kind of MMU you speak of is only present in high-end microcontrollers like PowerPC or Cortex A.

Low-end to mid-range microcontrollers do often have some simpler form of MMU though. Not as advanced as used to create virtual memory sections, but a simpler kind which allows remapping of RAM, flash, registers and so on. Similarly, they often have various mechanisms for protecting certain parts of the memory from accidental writes. They may or may not be smart enough to do a "MMU-like" realization that code is executing from data memory or when data access happens in code memory. Harvard vs von Neumann architecture also matters here.

As for multiple processes in a RTOS, it can't be compared with multiple processes in a desktop computer. Each process in a RTOS typically got its own stack but that's about it - the MMU isn't involved in that but it's handled by the RTOS. Code in embedded systems is typically executed directly from flash, so it doesn't make sense to assign chunks of RAM memory for executable code like in a PC. Several processes will simply execute code from flash and it might be the same code or different code between processes simply depending on whether they share common code or not.

Similarly, it is senseless to use heap allocation in embedded systems (see Why should I not use dynamic memory allocation in embedded systems?) so we don't need to create a RAM image for that purpose either. The only thing left as unique per process is the stack, as well as separate parts of .data/.bss.