No Teensy with MMU?

Indeed no MMU.

We do get a MPU which enforces access policies (eg, no executing code from data memory, no writing to code memory) but no virtual memory mapping.
 
I read something about the "Linux Tax" ARM is levying for a MMU. How long before some Chinese company will release a RISC-V MCU with a MMU so we can run some half-decent operating system on a microcontroller?
 
A (very) long time ago, I tryed uClinux on a Coldfire @ 66MHz, with no MMU.
But I don't know if it is still maintained.
 
I'm not talking about Linux per se. There are many other smaller and simpler operating systems out there which would greatly benefit from paging and memory protection.

Xous could be a candidate.
 
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BTW I sent an email to a company named WCH in China which makes RISC-V microcontrolers and suggested they add a MMU to their product. Their lowest offerings cost $0.20 (2K RAM and 16KB Flash) and their most expensive ones $2 (with 64K RAM and 256K Flash, somewhat comparable to a Teensy 4).

If they take up my suggestion it would be Earth-shattering.
 
What good would an MMU be for such tiny amounts of RAM? How are you going to remap anything when you don't even have enough space for page tables?
 
The number of pages would be limited, of course, but you'd gain memory and process protection. An MCU would only have limited number of processes and the small OS running. It's certainly not meant for running Linux or Windows.

Think of it as a more reliable MCU software stack.
 
But the MMU is only used by the CPU - half of the processing on an MCU is done by the peripherals, which bypass it. So there's no protection there and you have to manually translate every time you want a physical memory address.
You're not going to be running multiple processes with that tiny amount of memory.
Teensy 4.x has an MPU which can set access modes for regions, it is used to restrict execution to RAM/FLASH and disallow r/w to non-existent areas, it can easily be restricted further than the default if needed.
 
Speak of the devil: Espressif has just released a RISC-V ESP32 version with MMU support! I haven't seen any pricing yet but hopefully it will force ARM to lower theirs.

WCH also told me they would look into my suggestion to add a MMU to their offerings.
 
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