[NTLK] ARM MMU support for NewtonOS

David Arnold davida at pobox.com
Tue Mar 22 05:24:46 PDT 2022

Perhaps of interest to some here:

Walter Smith (wrs) posted a comment on Hacker News yesterday, describing some memory management features Apple and ARM added to their earlier processors to support NewtonOS.

Long ago, we in the Newton project at Apple had that idea. We (in conjunction with ARM) were defining the first ARM MMU, so we took the opportunity to implement “domains” of memory protection mappings that could be quickly swapped at a context switch. So you get multiple threads in the same address space, but with independent R/W permission mappings.

I think a few other ARM customers were intrigued by the security possibilities, but the vast majority were more like “what is this bizarre thing, I just want to run Unix”, so the feature disappeared eventually.

Here’s some ARM documentation if you want to pull this thread:
https://developer.arm.com/documentation/dui0056/d/caches-and-tightly-coupled-memories/memory-management-units/memory-access-permissions-and-domains?lang=en <https://developer.arm.com/documentation/dui0056/d/caches-and-tightly-coupled-memories/memory-management-units/memory-access-permissions-and-domains?lang=en>

This was in a thread discussing a new RISC-V implementation:
https://news.ycombinator.com/item?id=30755716 <https://news.ycombinator.com/item?id=30755716>


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