Comment by amiga386 | Hacker News Reader

Comment by amiga386 | Hacker News Reader

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It would be simple, but somewhat incomplete. It would not look like a virtual memory subsystem where merely accessing an address can allocate physical RAM to it.

The API is already there! AllocMem(, MEMF_PUBLIC) allocates shared memory, otherwise you get private memory.

You can then enforce memory protection around that, but obviously the entire OS is still built on message-passing between tasks where the message itself remains at exactly the same (public) address and isn't copied between private memory spaces, and there's a gentlemens' agreement as to who currently owns it (PutMsg/ReplyMsg)

Software like Enforcer and MuGuardianAngel already checked this, and dinged your program for accessing non-allocated memory.

Software that tried to implement memory protection or virtual memory on the Amiga had to have a huge long hit-list of software that set the AllocMem flags incorrectly and had to be treated differently. That's because the MEMF_PUBLIC flag didn't really do anything on the base OS, so programmers didn't bother using it correctly.

I think "somewhat incomplete" is underplaying it as such a large proportion of message exchange tends to include pointers to shared structures. That the message handling uses intrusive lists doesn't help. So "simple" really depends on where you set the bar for what you consider useful levels of memory protection.

I still think it's possible but doing it to a meaningful extent would be a long tedious effort, at least for my definition of meaningful.

You have Janus-UAE which sandboxes/emulates M68k apps for AROS "rootlessly" in that it bridges a number of structures etc. That mostly solves the problem on modern hardware for legacy apps, though it's a painful solution.

Then you could aggressively upgrade message formats for "native" apps to replace pointers with opaque handles.

If done at the time you could of course had relied on more of the developers of various apps still being active and willing to update their code and might have been able to avoid the Janus-UAE approach, but there was never a point at that time where anywhere close to a majority of users had MMU's,

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