Messages in this thread | | | From | Gabriel Paubert <> | Date | Fri, 7 Mar 2003 11:03:31 +0100 | Subject | Re: [PATCH] move SWAP option in menu |
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On Thu, Mar 06, 2003 at 09:06:49PM +0000, Alan Cox wrote: > On Thu, 2003-03-06 at 19:33, Gabriel Paubert wrote: > > I'd be very surprised if it were possible to have swap on a MMU-less > > machine (no virtual memory, page faults, etc.). Except for this nitpick, > > the patch looks fine, but my knowledge of MM is close to zero (and > > also of the new config language, but I'll have to learn it soon). > > You can, and people have had swapping long before virtual memory.
Indeed, I was unclear. A long time ago some OS I used distinguished between swapping (getting rid of a whole process' address space) and paging. The former one you can implement on any machine (with restrictions), the second one needs an MMU and that's what CONFIG_SWAP means AFAICT.
> Most ucLinux platforms can't swap because they can't dynamically relocate code.
I believe that dynamically relocating code is fairly easy (PIC may help too), but data is not: how do you relocate pointers of a swapped out process when you swap it in at a different address?
I have fuzzy memories of a system in which you had a pair of privileged registers (base and limit) which allowed you to implement swapping and moving programs around in physical memories: all addresses were checked against the limit and the base was added to perform physical accesses. I might be wrong: it was about 20 years ago and I've used so many different systems since then. But there is no such mechanism on a 68000 for example (you could add it externally) and it has its own problems (no easy way of sharing library code).
(Yes we're drifting way off-topic.)
> Linux 8086 can swap because it can use CS/DS updates to relocate code/data.
Unless I miss a subtle trick, that's using the segment registers as a poor man's MMU. You can share library code with far calls but you can't use "far" data pointers, can you?
> The way it worked on older systems is that you never run a program which > isnt entirely in memory. With that constraint you know it won't suddenely > want data you don't have.
Oh yes, I've used such systems a loooong time ago. But I can't remember the details well enough.
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