Simple toy OS memory management

I'm developing a simple little toy OS in C and assembly as an experiment, but I'm starting to worry myself with my lack of knowledge on system memory.

I've been able to compile the kernel, run it in Bochs (loaded by GRUB), and have it print "Hello, world!" Now I'm off trying to make a simple memory manager so I can start experimenting with other things.

I found some resources on memory management, but they didn't really have enough code to go off of (as in I understood the concept, but I was at a loss for actually knowing how to i开发者_JAVA百科mplement it).

I tried a few more or less complicated strategies, then settled with a ridiculously simplistic one (just keep an offset in memory and increase it by the size of the allocated object) until the need arises to change. No fragmentation control, protection, or anything, yet.

So I would like to know where I can find more information when I do need a more robust manager. And I'd also like to learn more about paging, segmentation, and other relevant things. So far I haven't dealt with paging at all, but I've seen it mentioned often in OS development sites, so I'm guessing I'll have to deal with it sooner or later.

I've also read about some form of indirect pointers, where an application holds a pointer that is redirected by the memory manager to its real location. That's quite a ways off for me, I'm sure, but it seems important if I ever want to try virtual memory or defragmentation.

And also, where am I supposed to put my memory offset? I had no idea what the best spot was, so I just randomly picked 0x1000, and I'm sure it's going to come back to me later when I overwrite my kernel or something.

I'd also like to know what I should expect performance-wise (e.g. a big-O value for allocation and release) and what a reasonable ratio of memory management structures to actual managed memory would be.

Of course, feel free to answer just a subset of these questions. Any feedback is greatly appreciated!

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If you don't know about it already, http://wiki.osdev.org/ is a good resource in general, and has multiple articles on memory management. If you're looking for a particular memory allocation algorithm, I'd suggest reading up on the "buddy system" method (http://en.wikipedia.org/wiki/Buddy_memory_allocation). I think you can probably find an example implementation on the Internet. If you can find a copy in a library, it's also probably worth reading the section of The Art Of Computer Programming dedicated to memory management (Volume 1, Section 2.5).

I don't know where you should put the memory offset (to be honest I've never written a kernel), but one thing that occurred to me which might work is to place a static variable at the end of the kernel, and start allocations after that address. Something like:

(In the memory manager)
extern char endOfKernel;
... (also in the memory manager)
myOffset = &endOfKernel;
... (at the end of the file that gets placed last in the binary)
char endOfKernel;

I guess it goes without saying, but depending on how serious you get about the operating system, you'll probably want some books on operating system design, and if you're in school it wouldn't hurt to take an OS class.

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If you're using GCC with LD, you can create a linker script that defines a symbol at the end of the .BSS section (which would give you the complete size of the kernel's memory footprint). Many kernels in fact use this value as a parameter for GRUB's AOUT_KLUDGE header.

See http://wiki.osdev.org/Bare_bones#linker.ld for more details, note the declaration of the ebss symbol in the linker script.