Building the Memory Manager

I've started work on the memory manager implementation for SharpOS. Mostly this is because it hasn't been done yet and I will need the functionality to test any of the other code (SharpOS.EIC and SharpOS.Runtime) using the SharpOS AOT compiler.

Using various internet resources I have hacked together what is a mega-basic page allocator. It uses the provided Multiboot information to determine the amount of RAM, and the offset/size of the kernel must be passed to MemoryManager.Init(). Using this information, it sets up a bitmap for control information on the first page past the end of the kernel, and reserves the pages below 0x100000 as well as the pages occupied by the kernel. The only thing left to implement for a basic page allocator is the Alloc/Dealloc, which is a matter of binary search.

The allocator also handles setting up 2-level paging and enabling it, but right now it does not relocate anything--so physical == virtual. I'd like to put the kernel in the higher half, but that requires modifying the segment base pointer so that we don't have to relocate pointers (would be tedious). I think I need the AOT's help to manipulate the segment base pointer at the right time (compile-time?).

Of course, the bitmap is not an ideal method of memory management. The bitmap search will be slow once lots of memory is allocated, and I'm looking into various ways of improving the performance. One method I've seen is to use another control bitmap for 'superpages' which represent large blocks of pages. Another method is to use a stack of free pages and just pop and push pages to it for allocation. This is a much faster way of allocating, but apparently gets difficult when allocating contiguous pages. I believe it is the method used by Linux and Windows NT though, which lends credence to the concept.

More later!