Wednesday, January 17, 2007

More on OS development...

I'm just playing around with the toy OS and learning quite a lot about how a computer works and what the kernel for a computer really has to do. I am not even working on device I/O or advanced features at all yet. Just memory management.

Memory management is subject to the following considerations:
  • Where to put the kernel code and data (you do not want to overwrite this)
  • Where to put the GDT and IDT
  • Where to put the memory "page directory" (full 4GB directory == 8 MB)
  • The ratio between kernel space vs. user space (linux = 1:3)
  • How to implement an optimal "malloc" routine
  • Page swapping (but not required, a kernel may also fault)
  • Optimization of the above requirements, like is done in Linux > 2.4
  • Memory page sharing through copy-on-write for example (by default the process shares the same code + data, but on any attempt to write to the page, the kernel makes a copy of that page)
So... great... lots of considerations. I am just at the part of where to put everything. My decisions haven't been ideal yet, because of boot-up limitations mostly. But slowly I should be able to work towards page allocation for a process, assigning it to a process and then subsequently the ability to run a certain process (albeit hardcoded in the kernel itself for the time being).

As said before, this is not going to be anything big, but will greatly assist in understanding other OS's a lot better. Maybe even I could use this knowledge to jump-start another project for a limited device and make the machine do something really specific in an easy way.


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