M3 Operating System Development

A chronicle of the development of the M3 operating system

New goals for the M3 boot loader and startup code

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I’ve taken a bit of a break from coding this past week, and have instead been reading, studying, and doing a bit of design.

After reading more deeply through the Intel IA-32 System Programmer manual and thinking about things a bit, I’ve changed my mind about the structure of the M3 startup code. I think that I’ve left out some things that need to be done before I jump to protected mode, and the GDT that I’ve currently set up doesn’t give me quite the memory model that I would like for M3.

It’s very interesting to read through the Intel documentation. As documentation goes, it’s well-written and contains many useful diagrams. It’s very tempting to try to cobble together an OS using only the various tutorials and source code available on the ‘net, but I think those that try to do this are really missing out, and their OSes are destined to be simple one-offs of existing systems at best. Without a full understanding of how the processor works, and what features are available, you’re handicapping yourself. Besides, this stuff is pretty fascinating.

OK, enough of the soapbox speech. As I was saying, I want to add some stuff to the M3 boot loader and OS initialization code, and do some code restructuring. Here’s a list of the things that I want to accomplish before I hand over control to my kernel:

  • Load kernel into memory
  • Enable A20
  • Setup GDT (for both system and user segments)
  • Setup protected-mode IDT
  • Setup TSS for multitasking
  • For paging, set up at least one page directory and page table.
  • Turn on pmode and paging at the same time (both in CR0)
  • Setup interrupt handlers
  • Jump to kernel main()

This stuff isn’t in order – it’s just a checklist.

As an additional goal, I’d like to do as much of this as possible in C, so that the code is more easily understood. Right now, I do things like setting up the GDT in assembly, and I’d like to rewrite that so that I have the bare minimum of assembly code.

There’s a lot of stuff here, and way too much to fit into a boot sector. So my thinking is that the boot sector will do very little – basically it will just load what I’m calling the OS environment initialization code and then hand over control to that code.¬†

The environment intialization code will be a mix of assembly and C, and will handle the tasks I listed above. It will also load the kernel into memory. When all its tasks are complete, it will hand over control to the kernel.

This design will keep the initialization code out of the kernel, since it really doesn’t belong in there, in my opinion. I think that this new design is clearer, and will therefore be more easily understood and easier to maintain.

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Written by m3os

April 8, 2009 at 7:40 am

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