Friday, November 29, 2013

Rolling Your Own Operating System

I started an extremely ambitious project a while back -- working on my own operating system. This isn't one that I ever intend to gain any ground or even boot on real hardware. (I'm targeting VirtualBox, VirtualPC/HyperV, and Bochs emulator environments right now.) It's just a place for me to play around with OS concepts. At work I write driver code for windows, but this code relies on a system of interfaces and mechanisms that have been designed and implemented by people much smarter than I. I'd like to start working on those more basic concepts. Sort of like peeling back the layers of an onion by recreating it. I anticipate I will learn much not only about how my OS works and can work, but about the tradeoffs that most OS designers have encountered along the way.

My current "OS" is very limited. It uses the standard DOS (well, Windows 7 -- because there are differences...) bootloader from the partition table. I created my own volume boot record to receive control from there. I'm able to load from the beginning of the partition. I'm still working on the loader. This is not be choice, but actually some unexpected first bit of learning. Here's the deal...

The BIOS loads and calls into the MBR code. The MBR code loads and calls into my volume boot record code. Now the fun starts. There are numerous extremely important interfaces the BIOS provides to early boot code. Before protected mode, paging, multi-tasking, and all the other things that really make an operating system an operating system you could just happily keep calling these BIOS interfaces to get this functionality. However, the BIOS code is legacy... It's so legacy that it isn't all that compatible with the way a modern OS sets up the CPU. (There are a few more compatible extensions -- VESA graphics BIOS stuff for example, but even this is severely limited)

So, it's already decision time. A lot sooner than I thought it would be. How do I start laying the ground work for the more modern features of my OS when I need to use the BIOS routines to talk to the hardware? Well, the short answer is I need to stop at some point, and probably the sooner the better. I don't want to litter my kernel code with a bunch of BIOS dependencies -- anything I write that requires this cannot really be reused anywhere else. Ideally, I would switch over from BIOS interfaces to my own all at once, however that will be difficult. Instead, I've decided a better approach is to rely on the BIOS interfaces until I'm ready to create my own. So, what does the BIOS give me that I need to reproduce?

  1. VGA/VESA support -- in order to know my OS is doing something, I should probably have a way to see it working. 1
  2. Keyboard support.
  3. Disk IO support.
  4. ...

I'm sure there are many more but these are the interfaces I'm immediately aware of. The first item, VGA support was easiest. This is probably because it's also the only one that's moved past legacy support. I am able to select a video mode which allows me to draw 24/32 bit RGB directly into a flat buffer space. This buffer space is directly mapped to the screen buffer, so I'm set as far as rudimentary drawing is concerned. I don't get any hardware acceleration and I'm stuck having to implement all my own line drawing, bit-blitting, and other APIs, but at least it's doable. The most annoying thing about this is I cannot adjust for changing the monitor resolution, but if I look at my above goals of not really caring if this runs on real hardware this is acceptable.

Now comes disk IO. This is a bit sticky, since there is no easy way to go about this. I'm probably going to have to support at least IDE and AHCI. I'm not exactly sure if I can get away with IDE only and rely on hard disks to support the legacy commands... I think so, but not sure. I also cannot keep thunking back and forth between BIOS code for this support -- every time I do that I'll probably have to tear down the things I really wanted to learn about -- memory paging, scheduling, etc. I see some work on a storage driver in my near future... It does indeed look like I can just go with IDE. Hopefully, IDE will be pretty easy to work with. It looks like the specs are open as well --

Here's what my volume boot record code looks like so far:

1 I can just use a COM port interface for logging what is actually happening in the OS, but there is going to come a time relatively early in the OS where I'll actually want to SEE what I've created. Maybe this isn't as big of a requirement as I think it is right now...