Monday, 18 May 2009

The devel's in the details

Well the last post was written before the weekend, and it's been a long snotty weekend since then. I've become a little obsessed with it lately although I found a couple of hours to fix a sticking door.

I spent the rest of the weekend glued to the screen hacking away feverishly - although I'm getting stuck doing more support code than anything else.
  • Had another stab at MMU code, but i'm still not comfortable with what i'm coming up with so again I've shelved it for now. I'm getting too caught up with trying to write neat re-usable code or worrying about efficiency.
  • Wrote an extremely rudimentary ELF relocator, which will be required for shared library support at least. The doco's a bit slim so it was a bit of mucking about to get anything going.
  • More work on re-arranging the code structure to isolate platform-dependent parts and create a practical file heirarchy. Lost of makefile crap.
  • Added the ability for user processes to attach to interrupts. They just get translated into signals. I'm not sure if this will be flexible or efficient enough.
  • Came up with basic process and task (thread) objects and creation primitives and more thoughts on how they might work. Yet to test.
  • More thinking and research about the kickstart process after the initial entry point. How to set-up exec, initialise ram, initialise included modules, etc. AmigaOS has a nifty extensible mechanism for automatic module-discovery and initialisation which i think i'll mimic.
  • Broke the context switcher for a few hours with a silly mistake. Bloody hard to debug. I started building with optimisation turned on too - about halves the number of instructions i have to step through.
  • Worked on implementing the user-level interfaces as object references rather than linked calls. Lots of scafolding support work required here. I also need to decide how I handle in-kernel calls, although they will be isolated to a single library.
  • More thoughts on how shared libraries and devices might work, although i've still got a way to go there.
Before one of the more major re-orgs I burnt the iso image to a dvd and loaded it up on an old pc. As much as bochs and qemu are handy, it was quite inspiring to see it work on real hardware even if it didn't do much.

Although they're not particularly efficient, i'm thinking of using taglists a lot for many of the interfaces - anywhere where you might need to extend functionality. At least on x86 32 bit they are very simple. If a syscall is required, the user-level code will then marshal the taglist into a more compact private structure and invoke the system handler. This would let it perform some validation checks too although the supervisor code will probably still have to do the same to avoid malicious code; which is an expensive pita. Maybe I can let the 'kernel' crash in this case and just throw away the calling task 'safely' instead - let the hardware do the checking.

I guess i'll be stuck doing some of this housekeeping work for a while, and then I can get back to trying to get a device going, or something else more 'interesting'.

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