Aug 17 2009
Writing a Small Operating System
I’ve just rediscovered a project I started in 1997 and have always intended to go back and complete one day. The project is a small operating system suitable for microcontrollers.
The OS will provide the following functionality, similar to a MicroKernel:
- memory management (done)
- preemptive task switching (done)
- process management (done)
- inter process communication
Memory Management
Memory is divided into variable size block, each preceded by a small memory control stucture. The structure contains the following:
- link to previous block
- link to next block
- link to owner of block
- length of block
- in use flag: is this block available?
- ready flag: is process ready to execute?
- stack pointer: sp saved when not active
Functions to allocate / free memory are provided. Have I missed anything important?
Task Switcher
The task switcher is called by the timer interrupt and switches to the next available task. Switching is disabled while memory is being allocated / freed.
Process Management
Functions are provided to create new processes, kill processes, mark a process as available or blocked. Switching is disabled while processes are being manipulated.
Inter Process Communication
Semaphores and message passing will be implemented. Unfortunately, I stumbled across a problem. How do I get two processes to agree which semaphore / message to use?
Should they be numbered, named or referred to by an address in memory? Should a semaphore / message buffer be allocated to each process or block of memory? If named, how should I handle name collisions?
Any advice on implementing IPC would be appreciated.
