Summary
PHOBOS is a message passing system; that is, it consists of a
number of components (called tasks) which communicate by
sending short messages to each other. The transmission and routing of
messages is carried out by the system kernel. There are a
number of built in tasks which carry out essential system functions, and
additional external tasks can be loaded and run as required. All tasks
have the ability to access everything and indeed do anything.
The kernel
The kernel consists of:
- A scheduler. This selects the next task to run, and
gives it an allocation of time before the scheduler regains control. The
scheduler operates a simple four level priority system using four
separate CPU queues.
- A system call handler. This handles software
interrupts generated by tasks, and uses the register contents at the
time of the interrupt to determine the service that has been requested
by the task.
- An interrupt handler. This simply routes each
interrupt to the task that has chosen to link to it, except in the case
of the clock interrupt.
- A clock handler. For reasons of efficiency and timing, the clock
interrupt handler is built into the kernel. It maintains a record of
time since the system was started, and can send timed messages to tasks
if requested to do so.
There is also a quantity of one-time initialisation code; in time, it is
planned to make the space occupied by this code recoverable.
The built-in tasks
The current built-in tasks are:
- The console task. This task handles all input from
the keyboard, and output to the screen (except emergency messages frpom
the kernel).
- The diskette task. This provides a message-based
interface for low level access to the diskette drive; it does not
provide any kind of filing system, merely a method of reading and
writing sectors. At present, this task is in its infancy, and it does no
more than turn off the diskette drive motor after boot.
The external tasks
At this time, no external tasks exist.
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