AOS Design Guide
The AOS project is a very large and complex system. Often the system is
built in an ad-hoc manner with little or no design beforehand, frequently
with the attitude that it will be cleaned up later.
This design methodology typically produces a system with only partial
functionality. This also involves a lot of writing and re-writing the same
code. It is usually also the case that "I'll do it later" never eventuates.
This page lists some of the design choices involved when building SOS to
allow you to better understand the difficulties before they occur. You are
free borrow or ignore as much or as little of this information as you like.
This is only a guide, and any SOS project you want to produce is fine.
Data Structures and Subsystems
While designing SOS you will need to consider a number of different
subsystems and data structures, and how they inter-relate.
Some systems you will need to consider include:
- A task/thread startup protocol
- File system access
- Console access
- A VFS-like layer
- [Virtual] Memory
- Page tables
- Frame management
- Heap management
- Thread and Process management
- Scheduling, run queues and blocking system calls
Some systems you probably don't need to implement, but still might like to
- Resource management
- Access control / multi-user issues
- A page-cache
In Milestone 0 you are required to pass data from an application to the OS
in the form of text data for printing. This is an example of passing
parameters and data between seL4 address spaces.
There are many ways to pass data between applications, not limited to the
- UNIX-like copyin/copyout
- Many short-IPC messages
- Long IPC strings (NB: These are not be supported by your kernel)
- frame mapping
- Memory objects
Which model you use is up to you, however some mechanisms are
more appropriate for some systems and not others (eg. copyin/copyout
will not work well with a multi-server OS, and tricky to implement
as a recovery-on-fault approach). A poorly designed IPC primitive is
a common problem and leads to an inefficient system. You must ensure
it is capable of providing fast communication (eg. simple system
calls) and also efficient when dealing with large amount of data
(eg. file access). You must also ensure your IPC mechanism is free
of denial of service (DOS).
Debugging your SOS project can often be a very difficult
task. For more information on debugging in seL4 have a look at
L4 Server Thread Management
Regardless of what kind of OS personality you choose to write,
you will need to deal with multiple asynchronous requests to your
seL4 server(s). There are a number of ways this can be
achieved. Once again, this list is neither exhaustive nor truly
- Single threaded servers
Have a single server thread and use continuations to avoid blocking
- Single thread per 'application'
At the cost of extra resources you can remove some of the problems of
- Single thread per 'session'
Having more than one seL4 server can create a headache for
multi-threaded solutions. Using sessions semantics can help to solve
- Worker threads per sub-system
Dedicating a thread to each subsystem can help place an upper-bound
on the number of threads that need creating (and deleting).
A light-weight alternative to multi-threading is to have a separate
stack per client/application. This can be simpler than multiple threads
to manage and easier than explicit continuations. Debugging, however, can
be ... interesting.
- Worker thread pool
Using a multi-threaded solution can leave to wasting of resources. By
creating a pool of worker threads that can grown and shrink resource
usage can be controlled.
All the above techniques are valid for use in SOS. Each has
their own advantages that make them
easier/nicer/faster/better/whatever. They also have their own
problems with implementation and testing. Make sure your solution is
deadlock free, DOS free, thread safe, efficient and easy to
In AOS you are encouraged to do extra work that you are
specifically interested in. This can be in the form of additions to
the SOS project, or entirely different OS related projects. Some
suggested SOS enhancements for extra marks are listed below. Feel
free to come up with your own ideas. If you would like to do one of
these (or your own suggestion), talk to us about what is to be done
and how marks will be awarded.
Disclaimer: None of
these advanced features is easy or trivial. You should avoid
making your project dependent on any of these as there is a good
chance you will not complete it. You should try to make sure you
at least one to two milestones ahead of the marking schedule
before you attempt one of these. The motto is "make it work,
then make it fancy".
- Device Driver
Write a more complex device driver than the OSTS
timer. Examples include a USB controllers, USB disk drives
etc. Marks can be awarded based on a partially functional
- Protocol Implementation
Implement a common network protocol within your SOS project. For
example, port (or implement your own) ssh daemon to SOS. The difficulty
of the protocol or port will dictate the bonus marks for it.
- Virtualise SOS
OS virtualisation is a hot topic right now. Demonstrate two
copies of SOS running on seL4, each running user-land
applications. Each copy of the OS must be segregated from
the other, and share device drivers securely.
- Distributed SOS
shared memory and cross-node process creation support to
- Orthogonal Persistence
Filesystems are passé. Implement orthogonal persistence in SOS so
applications restart back to the point when the system was shutdown.
Fault tolerance is not necessary.
- Multi-user SOS
The standard SOS project has memory protection, but no access control.
Implement ACL or capability based access control and provide resource
accounting. Multiple concurrent users can be provided with a trivial
Alternate System Models
There are a number of different general system structures you can
use when building an OS personality on top of seL4. A few major
categories are listed here. Be aware that this is in no way an
exhaustive list, and you are encouraged to come up with or design
your own. These categories are also very rough, and in no way well
- Monolithic System
Most operating systems, eg. Windows,
Linux and any BSD (Net, Free, Open, ...) use a monolithic system
model, with most, if not all, of the system implemented in a monolithic
By placing all system components in the same address space communication
is done trivially with shared memory and function calls. Of course, this
means that all code in that address space is trusted and can bring down
the whole system.
Example of how a typical
monolithic (UNIX) address-space layout
This model is the most commonly used for SOS
as it is the easiest to implement and debug. Within
this model there is plenty of scope for creativity and ample
work for two people. Other models are presented here so you
can incorporate ideas and abstractions. Students with a keen
caffeine dependence are welcome to try them for a system
model. There are very few systems to source examples from
and many have problems which are as yet
unsolved. If you choose a system
model other than monolithic, you have been warned.
- Single Address Space (SAS)
Single-address-space systems are designed to make sharing between
applications easier. By placing all applications and the kernel in the
same address space (translation) pointers can be passed around while
maintaining their meaning. To preserve security the system needs to
implement protection as an orthogonal abstraction.
While a SAS is not necessarily an entirely different model (it
could be monolithic or multi-server), it does offer some
interesting design decisions. Instead of the typical SOS
filesystem you could create a persistent system. A SAS also
helps in making a distributed shared memory system.
Example single address space operating systems include Mungi, Nemesis
A multi-server OS is the holy-grail of microkernel systems. By
decomposing the system into components (eg. VFS, file systems,
memory management, naming), each in their own address space, the
system can be constructed in a more flexible way. Multi-server
OSes have a strong tendency to move more work into application
libraries rather than the OS modules, but still preserving
security. An example Multi-Server OS
is Minix 3.
Example multi-server structure on seL4
L4 is a flexible and fast microkernel, however there are other
designs for microkernel (and micro-kernel-like) systems including L3, EROS, Mach,
Exokernel and K42.
You may choose to pick an existing microkernel (or come up
with your own abstractions) and implement this on top of
seL4. You will also need to implement elements of SOS on this
system to demonstrate its usefulness.
- OS161 on L4
Many students are probably familiar with OS161 used in
the introductory operating systems course. OS161
is a specific example of a (simple) monolithic kernel written for the
One option for the project is to port OS161 to seL4 by adding
a new 'seL4' architecture. Because OS161 is already quite
complete, it will also be necessary to add extensions and
features to make this project in-line with writing SOS from
29 Jul 2014.