Advanced Operating Systems
M3: System call interface
The aim of this milestone is to design the RPC protocol for the
system call interface. You should implement both the client and
system side of this interface. This client side system call
interface must conform to the interface provided in include/sos.h.
You should create a
At this stage you will not actually be able to implement most
of the system calls, however you should be able to partially
Other system calls should output
When a program opens the file
Reading the console is a blocking operation. If a process reads from the console and there is no data available it should block until data becomes available.
Be careful not to implement the console device as a 'hack'. You should think about being able to support multiple serial ports and other stream devices in your design (although not necessarily implement them).
You may once again find the documentation on libserial handy.
Changing your SConstruct
You will need to create a new library and to change your top-level
To add a new library you should create a new directory in the
from: milestone = 0 to: milestone = 3
Once you have your libs/sos library set up, it should like like this:
libs/sos libs/sos/SConscript libs/sos/src/ libs/sos/include/ libs/sos/include/sos.h
You should copy the contents of tty_test/ttyout.c to libs/sos/src. You can consider this new file as the acorn from which the entire grand tree of -lsos will flourish.
At this stage of the project you will need to decide whether you want to have a simple single-threaded server, or to multi-thread it. A multi-threaded design could be advantageous to deal with the inherent concurrency your system will have (e.g. between paging, system calls, asynchronous I/O and clock interrupts), but it will require careful design of synchronisation in order to avoid race conditions and deadlocks. A single threaded model will require extra attention to ensure liveness.
Another design descision is how to transfer data between the kernel and user processes. Some options you have are:
Whatever you do, remember the basic engineering rule: keep it simple, stupid!(KISS).
This milestone is larger than it seems. The system call interface of an OS determines how user applications receive data they request. You will need to consider how you can move data in various quantities between your root server and clients. Scenarios to consider include:
Think about how your OS/161 did it. Was it copy-in-copy-out? Did it reach into application address spaces based on app-supplied pointers? If so, what are the implications of this approach in the face of paging? (That is, what happens if the app's buffer has been paged out? How do you arrange to block the syscall, page the relevant data back in, and then restart the syscall?)
For this assessment you should be able to demonstrate
As always, you should be able to explain both the design and your implementation.
Note: In line with the comments in the advice section, streaming large blocks of data over multiple IPCs is a sure-fire way not to get full marks!
Last modified: 09 Aug 2010.