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 interace. This client side system call interface must conform to the interface provided in include/sos.h. You should create a libsos.a library that your applications can link against.

At this stage you will not actually be able to implement most of the system calls, however you should be able to partially implement open/close, read/write for the console device. This will allow you to run a simple shell on your system, which will allow you to perform interactive testing.

Other system call should output system call not implemented.

Console device

When a program opens the file console it should access the console on the serial device. The console is a multiple writer, single reader device, i.e., more that one process can concurrently open the device for writing, but only one process can open the device for reading.

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 carefult 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 Makefile

You will want to change your Makefile to make it build both your libsos.a and the provided sosh application.

To add a new library you should create a new directory in the lib/ directory, and place any source files into it. You will also need to create a file subdir.mk to tell make how to build your new library. Its format should be self-evident from the subdir.mk files in other provided libraries. Finally you should change the line:

LIBS := l4e c device serial

LIBS := l4e c device serial sos

To make sure a new application gets compiled you should change the APPS in your makefile. You will most likely want to remove the tty_test application from your build. Building more than one application may require extra makefile hacking on your part.

Design alternatives

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.

Another design descision is how to transfer data between the kernel and user processes. Some options you have are:

• Transfer data in message registers (like you did in M0).
• Use fpage mappings to transfer data.
• Set up a region of memory which is shared between the kernel and each user process and memcpy data into it when you need to transfer data.
• Use copyin-copyout method, where you lookup the page table and your system copies and data in to/out of physical memory.

Whatever you do, remember the basic engineering rule: keep it simple, stupid!(KISS).

Advice

This milestone is larger than previous ones, and it is advisable that you split the workload between the group.

Assessment

For this assessment you should be able to demonstrate sosh running, and the SOS outputting system call not implemented for the relevant system calls.

sosh makes use of the the SOS system calls in its ls and ps commands. This should be sufficient to demonstrate that your system calls work, alternatively you can create your own test code.

As always you should be able to explain both the design and your implementation.