[SOLVED] OSlab-Assignment 0

Description

CSE3201 Operating Systems

Assignment 0 Part 2             

OS/161

OS/161 is an educational operating system. It aims to strike a balance between giving students experience working on a real operating system, and potentially overwhelming students with the complexity that exists in a fully fledged operating system, such as Linux. Compared to most deployed operating systems, OS/161 is quite small (approximately 20,000 lines of code and comments), and therefore it is much easier to develop an understanding of the entire code base, as you will begin to do during this assignment.

The source code distribution contains a full operating system source tree, including the kernel, libraries, various utilities (ls, cat, etc.), and some test programs. The OS/161 boots on the simulated machine in the same manner as a real system might boot on real hardware.

System/161

System/161 simulates a “real” machine to run OS/161 on. The machine features a MIPS R2000/R3000 CPU including an MMU, but no floating point unit or cache. It also features simplified hardware devices hooked up to lamebus. These devices are much simpler than real hardware, and thus make it feasible for you to get your hands dirty, without having to deal with the typical level of complexity of physical hardware.

Using a simulator has several advantages. Unlike software you have written thus far (Windows excluded :-)), buggy software may result in completely locking up the machine, making to difficult to debug and requiring a reboot. A simulator allows debuggers access to the machine below the software architecture level as if debugging was built into the CPU chip. In some senses, the simulator is similar to an in circuit emulator (ICE) that you might find in industry, only it’s done in software. The other major advantage is speed of reboot, rebooting real hardware takes minutes, and hence the development cycle can be frustratingly slow on real hardware.

 

The group Reading/Discussion Part 2 of Assignment 0

This is probably the first time most of you will attempt to understand, and carefully modify, a large body of code that you did not write yourself. It is imperative that you take the time to read through the code to get an understanding of the overall structure of the code, and what each part of the code does.

This assessable, code reading component of this assignment aims to guide you through the code base to help you comprehend its contents, identify what functionality is implemented where, and be able to make intelligent decisions on how to modify the code base to achieve the goals of the assignments.

You don’t need to understand every line of code, but you should have a rough idea of what some files do.

Invest the time now in gaining an overall understanding of the code base. Now is probably the least busiest part of the year for you. Don’t waste it and struggle later.

The top-level Directory

The os161-ASST0 directory contains the top-level directory of the OS/161. It contains a few files, and subdirectories containing distinct parts of OS/161. The files are:

1. * Makefile this makefile builds the OS/161 distribution, including all the provided utilities. It does not build the operating system kernel.
2. * configure: this is a configuration script, similar to autoconf, but not generated by autoconf. You shouldn’t need to understand or tamper with it.
3. * defs.mk: this file is generated by running ./configure. Unless something goes wrong, you shouldn’t need to do anything with it.
4. * defs.mk.sample: this is a sample defs.mk file in case something does go wrong with configure. If configure does fail, you can fix def.mk using the comments in this file.

os161 contains the following directories:

1. * bin: contains the source code for the user-level utilities available on OS/161. They are a subset of the typical unix /bin tools, e.g. cat, cp, ls.
2. * include: these are the include files used to build user-level programs on OS/161, they are not the kernel include files. Among other things, they contain appropriate definitions for using the C library available on OS/161.
3. * kern: contains the sources to the OS/161 kernel itself. We will cover this in more details later.
4. * lib: the user-level library code for libc is here.
5. * sbin: contains the source code for the user-level system management utilities found in /sbin on a UNIX machine (e.g. halt, reboot, etc.)
6. * testbin: these are pieces of test code. They are most relevant to the course given at Harvard, but are included here for your perusal and potential use.

 

The Kern Subdirectory

This directory and its subdirectories are where most (if not all) of the action takes place. The only file in this directory is a Makefile. This Makefile only installs various header files. It does not actually build anything.

We will now examine the various subdirectories in detail. Take time to explore the code and answer the questions, include questions and answer in your report.

kern/arch

This directory contains architecture-dependent code, which means code that is dependent on the architecture OS/161 runs on. Different machine architectures have their own specific architecturedependent directory. Currently, there is only one supported architecture, which is mips.

kern/arch/mips/conf

conf.arch: This tells the kernel config script where to find the machine-specific, low-level functions it needs (see mips/mips).

 

Question 1: What is the vm system called that is configured for assignment 0?

 

Makefile.mips: Kernel Makefile; it copies this when you “config a kernel”. kern/arch/mips/include

These files are include files for the machine-specific constants and functions.

Question 2. Which register number is used for the stack pointer (sp) in OS/161?

Question 3. What bus/busses does OS/161 support?

Question 4. What is the difference between splhigh and spl0?

Question 5. Why do we use typedefs like u_int32_t instead of simply saying “int”?

Question 6: What must be the first thing in the process control block?

 

kern/arch/mips/mips

These are the low-level functions the kernel needs that are machine-dependent.

 

Question 7. What does splx return?

Question 8. What is the highest interrupt level?

Question 9. What function is called when user-level code generates a fatal fault?

kern/compile

This is where you build kernels. In the compile directory, you will find one subdirectory for each kernel you want to build. In a real installation, these will often correspond to things like a debug build, a profiling build, etc. In our world, each build directory will correspond to a programming assignment, e.g., ASST1, ASST2, etc. These directories are created when you configure a kernel (described in the next section). This directory and build organisation is typical of UNIX installations and is not necessarily universal across all operating systems.

kern/conf config is a shell script that takes a config file, like ASST1, and creates the corresponding build directory. Later (not now), in order to build a kernel, you will do the following:

 

% cd kern/conf

% ./config ASST0

% cd ../compile/ASST0

% bmake depend

% bmake

 

This will create the ASST0 build directory and then actually build a kernel in it. Note that you should specify the complete pathname ./config when you configure OS/161. If you omit the ./, you may end up running the configuration command for the system on which you are building OS/161, and that is almost guaranteed to produce rather strange results! kern/include

These are the include files that the kernel needs. The kern subdirectory contains include files that are visible not only to the operating system itself, but also to user-level programs. Now answer the following questions.

 

Question 10. How frequently are hardclock interrupts generated?

Question 11. What functions comprise the standard interface to a VFS device?

Question 12. How many characters are allowed in a volume name?

Question 13. How many direct blocks does an SFS file have?

Question 14. What is the standard interface to a file system (i.e., what functions must you implement to implement a new file system)?

Question 15. What function puts a thread to sleep?

Question 16. How large are OS/161 pids?

Question 17. What operations can you do on a vnode?

Question 18. What is the maximum path length in OS/161?

Question 19. What is the system call number for a reboot? Question 20. Where is STDIN_FILENO defined?

 

kern/main

This is where the kernel is initialised and where the kernel main function is implemented.

 

Question 21. What does kmain() do?

kern/thread

Threads are the fundamental abstraction on which the kernel is built.

 

Question 22. Is it OK to initialise the thread system before the scheduler? Why (not)?

Question 23. What is a zombie?

Question 24. How large is the initial run queue?

 

kern/lib

These are library routines used throughout the kernel, e.g., managing sleep queues, run queues, kernel malloc, etc. kern/userprog

This is where to add code to create and manage user level processes. As it stands now, OS/161 runs only kernel threads; there is no support for user level code. kern/vm

This directory is also fairly vacant. Virtual memory would be mostly implemented in here. kern/fs

The file system implementation has two subdirectories. We’ll talk about each in turn. kern/fs/vfs

This is the file-system independent layer (vfs stands for “Virtual File System”). It establishes a framework into which you can add new file systems easily. You will want to review vfs.h and vnode.h before looking at this directory.

Question 25. What does a device name in OS/161 look like?

Question 26. What does a raw device name in OS/161 look like?

Question 27. What lock protects the vnode reference count?           Question 28. What device types are currently supported?

 

kern/fs/sfs

This is the simple file system that OS/161 contains by default. You may augment this file system as part of a future assignment, so we’ll ask you questions about it then. kern/dev

This is where all the low level device management code is stored. You can safely ignore most of this directory.

This concludes the assessable group reading/discussion report writing component of the assignment 0. Feel free to discuss your answers with fellow group member, and your teacher. You can discuss with any student to understand but your report should be different from other groups. You have to prepare a report groupwise as assigned previously. You are not allowed to copy all or any part of othe report, if you thus so, you will be given 0. Submit the final report as Latex source code. See near the end of this document which will discuss about format, name and other specification of your report which must be followed.

Building a Kernel

First download os161-ASST0.zip from course website and unzip     . Now to the business end of this assignment. You will now build and install a kernel.

 

• You first have to configure your source tree.  % cd os161-ASST0

% ./configure

• Now you must configure the kernel itself.

% cd os161-ASST0/kern/conf   % ./config ASST0

• The next task is to build the kernel.

% cd ../compile/ASST0

% bmake depend

% bmake

• Now install the kernel

% bmake install

Running your Kernel

If you have made it this far, your have built and installed the entire OS. Now it is time to run it.

 

• cp the sample sys161.conf.sample to sys161.conf in ~/cs161/root/ directory
• Change to the root directory of your OS.

 

% cd ~/os161/root * Now run system/161 (the machine simulator) on your kernel.

% sys161 kernel

• Power off the machine by typing q at the menu prompt.

 

Using GDB

I cannot stress strongly enough to you the need to learn to use GDB. You can find directions and a short tutorial on using GDB with os161 here. Note: the version of gdb used for these assignments is cs161-gdb.

Modifying your Kernel

We will now go through the steps required to modify and rebuild your kernel. We will add a new file to the sources. The file contains a function we will call from existing code. We need to add the file to the kernel configuration, re-config the kernel, and the rebuild again.

 

• Begin by downloading c from course website and place it in kern/main/.
• Find an appropriate place the in the kernel code, and add a call to complex_hello() (defined in hello.c) to print out a greeting (Hint: one of the files in kern/main is very appropriate). It should appear immediately before the prompt.
• Since we added new file to the kernel code, we need to add it to the kernel configuration in order to build it. Edit kern/conf/conf.kern appropriately to include hello.c.
• When we change the kernel config, we need to re-configure the kernel again.

 

% cd ~/os161-ASST0/kern/conf

% ./config ASST0

 

• Now we can rebuild the kernel.

 

% cd ../compile/ASST0

% bmake depend

% bmake

% bmake install

 

• bmake treats warning as error
• If you find error (warning) using ‘bmake’, then try with     bmake WERROR=
• Run your kernel as before. Note that the kernel will panic with an error message.
• Use GDB to find the bug (Hint: the display, break, and step commands will be very useful).
• Edit the file containing the bug, recompile as before and re-run to see the welcome message.