Systems
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Introduction Operating to Nachos: Our Objective
Kernel Assignments
Provide a basic understanding of the structure of Nachos
not a replacement of your own practice and learning
Jump start you on Programming Assignment #3
CS 256/456 Dept. of Computer Science, University of Rochester
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Motivation for Nachos
Motivation: need good projects for teaching operating systems Assignments so far are all at user level
OK with understanding OS concepts Not real OS development experience
Overall Nachos Structure
Nachos user programs
user programs
Nachos syscalls MIPS CPU I/O & mem devices Simulated Nachos HW
Develop an OS from scratch on a real hardware (e.g., Intel IA32)
Real hardware is too complex to work with Very hard to support even the most basic user programs
System calls OS kernel
Nachos kernel
Making enhancement/adjustment of an existing OS (e.g., Linux)
Modern OSes are too complex to understand Not complete view about OS development
Host Syscalls Host OS kernel I/O devices CPU & mem I/O devices Hardware machine CPU & mem Host hardware machine
Host user programs
Nachos is a toolkit for teaching operating systems; not an operating system itself
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Normal computer structure
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Nachos structure
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Operating Systems
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Nachos Kernel and User Programs
Nachos user programs
Simulated Nachos Hardware
Nachos user programs
Nachos syscalls Nachos kernel
MIPS CPU I/O & mem devices Simulated Nachos HW
The Nachos kernel runs directly as a user program on the host machine. Kernel code written in C++. User programs run on the Nachos kernel and the simulated hardware (with a MIPS CPU). User programs access simulated I/O devices through Nachos syscalls. User programs written in a stripped-down C, compiled into MIPS executables.
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Simulated MIPS CPU
A MIPS instruction interpreter
Nachos syscalls Nachos kernel
MIPS CPU I/O & mem devices Simulated Nachos HW
Simulated memory
Basically an array of words
Host Syscalls Host OS kernel
Host user programs
Host Syscalls Host OS kernel
Host user programs
Simulated I/O devices
Console terminal Disk Timer
I/O devices
CPU & mem
I/O devices
CPU & mem
Host hardware machine
Host hardware machine
Network interface
Nachos structure
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Nachos structure
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Nachos as An Instructional Tool
The Nachos toolkit contains:
The simulated hardware (MIPS CPU, memory, I/O devices) A cross-complier/linker that compiles your C user programs into Nachos executables
instructions are in MIPS the segment layout and header format can be recognized by the Nachos kernel
The Most Common Confusion
Nachos kernel and Nachos user programs do not run on the same machine (real or virtual) Implications:
they dont run on the same CPU (real or virtual) they dont share the same memory (real or virtual)
A skeleton OS kernel supporting
a single system call: halt limited synchronization primitives running a single user program at a time no inter-process communication
This is a big weakness
why not let Nachos kernel run on the simulated hardware? the projects become too challenging a pain to develop code on the simulated hardware, e.g., cant run gdb
Your job is to augment the OS step by step
dont touch the simulated hardware shouldnt need to touch the compiler
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Operating Systems
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Nachos Assignments
Start from a given skeleton OS, filling the missing pieces step by step Assignment threads #3: and synchronization Assignment #4: supporting multiple user programs Assignment #5: virtual memory Assignment #6: file system and disk scheduling
Assignment #3: Threads and Synchronization
Become familiar with Nachos and the behavior of a working (but incomplete) thread system Use what is supplied to study concurrent programming Threads you use for this assignment are: Nachos threads in kernel user-level threads from the host machines perspective Synchronization primitives are used for synchronization between Nachos threads in kernel
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Learning about Nachos
You cannot learn all about software systems from textbooks
Traversing the Nachos Files
starting from the Makefiles threads/
read the source code for systems that other people have written it is not sufficient to just read the textbook, lectures, and information on the assignment Web pages read over the Nachos source code try to understand where the various pieces of the system live, and how they fit together
For Nachos:
support threads and synchronization the threads support is fully functional, though some of the synchronization primitives have not been implemented. support the loading and execution of user programs basic memory management virtual memory subsystem
userprog/
It will take a while to develop an understanding. Start early and be patient!
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vm/
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Operating Systems
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Traversing the Nachos Files
test/
some simple Nachos user programs Makefile for compiling these programs and converting them to NOFF support machine simulation. You need to read some header files to know how to access the simulated machine. It might also be instructive to look at the implementation of the machine simulation. But you shouldn't have to modify anything here.
Administrative Issues
group assignments
group of two or three dont form the same group for all four assignments high-level languages (e.g., Java, perl) are not suitable for operating system development
machine/
C/C++ programming
managing TA for each assignment start early
you cant cut corners on t...