Systems
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Recap Operating of Last Class
Introduction to Nachos
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Classic synchronization problems
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Bounded buffer (producer/consumer) Dining philosopher Monitor Condition variables (Hoare/Mesa semantics) OS kernel synchronization User program synchronization for threads User program synchronization for processes
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High-level synchronization primitives
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Synchronization in practice
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Introduction to Nachos: Our Objective
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Motivation for Nachos
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Provide a basic understanding of the structure of Nachos
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Motivation: need good projects for teaching operating systems Assignments so far are all at user level
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not a replacement of your own practice and learning
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OK with understanding OS concepts Not real OS development experience Real hardware is too complex to work with Very hard to support even the most basic user programs Modern OSes are too complex to understand Not complete view about OS development
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Jump start you on Programming Assignment #3
Develop an OS from scratch on a real hardware (e.g., Intel IA32)
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Making enhancement/adjustment of an existing OS (e.g., Linux)
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Nachos is a toolkit for teaching operating systems; not an operating system itself
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Operating Systems
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Overall Nachos Structure
Nachos user programs
Nachos Kernel and User Programs
Nachos user programs
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user programs
Nachos syscalls I/O MIPS CPU & mem devices Simulated Nachos HW Nachos syscalls Nachos kernel I/O MIPS CPU & mem devices Simulated Nachos HW
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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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System calls OS kernel
Nachos kernel
Host Syscalls Host OS kernel I/O devices CPU & mem I/O devices Hardware machine CPU & mem
Host user programs
Host Syscalls Host OS kernel
Host user programs
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I/O devices
CPU & mem
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Host hardware machine
Host hardware machine
Normal computer structure
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Nachos structure
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Nachos structure
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Simulated Nachos Hardware
Nachos user programs
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Nachos as An Instructional Tool
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Simulated MIPS CPU
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The Nachos toolkit contains:
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Nachos syscalls Nachos kernel
A MIPS instruction interpreter Basically an array of words
MIPS CPU I/O & mem devices Simulated Nachos HW
The simulated hardware (MIPS CPU, memory, I/O devices) A cross-complier/linker that compiles your C user programs into Nachos executables
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Simulated memory
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instructions are in MIPS the segment layout and header format can be recognized by the Nachos kernel a single system call: halt limited synchronization primitives running a single user program at a time no inter-process communication
Host Syscalls Host OS kernel
Host user programs
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Simulated I/O devices
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A skeleton OS kernel supporting
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I/O devices
CPU & mem
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Host hardware machine
Console terminal Disk Timer Network interface
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Your job to is augment the OS step by step
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Nachos structure
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dont touch the simulated hardware shouldnt need to touch the compiler
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Operating Systems
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The Most Common Confusion
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Nachos Assignments
Start from a given skeleton OS, filling the missing pieces step by step
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Nachos kernel and Nachos user programs do not run on the same machine (real or virtual) Implications:
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they dont run on the same CPU (real or virtual) they dont share the same memory (real or virtual) 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
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This is a big weakness
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Assignment #3: threads and synchronization Assignment #4: supporting multiple user programs Assignment #5: inter-process communication and I/O Assignment #6: virtual memory
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Assignment #3: Threads and Synchronization
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Learning about Nachos
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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:
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You cannot learn all about software systems from textbooks
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read the source code for systems that other people have written
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For Nachos:
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Nachos kernel threads user-level threads from the host machines perspective
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Synchronization primitives are used for synchronization between Nachos kernel threads
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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
It will take a while to develop an understanding. Start early and be patient!
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Operating Systems
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