NacHos路标：操作系统学习指南

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"NacHos是一个用于教学目的的软件平台，它允许学生深入研究、修改并执行操作系统软件。这个系统的核心特点包括支持线程、用户级进程、虚拟内存以及中断驱动的输入输出设备。作为一款复杂的软件，理解和掌握NacHos各个组件之间的交互与协作对于初学者和教师来说是一项挑战。本文档旨在提供一个理解NacHos系统的路线图，它以高瞻的视角概述了源代码，重点在于整体架构而非细节。它并不是阅读源代码的替代品，而是作为辅助工具，帮助学习者在探索NacHos的复杂性时，更好地把握整个系统的结构和工作原理。文档的内容分为以下几个部分： 1. **Introduction to Nachos**：这部分会介绍NacHos的基本概念和目标，让读者对整个系统有一个初步的认识。 2. **Nachos Machine**：讲解NacHos的虚拟机环境，包括硬件抽象层和处理器模型，如MIPS架构。 3. **Nachos Threads**：详细阐述线程管理，包括创建、调度和同步机制。 4. **User-Level Processes**：探讨用户空间的进程管理和通信，强调其在虚拟内存中的运作。 5. **Nachos Filesystem**：涵盖了文件系统的设计和实现，解释数据存储和I/O操作的处理。 6. **Experience With Nachos Assignments**：分享实际操作和项目经验，让学生了解如何通过实践来深化对NacHos的理解。 7. **Source Code Overview**：提供源代码的概览，关注关键模块和功能，帮助读者理解软件结构和逻辑。通过这些章节，读者将获得一个系统性的框架，有助于他们逐步解开NacHos的奥秘，提升对操作系统内部工作流程的深入认识。无论是作为教学材料还是个人学习资料，这份路线图都是一份宝贵的资源。"

One point that should be noted is that

ReadMem

fails (returning FALSE), if the

address translation fails (for whatever reason). Thus, if the page is not present in

physical memory,

ReadMem

fails.

ReadMem

do es

not

distinguish temp orary failures

(e.g., page not in memory) from hard errors (e.g., invalid virtual address)

ReadMem

is used (for instance) when dereferencing arguments to system calls.

bool WriteMem(int addr, int size, int value)

writes 1, 2, or 4 bytes of

value

into mem-

ory at virtual address

addr.

The same warnings given for

ReadMem

apply here as well.

2.2 Interrupt Management

Nachos simulates interrupts bymaintaining an event queue together with a simulated clo ck.

As the clo ck ticks, the event queue is examined to nd events scheduled to take place now.

The clo ck is maintained entirely in software and ticks under the following conditions:

1. Every time interrupts are restored (and the restored interrupt mask has interrupts

enabled), the clock advances one tick. Nachos co de frequently disables and restores in-

terrupts for mutual exclusion purp oses by making explicit calls to

interrupt::SetLevel()

2. Whenever the MIPS simulator executes one instruction, the clo ckadvances one tick.

3. Whenev

er the ready list is empty, the clo ck advances however many ticks are needed

to fast-forward the current time to that of the next scheduled event.

Whenever the clo ck advances, the event queue is examined and any p ending interrupt

events are serviced by invoking the procedure asso ciated with the timer event (e.g., the

interrupt service routine). All interrupt service routines are run with interrupts disabled,

and the interrupt service routine maynot re-enable them.

Warning:

in interrupt handler may

not

call any routines that lead to a context switchof

the current thread (e.g.,

scheduler::Run()

SWITCH()

). Doing so may lead to deadlo ck.

This restriction is an artifact of the way interrupts are simulated under Nachos, and should

not be taken as an indication of the way things are done on real machines. Specically,

consider the case where multiple events happ en to be scheduled at

exactly

the same time.

If the handler for the rst event invokes

sleep

(which calls

SWITCH

), the others won't

serviced at the right time. In fact, the thread that called

sleep

may actually be waiting for

one of the other events that is supp osed to take place now, but is delayed b ecause of the

SWITCH

.We now have a deadlo ck

All routines related to interrupt management are provided by the

Interrupt

ob ject. The

main routines of interest include:

See Section 1 for a description of one common problem students encounter due to this.

To correctly implement preemption, the interrupt handler invoked when a running threads quantum

expires needs to switch to another thread. This is handled byhaving the interrupt service routine invoke

Thread::YieldOnReturn()

,whichdelays the actual preemption until it is safe to do so.

void Schedule(VoidFunctionPtr handler, int arg, int when, IntType typ e)

schedules

a future event to take place at time

when

. When it is time for the scheduled event to

take place, Nachos calls the routine

hand ler

with the single argument

arg

IntStatus SetLevel(IntStatus level)

Change the interrupt mask to

level

, returning the

previous value. This routine is used to temp orarily disable and re-enable interrupts

for mutual exclusion purp oses. Only two interrupt levels

are supp orted:

IntOn

and

IntO

OneTick()

advances the clo ck one tick and services any p ending requests (by calling

Check-

IfDue

). It is called from

machine::Run()

after each user-level instruction is executed,

as well as by

SetLevel

when the interrupts are restored.

bool CheckIfDue(bo ol advanceClo ck)

examines the event queue for events that need

servicing now. If it nds any, it services them. It is invoked in such places as

OneTick

Idle()

\advances" to the clo ck to the time of the next scheduled event. It is called by the

scheduler (actually

Sle

ep()

) when there are no more threads on the ready list and we

want to \fast-forward" the time.

2.3 Real-Time Clo ck Interrupts

Nachos provides a

Timer

ob ject that simulates a real time clock, generating interrupts

at regular intervals. It is implemented using the same event driven interrupt mechanism

describ ed ab ove.

Timer

supp orts the following operations:

Timer(VoidFunctionPtr timerHandler, int callArg, bool doRandom)

The

Timer

con-

structor creates a real-time clo ck that interrupts every

TimerTicks

(100)

time units.

When the timer go es o, the Nachos simulator invokes pro cedure

timerHand ler

, pass-

ing it

cal lArg

as an argument.

To add a bit of non-determinism to the system, argument

doRandom

sp ecies that

the time between interrupts should be taken from a uniform interval between 1 and



T imer T icks

The real-time clo ck can b e used to provide preemption.

Note that starting Nachos with the \-rs" option creates a timer ob ject that interrupts at

random intervals and preempts the currently running thread.

2.4 Address Translation

Nachos supp orts two types of VM architectures: linear page tables, or a

software managed

TLB. While the former is simpler to program, the latter more closely corresp onds to what

currentmachines supp ort. Nachos supp orts one or the other, but not b oth (simultaneously).

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