Java版Nachos 5.0教程：从C++到Java的迁移指南
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《NACHOS 5.0j指南》
这本指南由Dan Hettena和Rick Cox合作编写，着重介绍了将NACHOS（一个用于教学的轻量级操作系统）从C++版本移植到Java的过程。NACHOS最初是由伯克利大学开发，旨在提供一个实践操作系统的平台，让学生了解操作系统的基本原理。在将系统转换为Java后，虽然保留了原有C++版本NACHOS的优势，但为了反映Java版本与C++版本之间的差异，许多细节有所调整。
该指南首先概述了NACHOS及其Java版的主要特点，强调了这种转变如何适应Java语言的特性。它指出，尽管Narten的《NACHOS主程序》(http://www.cs.duke.edu/~narten/110/nachos/main/main.html)仍然是学习C++ NACHOS的重要资源，但随着Java版本的出现，有必要提供更新的内容来帮助读者理解两者间的异同。
接着，指南深入探讨了以下几个关键部分：
1. **NACHOS与Java端**：这部分会解释Java版NACHOS如何构建，以及它是如何利用Java的面向对象特性和垃圾回收机制来简化操作系统的实现。
2. **NACHOS机器**：介绍Java版本中的虚拟机结构，可能包括处理器模型、内存管理以及与硬件交互的抽象层。
3. **线程和调度**：讲解Java中的并发处理和调度策略，包括用户级线程、内核级线程以及调度算法的调整。
4. **模拟的MIPS机器**：说明Java版NACHOS是如何模拟MIPS架构，这对于理解操作系统底层工作原理至关重要。
5. **用户级进程**：讨论Java环境下的进程创建、通信和同步机制，以及如何通过Java类和接口实现这些功能。
《NACHOS 5.0j指南》不仅提供了对NACHOS核心概念的深入理解，还展示了如何将这些概念应用到Java环境中，以便于初学者和开发者学习和实践操作系统设计与实现。无论是对C++还是Java背景的读者来说，这都是一份极具价值的学习资料。
A Guide to Nachos 5.0j
A Nachos kernel is just a subclass of nachos.machine.Kernel. For instance, the thread project uses
nachos.threads.ThreadedKernel (and later projects inherit from ThreadedKernel).
2.3. Nachos Hardware Devices
The Nachos machine simulation includes several hardware devices. Some would be found in most
modern computers (e.g. the network interface), while others (such as the elevator controller) are unique
to Nachos. Most classes in the machine directory are part of the hardware simulation, while all classes
outside that directory are part of the Nachos operating system.
2.3.1. Interrupt Management
The nachos.machine.Interrupt class simulates interrupts by maintaining an event queue together with a
simulated clock. As the clock ticks, the event queue is examined to find events scheduled to take place
now. The interrupt controller is returned by Machine.interrupt().
The clock is maintained entirely in software and ticks only under the following conditions:
• Every time interrupts are re-enabled (i.e. only when interrupts are disabled and get enabled again), the
clock advances 10 ticks. Nachos code frequently disables and restores interrupts for mutual exclusion
purposes by making explicit calls to disable() and restore().
• Whenever the MIPS simulator executes one instruction, the clock advances one tick.
Note: Nachos C++ users: Nachos C++ allowed the simulated time to be advanced to that of the next
interrupt whenever the ready list is empty. This provides a small performance gain, but it creates
unnatural interaction between the kernel and the hardware, and it is unnecessary (a normal OS uses
an idle thread, and this is exactly what Nachos does now).
Whenever the clock advances, the event queue is examined and any pending interrupt events are serviced
by invoking the device event handler associated with the event. Note that this handler is not an interrupt
handler (a.k.a. interrupt service routine). Interrupt handlers are part of software, while device event
handlers are part of the hardware simulation. A device event handler will invoke the software interrupt
handler for the device, as we will see later. For this reason, the Interrupt class disables interrupts before
calling a device event handler.
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