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# 计算机代写|OS操作系统代考OS operating system代写|COMP7308 Memory and storage

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## 计算机代写|OS操作系统代考OS operating system代写|Memory and storage

While a process is running, most of its data is held in “main memory”, which is usually some kind of random access memory (RAM). On most current computers, main memory is volatile, which means that when the computer shuts down, the contents of main memory are lost. A current typical desktop computer has 1-8 GiB of memory. GiB stands for “gibibyte,” which is $2^{30}$ bytes.

If the process reads and writes files, those files are usually stored on a hard disk drive (HDD) or solid state drive (SSD). These storage devices are nonvolatile, so they are used for long-term storage. Currently a typical desktop computer has a HDD with a capacity of $500 \mathrm{~GB}$ to $2 \mathrm{~TB}$. GB stands for “gigabyte,” which is $10^{9}$ bytes. TB stands for “terabyte,” which is $10^{12}$ bytes.
You might have noticed that I used the binary unit GiB for the size of main memory and the decimal units GB and TB for the size of the HDD. For historical and technical reasons, memory is measured in binary units, and disk drives are measured in decimal units. In this book I will be careful to distinguish binary and decimal units, but you should be aware that the word “gigabyte” and the abbreviation GB are often used ambiguously.
In casual use, the term “memory” is sometimes used for HDDs and SDDs as well as RAM, but the properties of these devices are very different, so we will need to distinguish them. I will use “storage” to refer to HDDs and SDDs.

Each byte in main memory is specified by an integer “physical address.” The set of valid physical addresses is called the physical “address space.” It usually runs from 0 to $N-1$, where $N$ is the size of main memory. On a system with $1 \mathrm{~GB}$ of physical memory, the highest valid address is $2^{30}-1$, which is $1,073,741,823$ in decimal, or 0x03ff ffff in hexadecimal (the prefix 0x indicates a hexadecimal number).

However, most operating systems provide “virtual memory,” which means that programs never deal with physical addresses, and don’t have to know how much physical memory is available.

Instead, programs work with virtual addresses, which are numbered from 0 to $M-1$, where $M$ is the number of valid virtual address. The size of the virtual address space is determined by the operating system and the hardware it runs on.

You have probably heard people talk about 32-bit and 64-bit systems. These terms indicate the size of the registers, which is usually also the size of a virtual address. On a 32-bit system, virtual addresses are 32 bits, which means that the virtual address space runs from 0 to 0xffff ffff. The size of this address space is $2^{32}$ bytes, or $4 \mathrm{GiB}$.

On a 64-bit system, the size of the virtual address space is $2^{64}$ bytes, or 4 . $1024^{6}$ bytes. That’s 16 exbibytes, which is about a billion times bigger than current physical memories. It might seem strange that a virtual address space can be so much bigger than physical memory, but we will see soon how that works.

When a program reads and writes values in memory, it generates virtual addresses. The hardware, with help from the operating system, translates to physical addresses before accessing main memory. This translation is done on a per-process basis, so even if two processes generate the same virtual address, they would map to different locations in physical memory.
Thus, virtual memory is one important way the operating system isolates processes from each other. In general, a process cannot access data belonging to another process, because there is no virtual address it can generate that maps to physical memory allocated to another process.

## MATLAB代写

MATLAB 是一种用于技术计算的高性能语言。它将计算、可视化和编程集成在一个易于使用的环境中，其中问题和解决方案以熟悉的数学符号表示。典型用途包括：数学和计算算法开发建模、仿真和原型制作数据分析、探索和可视化科学和工程图形应用程序开发，包括图形用户界面构建MATLAB 是一个交互式系统，其基本数据元素是一个不需要维度的数组。这使您可以解决许多技术计算问题，尤其是那些具有矩阵和向量公式的问题，而只需用 C 或 Fortran 等标量非交互式语言编写程序所需的时间的一小部分。MATLAB 名称代表矩阵实验室。MATLAB 最初的编写目的是提供对由 LINPACK 和 EISPACK 项目开发的矩阵软件的轻松访问，这两个项目共同代表了矩阵计算软件的最新技术。MATLAB 经过多年的发展，得到了许多用户的投入。在大学环境中，它是数学、工程和科学入门和高级课程的标准教学工具。在工业领域，MATLAB 是高效研究、开发和分析的首选工具。MATLAB 具有一系列称为工具箱的特定于应用程序的解决方案。对于大多数 MATLAB 用户来说非常重要，工具箱允许您学习应用专业技术。工具箱是 MATLAB 函数（M 文件）的综合集合，可扩展 MATLAB 环境以解决特定类别的问题。可用工具箱的领域包括信号处理、控制系统、神经网络、模糊逻辑、小波、仿真等。