Memory Systems

keywords:
hierarchical memory 存储器层次结构
cache 高速换冲存储器 高速缓冲
chip 芯片
on-chip cache 单片高速缓存
sillicon die 硅片
magnetic disk 磁盘
main memory 主存储器
paged virtual memory 页式虚拟存储器
random access memory 随机存储器
read only memory 只读存储器
boot 引导 启动 自举
compact disk rom 只读光盘
disk drive 磁盘驱动器
floppy disk(diskette) 软磁盘
write one read more 一次写多次读
magnetic tape 磁带
register file 寄存器组
latency 潜伏时间 等待时间
page frame 页帧
real memory(storage)实存储器
Dynamic RAM 动态随机存储器
benchmark 基准测试程序 基准程序， 测试标准 基准
volatile 易失性
laser storage 激光存储器

1.memory system desiderata
存储系统需求
The memory system has three desiderata.
存储系统有一下3项需求
(1)Size: infitely large, no constraints on program or data set size.
容量：无限大 对程序和数据集大小没有任何约束
(2)Speed: infinitely fast, latency equal to the fastest memory technology available.
速度：无限快，等待时间在现有的存储技术下是最短。
(3)Cost: the per bit cost should approach the lowest-cost technology available.
价格：每单位价格在可使用的技术中接近最低。
Clearly theses specifications cannot all the be achieved as they are mutually exclusive.However, with the semiconductor and magnetic memory technology of today, thesr specifications are closely approximated.
很明显，由于三个需求的制约，所以很难全部满足，不过随着半导体和磁存技术的发展，这些需求近乎接近。

2.memory hierarchy
存储的层次结构
various memory elements grow exponentially in size and decrease exponentially in access time. Cloest memory to the cpu is the smallest, typically in the range of 1K byte of data (10008-bit characters) to 1M byte of data(1000000characters). These on-chip cache memories can have access time as short as 1/2 to nanosecond or one-billionsecond of a second, As a benchmark, light can travel about one foot through the air in one nanosecond. we call this cace the level 1 or L1 cache.
各种存储原件在规模上以几何形式增长，在存取时间上以几何形式减少，最接近cpu的存储器是最小的。典型的是从1KB（1千个8bite字节）到 1M（一百万字符）。这些芯片上的缓冲存储器，其获取时间可以短到1/2-1ns或者或者十亿分之一秒，作为基准。这种缓冲存储器成为一级存储器.
Below the L1 cache is the Level 2 or L2 cache. In today’pertium-class processor,The L2 cache is usually on the processor chip itselef,In fact, If you could lift the lid of a pentium or Athlon processor and look at the silicon die itself under a microscope you might be surprised to find that the biggest percentage of chip area was taken up by the cache memories.
在一级存储器下面是二级存储器，在当今奔腾处理器中，二级存储器通常在芯片上，事实上，如果你能够打开奔腾处理器或者athlon处理器的盖子在显微惊下观察芯片，你会惊奇的发现芯片的把部分面积被缓冲存储器占据着。
The secondary cache sits your computer’s main memory, These are the “memory sticks” that you can purchase in a computer shop to increase your computer’s perfromance.This memory is considerably slower than on-chip cache memory,but you usually have a much larger main memory than the on-chip memory.
二级缓存在计算机的主缓存上，这是在计算机商店购买的“存储器栈”，用于增加计算机的性能，这些存储器比在芯片上的存储器速度要低好多，但是更大的主存而不是芯片上的存储器。

3.Paged Virtual Memory
页式虚拟存储器
Paged virtual memory provide the solution to the first desideratum of a very large memorry’s bening available to the processor,Because of the importance of this desideratum, the relationship between virtual and real memory is discussed first.with virtual memory,the processor address the disk,the large,slow end of the hierarchy,the memories between the processor and the disk are there to increase the effective performace(reduced latency and increased bandwidth) of the very slow disk.if every instruction and data reference were made to the disk,the processor performance would be slow indeed.
页式虚拟存储器为处理器使用超大容量存储器提供解决办法，由于这一需求很重要，故先对虚拟内存和实存储器之间的关系进行讨论。处理器以访问虚拟存储器的方式访问位于层次结构位于最底层的大容量低速磁盘，在处理器和磁盘之间放置存储器是为了提高低速磁盘的有效性能（减少的等待时间和增加带宽） 如果每条数据应用都要访问磁盘，则处理器的性能一定很差。
Then why is vitual memory so important? Large memory is needed for large programes and data sets.Early computers with small real memory required that the transfer of data between real memory and disk be managed explicitly by the operating system or the user. Virtual memory provides for automatic explicitly by the operation of the memory herarchy through a combination of hardware and software aids.
那么为甚虚拟存储器如此重要？大型存储器需要大的大型程序和数据集，早期的电脑有很小的实存储器，在实存储和磁盘中传送数据，需要用户和系统直接管理，虚拟存储器通过硬件和软件的结合自动管理存储器层次结构的这一部分。
Virtual-memory space is divided into equal-sized proups call pages.A page in a modern computer is 1K ,2K,or 4K bytes.real memory is also divided into the same equal-sized groups.called page frames.when information is moved between virtual-memory space and real-memory space. a complete is moved.
虚拟存储器的空间被分为多个组，成为页面，现在计算机的页面大小是1K 2K 或者4k 。实存储器空间被分为多个相同的组，成为页帧，信息是整页整页地在虚拟存储器和实存储器之间传送。
4.Caches
高速缓存器
Section 3 dicussed how vitual memory extends the address space of a processor.However,the latency of the real memory is too long to support high-performace processors. Even with high-speed DRAMs used today for real memory,something must be done to overcome this latency problem.
第三部分讨论了虚拟存储器如果扩展处理器的地址空间，然而，是存储器的等待时间对于高性能处理器来讲仍然太久，即使目前很多实存储采用高速动态随机存储器的情况下，为了克服这些等待时间问题，还要做一些工作。
The solution to this performance problem is to add another level to the memory hierarchy,called a cache,As discussed in Section 3, the vritual-address space is divided into equal-size pages. These pages are placed in real-memory farms of the same size.Because a page can be placed in any vacant frame. there is no particular order to the allocation of pages to frames. with the addition of a cache,blocks of 16-32 bytes are taken from the page frames in real memory and placed into a block slot for access by this processor . for modern processor the cache usually has a latency of one processor clock. so that instructions and data may be fetched without delay except when the referenced item is not in the cache.
为了解决这一性能问题的方法是在存储层次结构中加入另外一层，成为高速缓冲存储器，正如在第三部分讨论的，虚拟地址空间被分为大小相同的页面，这些页面被放在大小相同的实存储器的帧中，每一页可以放置在任何空帧中，对于帧而言，没有具体的顺序要求。当加入高速缓存之后，实存储器中的页帧被分为16-32b的数据块，这些块别放置到高速缓存中的块存取窗口供处理器访问，在现实的处理器中，高速缓存的等待时间之占处理器的一个时钟周期，所以取数和和获取指令没有延迟，除非要访问的数据项没有在高速缓存中。
5.memory devices
存储设备
(1)Random access memory
随机访问储存器
random access memory, or ram, is the kind of memory we usually refer to when we speak of computer memory. it is the most widely used type. and consists of rows of chips with locations established in the tables maintained by control unit.
随机访问存储器是我们说的计算机储存器，用途广泛，有芯片构成，器存储单元由控制器部件中的地址表管理。
As the name suggests. item stored in ram can be gotten(accessed) both easily and in any order(randomly) rather than in some sequence,ram relies on electric current for all its operations; moreover, if the power is turned off or interrupted,ram quickly empties iteslf of all your hard work. thus , we say ram is volatile, or nonpermanent.
故名思意，ram中的内容很容易的以任意顺序读出，随机读取存储器始终靠电流进行操作，而且，如果关闭电源或中断，随机访问储存器很快丢失辛苦输入的内容，随机访问存储器称之为易失性和永久性存储器。
(2)Read-only memory
只读存储器
read-only memory, or rom,typically hold programs,these programes are manufactured or ‘hard-wired’ in place on the rom chips.for example, a microcomputer has a built-in rom chip(sometime called rom bios, for rom base input/ouput system) that stores electrical programes sunch as the one that start up,or boot ,the computer.rom us slower than ram memory, and as a result. items in rom are transferred to ram when needed for past processing.
只读存储器主要用来存储主程序，这些程序是在随机读取存储器芯片中或者通多硬件连接实现。例如在微型计算机中就内中随机读取芯片，（有时称rom bios 既基本输入和输出系统）用来存储一些关键的程序，如计算引导和启动程序，rom读取速度比ram慢，因此如果需要快速处理，则要将rom中的内容传送到ram中。
Items held in rom can be read, but they cannot be changed or erased by normal input methods. New items cannot written into rom, they only way to change items in most forms of ram is to change the actual circuits.
rom的内容是可读的，但是不可以通过一般的方法改变和擦除，新内容也不可以写入rom对于大多数rom而言，改变内容的唯一方法是切换事件电路。
(3)magnetic disks
磁盘
The magnetic disk is a circular platter with a smooth surface and a coating that can be magnetized. Data is stored on it as magnetized spots. the reading and recording devices. the disk drive.spin the disk past read/write heads that detect or write the magnetized spots on the disk.
磁盘是一个表面光滑而且涂覆有可磁化材料的圆盘，数据在磁盘上作为磁化点存入，独处与记录设备，既磁盘驱动器驱动磁盘转动，当通过读写磁头时，磁头从磁化点检测或写入出数据。
(4)CD-Roms
只读光盘
optical disks need thin beams of concentrated light to store and read data,it is a form of laser storage, called CD-ROM
光盘通过聚光光束存储和读取数据，cd-rom 是激光存储器的一种形式，
There are two types of optical disks that can be user recored :WORM and erasable optical .WORM stand for “write one, read many” :”data can be writen to this disk just one time, but the data can be read mang items. Erasable optical disk can be written to,read and erased.
用户使用可读写光盘分为两种，worm和擦除的光盘，worm代表一次写多次读，数据只能写入一次，但是可以读出多次，可擦除光盘可以读，写和擦除。
(5)Magnetic tape
磁带
A magnetic tape is a narrow plastic strip similar to the tape used in tape recoreded,the tape are read by tape drive moving the tape past read/write
head. which detects or writes magnetized spots on the iron-oxide coating of the tape, Each pattern of spots matches the bytes code for character bening stored.
磁带是一泓类似于录音磁带的窄塑料带，有磁带驱动器去读磁带内容，驱动器去读磁带内容，驱动器驱动磁带，由读写磁带头检入或写入由氧化铁涂覆的磁化点，以各磁化点的不同极性代表所存储字符的二进制代码。

后记：
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感受这个世界了解这个世界最本质的样子。
感觉有点乱的样子，但是要专注的去做自己。