计算机专业英语 学习笔记 3

Memory systems>

Memory system desiderata :

(1)   size: infinitely large, noconstraints on program or data set size.

(2)   Speed: infinitely fast, latencyequal to the fastest memory technology available.

(3)   Cost: the per bit cost shouldapproach the lowest-cost technology available. Clearly these specifications cannotall be achieved as they are mutually exclusive. However, with the semiconductorand magnetic memory technology of today, these specifications are closelyapproximated.

Hierarchical memory:

In this section it is shown how designersimplement a practical memory that approaches the performance of an ideal memoryat reasonable cost. This memory system has a hierarchy of levels: the memorycloset to the processor is fast and relatively small, but has a high cost perbit. This level is called the cache. The real memory, sometimes known as mainmemory, is slower, larger, and has a lower cost per bit than the cache. Thelowest level in the hierarchy is usually a magnetic disk that has the longestlatency and the lowest bandwidth, however, it can be very large and has a verylow cost per bit. This hierarchy is illustrated in fig. 1-3.

Note that fig. 1-3 does not include theprocessor register file in the memory hierarchy. The register file is aprogram-managed cache and is generally not included in the memory system. Also,there can be more than one cache in the hierarchy.

Paged virtual memory:

Paged virtual memory provides the solutionto the first desideratum of a very large memory’s being available to theprocessor. Because of the importance of this desideratum, the relationshipbetween virtual and real memory is discussed first. With virtual memory, theprocessor addresses the disk, the large, slow end of the hierarchy. Thememories between the processor and the disk are there to increase the effectiveperformance (reduced latency and increased bandwidth) of the very slow disk. Ifevery instruction and data reference were made to the disk, the processorperformance would be slow indeed.

Then why is virtual memory so important? Largememory is needed for large programs and data sets. Early computers with smallreal memory required that the transfer of data between real memory and disk bemanaged explicitly by the operating system or the user. Virtual memory providesfor automatic management of this portion of the memory hierarchy through acombination of hardware and software aids.

The virtual memory interface is shown inFig. 1-4. A real memory of 16M bytes and a virtual memory of 2G bytes are shownfor illustration. Many modern virtual-memory systems are much larger than this.Virtual-memory space is divided into equal-sized groups called pages. A page ina modern computer is 1K, 2K, or 4K bytes. Real memory is also divided into thesame equal-sized groups, called page frames. When information is moved betweenvirtual-memory space and real-memory space, a complete page is moved.(processor, real memory, virtual memory, page, page frame, virtual address,page allocation and address translation)

Caches:

Section 3 discussed how virtual memoryextends the address space of a processor. However, the latency of real memoryis too long to support high-performance processors. Even with the high-speedDRAMs used today for real memory, something must be done to overcome thislatency problem.

The solution to this performance problem isto add another level to the memory hierarchy, called a cache, show in Fig. 1-3.The allocation of spaces in a three-level memory is shown in Fig. 1-5. Asdiscussed in Section 3, the virtual-address space is divided into equal-sizedpages. These pages are placed in real-memory frames of the same size. Because apage can be placed in any vacant frame, there is no particular order to theallocation of pages to frames. With the addition of a cache, blocks of 16-32bytes are taken from the page frames in real memory and placed into a blockslot for access by the processor. For modern processors the cache usually has alatency of one processor clock, so that instructions and data may be fetchedwithout delay expect when the referenced item is not in the cache.

Memory devices:

Random-access memory

Random-access memory, or RAM, is the kindof memory we usually refer to when we speak of computer memory. It is the mostwidely used type, and consists of rows of chips with locations established intables maintained by the control unit.

As the name suggests, items stored in RAMcan be gotten(accessed) both easily and in any order(randomly) rather than insome sequence. RAM relies on electric current for all its operations, moreover,if the power is turned off or interrupted, RAM quickly empties itself of allyour hard work. Thus, we say RAM is volatile, or nonpermanent.

Read-only memory

Read-only memory, or ROM, typically holdsprograms. These programs are manufactured, or “hard-wired” in place on the ROMchips. For example, a microcomputer has a built-in ROM chip (sometimes calledROM BIOS, for ROM basic input/output system) that stores critical programs suchas the one that starts up, or “boots”, the computer. ROM is “slower” than RAMmemory, and as a result, items in ROM are transferred to RAM when needed forfast processing.

Items held in ROM can be read, but theycannot be changed or erased by normal input methods.

New items cannot be written into ROM. Theonly way to change items in most forms of ROM is to change the actual circuits.

Magnetic disks:

The magnetic disk is a circular platter(圆盘) with a smooth surface and acoating that can be magnetized. Data is stored on it as magnetized spots. Thereading and recording device, the disk drive, spins the disk past read/writeheads that detect or write the magnetized spots on the disk.

CD-ROMs:

Optical disks need thin beams ofconcentrated light to store and read data. It is a form of laser storage,called CD-ROM.

There are two types of optical disks(光盘) that can be user-recorded, WORMand erasable optical. WORM stands for “write once, read many”: data can bewritten to this disk just one time, but the data can be read many times. Erasableoptical disks can be written to, read, and erased.

Magnetic tape:

A magnetic tape is a narrow plastic stripsimilar to the tape used in tape recorders. The tapes are read by a tape drivemoving the tape fast a read/write head, which detects or writes magnetizedspots on the iron-oxide coating of the tape. Each pattern(模块) of spots matches the byte codefor character being stored.

Latency 等待时间  register file 寄存器组（栈）the processor addresses thedisk 处理器对磁盘编址（虚拟地址）locations 存储单元（在控制器中有一张内存地址列表）

Hierarchical memory 存储器层次结构

Cache 高速缓存

Magnetic disk 磁盘

Main memory 主存

Paged virtual memory 页式虚拟存储器

Random-Access Memory(RAM) 随机访问存储器

Read-only Memory(ROM) 只读存储器

Compact Disk ROM (CD-ROM)只读光盘

Disk drive 磁盘驱动器

Floppy disk 软磁盘

Write once, read many(WORM) 一次写多次读

Magnetic tape 磁带

Register file 寄存器组

Latency 等待时间

Page frame 页帧

Real memory(storage) 实存储器

Dynamic RAM(DRAM) 动态随机存储器

 

Fill in the blanks from using the sentencesand definitions found on text.

There are three desiderata in the memorysystem, they are (size, speed and cost).

Even with the high-speed DRAM as realmemory, the high-performance processors still have the latency problem.

A microcomputer has a built-in ROM chip, itis called sometimes ROM BIOS.

Items stored in RAM can be accessed botheasily and in any order (randomly).

Magnetic disk has two types, they are harddisks and floppy disks.

CD-ROM is abbreviated from CompactDisk-ROM.

Read/write head detects or writesmagnetized spots on the iron-oxide coating of the tape.

The locations of a RAM are maintained bythe control unit.

Main memory is known as real memorysometimes.

The processor addresses the disk withvirtual memory.

Cache is fast, relatively small, high cost.

Information stored in semiconductor RAM isnonpermanent and volatile.

We use paged virtual memory to extend thesize of memory, increase bandwidth of the disk, reduce latency of the disk,store large program and data set.

We read data on the disk by detecting themagnetized spots randomly, moving the read/write head.

The three desiderata of memory system areexclusive, closely approximated today.

The processor performance would be high aswe reference instructions and data from cache, memory.

Page frame is used in real memory.

The register file is a program-managedcache, called as a stack sometimes, included in the CPU.