Cad Guidebook: A Basic Manual for Understanding and Improving Computer-Aided Design by Stephen J. Schoonmaker Read Free Book Online Page A

Obviously, this is not always acceptable. In this case, a technique called “defrag- mentation” can be used. In this case, a special computer program is used to “shuffle” the data around on the disk (without doing the more drastic reformat- ting). The program reads a file into the memory system, then it deletes enough of the file to create some contiguous space on the disk. Then it rewrites the file from

22 Chapter 2
    the memory system back to this improved region. This process continues, file by file, until the all the files on the disk drive are in a more efficient arrangement. Of course, one must be certain that all the data on the disk drive has been backed up (as mentioned earlier) before anything like a “defrag” program is used. If the computer system should shut down for some reason during the running of the program, the result could be somewhat catastrophic.
    In the chapter on software, the concept of directories and subdirectories for files is presented. This technique of organizing larger numbers of files on a disk drive has some bearing on the performance, as well. It is the master files that need to keep track of the directories, and thus the same arguments of spreading data too thinly on the disk, or making the master file too large and cumbersome can apply to directories. For instance, if there are 10,000 files on a disk drive and they are in a single directory, then the master file for that directory will be rela- tively large. On the other hand, if these files are spread into 10 different subdirec- tories, and there is one master file for the top level directory, and then 10 smaller master files for each of the directories, then these master files will be much smaller, and therefore more efficient. Unfortunately, not all operating systems take complete advantage of this sort of efficiency, but it is still to be considered very beneficial for CAD programs, in particular, to keep files in a well-organized system of directories.
    2.5.5 Disk Cache
    The final concept to be mentioned with respect to the storage system is the disk cache. As already explained, the memory system (which is closely tied to the CPU) operates at a much higher rate of data transfer than the storage system. One way to improve the speed of data transfer for the storage system is to dedicate some memory chips to just communicate with the storage system. This relatively small amount of memory (perhaps a few percent of the overall memory system size) is called a cache or disk cache. It forms a buffer between the main opera- tions of the computer and the disk drives. And, special software may be running within the computer to attempt to keep the data “most likely” to be needed from the disk in that special memory area. This can certainly improve the performance of the storage system, and generally, the more memory that can be made avail- able for the cache, the better the performance. Also, many systems will use this cache technique not only between the disk drive and the memory, but also be- tween the memory and the CPU. These memory-type of caches may also be found on the CPU of the system.
    Table 2.3 presents a simple comparison between some common storage system’s characteristics. Some of this information will be explained in more de- tail in Section 3.2: The Operating System.

Computer Hardware Basics 23
    TABLE 2.3
    A Simple Comparison of Various Computer System’s Storage
    System type Disk drive designation Example file names
    PC A: (typically floppy disk)
    C: (main hard drive)
    F: through Z: (typically
    network drives)
    DRAWING.DAT Model_Of_A_Car.PRT
    Unix™ workstation /dev/hdisk0 drawing.dat
    model_of_a_car.prt
    VAX US E R1:
    USER2:
    DRAWING.DAT;1 MODEL_OF_A_CAR.PRT;32
    Mainframe A (user’s read–write area)
    H (shared “virtual” disks)
    (also referred to as DASD)
    DRAWING DATA A1 MODLCAR PART H/A
    2.6 NETWORKING
    So far, the basic internal workings of a “standard” computer system has been pre- sented. The