6. Read-only library – Complete files, and collections of associated files relating to a single application, held on read-only devices such as CD-ROM, or on a device with some form of write-protection control. Complete sets of files are read into the system from the read-only device, but for obvious reasons there are never any transfers from the system to the device.
The table indicates the typical access times, capacities, and unit costs for memory in each of these broad categories. Five main categories of system are considered:
desktop personal computers
laptop and fully portable personal computers
It is emphasized that this division is rather arbitrary, with no clear boundaries between systems. The capacity and performance of all systems continually increases, and there are many anomalies in the systems available in the marketplace, especially in the workstation, desktop, and laptop systems. The figures quoted are typical mid-range figures for performance, capacity, and costs (as at Spring 1995) and are clearly intended to be purely indicative of trends. With these caveats, the overall picture is reasonably consistent.
The following notes relate to the table: (1) In practice the registers on most current systems are part of a single chip, and it is difficult to identify accurately the cost of any single register.(2) On smaller systems the cache may be integrated with the processor proper, and again it is difficult to identify accurately the cost of any single register.(3) All systems use the same basic device for the actual storage. The higher unit costs on the larger systems arise from the inclusion of error-detection and error-recovery circuitry, and a wide data path between the memory and the cache.(4) The swapping device on large systems may take the form of semiconductor memory or high-performance disks, or a mix of the two.(5) The access time for any offline device is quoted as “minutes”. There are specific exceptions for offline tapes or cassettes held in some form of automated handler, where access time will typically be in the order of tens of seconds at worst. There is a similar exception for systems that handle multiple CD-ROMs. The unit cost for an online device includes the cost of the transport as well as the recording medium; for an offline device the cost quoted is purely for the recording medium.
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