1、 1 外文原文 Microcomputer systems Electronic systems are used for handing information in the most general sense ； this information may be telephone conversation , instrument read or a companys accounts , but in each case the same main type of operation are involved : the processing , storage and transmi
2、ssion of information . in conventional electronic design these operations are combined at the function level ； for example a counter , whether electronic or mechanical , stores the current and increments it by one as required . A system such as an electronic clock which employs counters has its stor
3、age and processing capabilities spread throughout the system because each counter is able to store and process numbers . Present day microprocessor based systems depart from this conventional approach by separating the three functions of processing , storage , and transmission into different section
4、 of the system . This partitioning into three main functions was devised by Von Neumann during the 1940s , and was not conceived especially for microcomputers . Almost every computer ever made has been designed with this structure , and despite the enormous range in their physical forms , they have
5、all been of essentially the same basic design . In a microprocessor based system the processing will be performed in the microprocessor itself . The storage will be by means of memory circuits and the communication of information into and out of the system will be by means of special input/output(I/
6、O)circuits . It would be impossible to identify a particular piece of hardware which performed the counting in a microprocessor based clock because the time would be stored in the memory and incremented at regular intervals but the microprocessor . However , the software which defined the systems be
7、havior would contain sections that performed as counters . The apparently rather abstract approach to the architecture of the microprocessor and its associated circuits allows it to be very flexible in use , since the system is defined almost entirely software.The design process is largely one of so
8、ftware engineering , and the similar problems of construction and maintenance which occur in conventional engineering are encountered when producing software. The figure1.1 illustrates how these three sections within a microcomputer are connected in terms of the communication of information within t
9、he machine . The system is controlled by the microprocessor which supervises the transfer of information between itself and the memory and input/output sections . The external connections relate to the rest (that is , the 2 non-computer part) of the engineering system. Fig.1.1 Three Sections of a Ty
10、pical Microcomputer Although only one storage section has been shown in the diagram , in practice two distinct types of memory RAM and ROM are used . In each case , the word memory is rather inappropriate since a computers memory is more like a filing cabinet in concept ； information is stored in a
11、set of numbered boxes and it is referenced by the serial number of the box in question . Microcomputers use RAM (Random Access Memory) into which data can be written and from which data can be read again when needed . This data can be read back from the memory in any sequence desired , and not neces
12、sarily the same order in which it was written , hence the expression random access memory . Another type of ROM (Read Only memory) is used to hold fixed patterns of information which cannot be affected by the microprocessor ； these patterns are not lost when power is removed and are normally used to
13、 hold the program which defines the behavior of a microprocessor based system. ROMs can be read like RAMs , but unlike RAMs they cannot be used to store variable information . Some ROMs have their data patterns put in during manufacture , while others are programmable by the user by means of special
14、 equipment and are called programmable ROMs . The widely used programmable ROMs are erasable by means of special ultraviolet lamps and are referred to as EPROMs , short for Erasable Programmable Read Only Memories . Other new types of device can be erased electrically without the need for ultraviole
15、t light, which are called Electrically Erasable Programmable Read Only Memories , EEPROMs. The microprocessor processes data under the control of the program , controlling the flow of information to and from memory and input/output devices . Some input/output devices are general-purpose types while
16、others are designed for controlling special hardware such as disc drives or controlling information transmission to other computers . Most types of I/O devices are programmable to some extent , allowing different modes of operation , while some actually contain special-purpose microprocessors to per
17、mit quite complex operations to be carried out without directly involving the main microprocessor . The microprocessor processes data under the control of the program , controlling the Memory (ROM,RAM) Microproce I/Ointerface 3 flow of information to and from memory and input/output devices . Some i
18、nput/output devices are general-purpose types while others are designed for controlling special hardware such as disc drives or controlling information transmission to other computers . Most types of I/O devices are programmable to some extent , allowing different modes of operation , while some act
19、ually contain special-purpose microprocessors to permit quite complex operations to be carried out without directly involving the main microprocessor . The microprocessor , memory and input/output circuit may all be contained on the same integrated circuit provided that the application does not requ
20、ire too much program or data storage. This is usually the case in low-cost application such as the controllers used in microwave ovens and automatic washing machines . The use of single package allows considerable cost savings to e made when articles are manufactured in large quantities . As technol
21、ogy develops , more and more powerful processors and larger and larger amounts of memory are being incorporated into single chip microcomputers with resulting saving in assembly costs in the final products . For the foreseeable future , however , it will continue to be necessary to interconnect a nu
22、mber of integrated circuits to make a microcomputer whenever larger amounts of storage or input/output are required. Another major engineering application of microcomputers is in process control . Here the presence of the microcomputer is usually more apparent to the user because provision is normal
23、ly made for programming the microcomputer for the particular application . In process control applications the benefits if fitting the entire system on to single chip are usually outweighed by the high design cost involved , because this sort if equipment is produced in smaller quantities . Moreover
24、 , process controllers are usually more complicated so that it is more difficult to make them as single integrated circuits . Two approaches are possible ； the controller can be implemented as a general-purpose microcomputer rather like a more robust version if a hobby computer , or as a packaged sy
25、stem , signed for replacing controllers based on older technologies such as electromagnetic relays . In the former case the system would probably be programmed in conventional programming languages such as the ones to 9 be introduced later , while in the other case a special-purpose language might b
26、e used , for example one which allowed the function of the controller to be described in terms of relay interconnections . In either case programs can be stored in RAM , which allows them to be altered to suit changes in application , but this makes the overall system vulnerable to loss if power unl
27、ess batteries are used to ensure continuity of supply . Alternatively programs can be stored in ROM , in which case they virtually become part of the electronic hardware and are often referred to as firmware . More sophisticated process controllers require minicomputers for their implementation , although the use if large scale integrated circuits the distinction
