If what has been covered to this point is clear, then a very reasonable understanding of the principles by which all Von Neumann architecture computers operate and are programmed should have been gained. In a later chapter we will examine the specific architecture and workings of the PC, but this should not now hold any surprises in store for us. To complete this chapter, we should now develop our two black boxes of processor and memory into a complete system.
Expanding on our two black boxes of Fig. 3.1, we now include an input device, an output device and a backing store to obtain the black box model of a complete infor- mation processing system,as shown in Fig.3.20.This is a typical black box model of a PC.
input output buses processor memory objects = data rules = program backing store
Fig. 3.20 Black box model of an information processing system.
A typical input device would be akeyboardand a typical output device would be a visual display unit (VDU) or monitor. Up until relatively recently most monitors would have been based on thecathode ray tube(CRT) display, but today a flat screen panelliquid crystal display(LCD), is much more likely. Many other kinds of input and output devices may of course be connected, ranging for example fromprintersandsound cardstomodemsandmice. All have access to the processor and memory, either directly or indirectly, through the address, data and control buses.
Because the main memory is usually volatile and loses all information at power down, it is essential that we are able to load previously developed programs and data from some external source into the main memory and save our programs and data that we wish to use again onto a more permanent medium. A device which provides this facility is called abacking store, and this is the lower black box in our diagram. In principle, at least, a backing store is very simple. When it is required to save a program or a piece of data, the binary patterns in memory representing the relevant information are transferred, for example, to a magnetic disk, where they are used to record equivalent magnetic patterns on the surface of the disk. When it is required to restore the program or piece of data, then the relevant part of the magnetic disk is played back and the resulting binary patterns are used to reset the memory addresses to their original values. In this way, any information may be saved and reloaded from disk. In Chapter 5 we will examine this process in much more detail.
Several forms of magnetic disk have become available over the years, including various sizes offloppy disk, various sizes ofhard diskand several versions ofZip disk. However, magnetic disk is not the only medium for backing store. Magnetic tape was first used, with rows of large reel-to-reel systems once typifying the layman’s image of a computer centre. Today magnetic tape is still used, though more usually in the form ofcartridge tapes.
Non-magnetic systems are also widely used, the most common of these being CD-ROM devices, where the binary patterns are implemented as pits in the surface of the disk and are read back by laser light. Originally these were classed as WORM14systems that could not be rewritten to, but nowadays so-calledrewritable
CD-ROM devices have become very common.DVD (which originally stood for Digital Video Disc but is now more usually called Digital Versatile Disc) has become the major substitute for CD-ROM drives on all modern systems. DVD operates in a similar manner to CD-ROM, using lasers to read pits on the surface of the disk, and it is of the same physical dimensions. The major differences, however, are that it uses a higher frequency laser, has a smaller track pitch and pit size and can be double-layered and double-sided. In comparison with a CD-ROM, which has a capacity of around 700 Mbyte, the single-sided, single-layered DVD has a capacity of 4.7 Gbyte, and a double-sided, double-layered DVD can hold up to 17 Gbyte.
References
Borland (1994) Borland C++ User’s Guide, Version 4.5, Borland International, Inc., Scotts Valley, CA 95067-0001.
Von Neumann, John (1945),First Draft of a Report on the EDVAC, 30 June 1945, Contract No W- 670-ORD-492, Moore School of Electrical Engineering, University of Pennsylvania, Philadelphia.
Exercises
3.1 Using Table 3.1 and a finger check table as required, determine the actions that will occur when the instructions shown in Fig. 3.21, from address 31 onwards, are executed.
3.2 Examine the diagram at Fig. 3.22. This is a more extensive piece of program code and data than we have used before. For this reason, a diagram such as Fig. 3.21 has not been used, but rather the program code and data segments have been written down in a tabular form, much as early programmers used programming sheets. Assuming that the program counter starts at 31, and using the doing codes that we have defined, determine the final value of memory address 09. What process is carried out by this code?
3.3 List a sequence of instructions, using only “jbnz” and those listed in Table 3.1, that will result in the value in memory address 05 beingmultipliedby the value in memory address 06 and the byte result left in memory address 07. Assume that memory address 05 contains the value 3 and that memory address 06 contains the value 7. Confirm, after execution of the program using a finger check, that memory address 07 contains 21. Because this is quite difficult, three hints are given as follows: (1) Note that multiplication can be implemented by
byte 7 byte 8 byte 31 byte 32 byte 33 byte 34 byte 35 byte 37 byte 36 byte 38 memory 00000011 00000001 00000001 00000111 00000100 00000111 00000100 00000010 00000111 00001000 processor counter gp doing using interpret update execute 31 fetch Fig. 3.21
successive addition. (2) You will need to use some additional memory bytes for temporary storage. (3) The program is at least 11 instructions long.
Memory address Memory address Value Doing code Using code 31 01 05 33 02 06 35 01 07 37 04 08 39 02 09 3b 01 08 3d 02 07 3f 01 09 41 02 08 43 01 06 45 05 0a 47 02 06 49 08 16 05 04 06 00 07 01 08 01 09 00 0a 01 program data Fig. 3.22
4.
PC Hardware and Inside the
Box
Introduction
In the previous two chapters we have looked first at how information can be repre- sented and then at how the principles of a von Neumann machine may be used to construct an information processing system. In this chapter we will take the black box model of chapter 3 and develop from that the actual hardware of a modern PC. As we do that, we will look inside the box so that we can identify all the pieces. This is an important part of an analyst’s task. A detailed internal examination of a PC is invariably going to be required since we will wish to know firstly that it is safe to operate, and then to see whether it is being used as a storage box for other items of possible evidential value, and finally whether all its components are connected up correctly. An ability to recognize what should be there and how the bits should be connected together is therefore of some considerable importance. This is summa- rized as follows:
● Revision of the black box model ● The mother board and all the buses ● Packaging of chips
● The 8088 and the design of the PC ● System resources
● The Intel processors
● Static RAM, dynamic RAM and ROM ● Connection of backing store devices ● Connection of peripherals
● Expansion cards