APPLICATION : Kscp Next Week

STEPPING OUT Our series on robotics ee continues with a look at the mechanics and | | we compare lap-helds from control of robot movement © | | iret Me ond Tandy = : : different versions of the same - 7 / _ | Basic computer. HARDWARE | @ Our robotics series : , = investigates and simulates th CUDDLY TOY The Koala-pad is a dedicated a graphics tablet that greatly enhances graphics _ © We introduce you to the on the Commodore 64 7 dynaturtle, in a LOGO-based

guided tour of vector algebra.

SUM OF THE PARTS We begin a new series on integrated software — one of the latest — ‘Innovations of computer design — by looking at the three governing principles

ORIGIN OF THE SPECIES Pacman versions are available for most of the popular home micros, but how well do they compare to the original? We review the game for the Vic-20 and Spectrum

DIVIDE AND RULE Our Loco series continues on the theme of recursion. In this instalment we design a number of complex patterns and shapes

FLOPPY DISK TO FORMAT A weekly _ glossary of computing terms

PROGRAMMING PROJECTS

ON YOUR BIKE We write a fast-action game in BASIC for two players on the Spectrum that 637 demands skill and concentration

Editor Vike Wesley. Art Director David Whelan: Technical Editor Brian Morris: Production Editor Catherine Cardwell, Art Editor Claudia Zeff; Chief Sub Editor Robert Pickering: Designer Julian Dorr Art Assistant Liz Dixon, Editorial Assistant Stephen Malone; Sub Editor Steve Wann: Researcher Vielanie Davis: Staff Writer steve Colwill, Contributors Geoff Bains, Harvey Mellor, Mike Curtis, Steve Colwill, Chris Naylor, Max Phillips, Matt Nicolson, Steve Malone; Software Consultants Pilot Software City, Group Art Director Perry Neville. Managing Director Stephen England, Published by Orbis Publishing Ltd: Editorial Director Brian Innes: Project Development | Peter Brookesmith, Executive Editors Chris Cooper, Maurice Geller, Production Controller Peter Taylor Medhurst, Gireulation Director David Breed, Marketing Director Michael Joyce: Designed and produced by Bunch Partworks Ltd; Editorial Office 14 Rathbone Place, London WIP 1DE. © APSIF Copenhagen 1984; © Orhis eet Lid 1984: Typeset by Universe; Reproduction by Mullis Morgan Ltd; Printed in Great Britain by Artisan Press Ltd, Leicester

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MACHINE CODE

JUMP LEADS Having mastered direct addressing, our 6809 code course progresses to 63/ indirect addressing | : |

TRAFFIC CONTROL Webring ourtwin- motor vehicle under the control of a joystick 634 using the two devices built in previous weeks

REFERENCE CARD A valuable reference INSIDE

, in | BACK any time when circumstances dictate. Binders depicted in this publication are those produced for the UK market card that comp lements the machine code COVE R only and may not necessarily be identical to binders produced for sale outside the UK. Binders and issues may be course Subject to import duty and/or local taxes, which are not included in the above prices unless stated.

COVER PHOTOGRAPHY BY MARCUS WILSON-SMITH

The Straight And Narrow |

Walking on two legs is an enormously difficult task for robots. Walking begins with a shift in the centre of gravityso — that the body topples forwards; the trailing leg is raised while the shoulder turns and the arms swing, bringing the body’s weight forward and sideways onto the leading foot; the body continues to topple as the lifted leg swings through and onto the heel. Balance is maintained throughout g by co-ordinating the movements of the large body masses, and by pushing the body sideways and forwards with the toes

In the first part of our series on robotics we looked at how robots have developed from science fiction fantasy to the ‘metal collar’ workers of today’s production lines. Now we discuss the three principal methods of robot movement and the most efficient n means of controlling it.

ee ee Le

Long before a child learns to walk it will be capable of picking objects up and will demonstrate intelligence in numerous other ways, but walking ‘is a skill that takes a long time to develop and

which involves considerable practice before it

becomes automatic.

Robots can be made to ‘walk’, but the techniques involved are very different from the methods used by people. ‘The robot may have legs, which it can swing backwards and forwards in an approximation of ahuman walk, but each of these legs is equipped with a foot that has wheels on its base. These wheels are fitted with ratchets to inhibit backward movement. So a robot of this type follows a set sequence of actions as it ‘walks’. The disadvantage of this method it that it is "difficult to develop a way of steering. the robot; it

will tend to move in a forward direction only and

its movements will be imprecise. A much better solution would be to make

HIP JOINT KNEE JOINT

ARMOURED KNEECAPS —

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Hibibese SURFACES ‘ WRAP-AROUND HEEL

‘STEVE CROSS.

robots walk by lifting first one leg and then the

other, as humans do, instead of simply swinging — Saale each limb through a limited arc. The major problem with this approach is that the robot must

be able to balance on one leg as it walks. Various -

solutions have been tried: thesé include tilting the

robot’s body sideways and even moving the entire

torso sideways on a rail so that the robot's centre of

gravity is kept directly above the leg that is bearing

its weight. If such a system were developed, robots

could walk in an efficient manner. In theory,

a walking robot could be designed to climb stairs .

and bring its owner a morning cup of tea. But in

practice, although a robot that can climb stairs is

quite feasible, a robot that ‘knew’ when it had

reached the top of the stairs would be harder to

develop because of the extra apparatus needed to

detect the top step.

An alternative approach has been to mount robots on tracks. The advantage of this system is that it allows the robot to travel over rough ground. The British Army uses tracked robots for carrying out hazardous bomb-disposal duties; these | machines can manoeuvre through debris and can cover reasonably rough ground.

Tracks are robust and are easily driven, but they have two main drawbacks. The first is that, as most robots are fairly small, so the size of the tracks is small and hence large obstacles cannot be

THE HOME COMPUTER ADVANCED COURSE 621

APPLICATION / ROBOTICS

\\oooO0]f *8\ 9 0 010 /F

negotiated. A full-size battle tank can climb over

almost any obstacle with ease — but this is only because tanks are so large, heavy and powerful. If

a tank tries to climb over an object so large that the tank’s centre of gravity is moved outside the area of its tracks it will fall over. This does in fact

happen occasionally when the ground is very

rough. The same thing will happen to your tracked robot if it tries to climb over surfaces that are too steep. The second disadvantage is that tracks cannot be controlled precisely. Steering is carried out by halting one track so that the other track continues

running; the robot (or tank) thus moves through

an arc. When this happens, the stationary track may easily slip slightly and the final position may not be what was expected. A battle tank that is driven by a person can readily correct any error of

this kind but, for a robot, the necessary course

corrections are considerably more complicated.

For robot control, it is obviously desirable to have a set of instructions that will always cause the robot to move to exactly the right place, facing in an exactly predictable direction. For these reasons, the most common form of robot movement uses wheels. Wheels have several obvious advantages, being simple, efficient and capable of producing a much smoother movement than legs could ever manage. |

Once it has been accepted that the robot should use wheels, the only problem is exact control of the movement. Consider, for instance, a clockwork toy motor car. This runs on wheels but

it is not a robot, as it has no means of ‘knowing’ its _

position at any given time. What is needed is a co- ordinate system that can be used to determine an object’s position on a surface — the most common system for this purpose uses Cartesian co- ordinates. With this system it is possible to locate a robot’s precise position and to specify the

Roll Those Robots!

622 THE HOME COMPUTER ADVANCED COURSE

movements needed to move it to another defined location. All that is then needed is a device to ensure that the robot can move precisely within this frame of reference.

Although hydraulic or pneumatic power is

occasionally used, the most common method of

moving robots is via an electric motor. As we have seen in our Workshop series (pages 585 and 612), a simple electric motor can provide movement and a modest amount of control over direction. This is not suitable for precise control — a simple electric motor always turns through at least 180 degrees before coming to rest, and inertia will often cause it to rotate a little more than that. So, for robot control, the stepper motor is

normally used. This is a motor that contains a large

number of coils and, although designs may vary widely, the general principle of the stepper motor allows very small, exact amounts of rotation to be specified, with little overshoot (rotating more than it should) or undershoot (rotating too little). Robots that use stepper motors are widely available. Such robots often have a pen attached, allowing them to draw a line on the surface they are travelling over. These pen-wielding robots are called ‘turtles’, and the designs they produce are

known as ‘turtle graphics’. These are capable of —

precise movement — their accuracy may be

_ judged by instructing them to draw a closed shape, such as a rectangle or a star, and checking to see whether or not the line drawn meets itself at the

Starting point. _ Stepper motors and Cartesian co-ordinates can therefore give us a relatively precise method of

controlling a robot’s movement. However, if the robot is to do more than simply roll around in a

given area, bumping into obstacles, it will need to be able to respond quickly and accurately to external conditions. We will consider oe further in the next instalment.

Stepping Measure

AXIS OF ROTATION oan

In the simplest electric motor, a flow of current in the rotor coil Creates a magnetic flux : opposed to that of the stator magnet's field; this opposition of forces causes the rotor to

| turnin the field

The stepper motor has

many (sometimes hundreds) of

coils. Switching current from one coil to another causes the

| whole to rotate in precisely

controllable increments of arc

KEVIN JONES

Robots must move, and not just on smooth laboratory floors. We show a few of the possible

_mechanisms.

racked eet give arip at the COSI 07 siecrabilily, bul allow a shiutiling gait without lea-liit, thus lessening balance problems.

Tracked robot vehicles are _ Commonplace — with bomb

Squads aiid planetary exploration teams, ior example.

The tr-axie formal is the only wheel adaptation that allows the robottocimp steps,

A large rolier-Dall Surrounded by ball-bearing stabilisers is very easily steered Dut is sensitive to irregular surfaces. The trolley alrangement of two fixed wheels and a steered castor is the minimum necessary (OF Stability.

slinging the load inside large driving wheels is an attractive idea, bul it raises the centre of gravily and lessens stability

KEVIN JONES

DIVIDE AND RULE

In this instalment of our LOGO course, we introduce a number of turtle graphics procedures that produce interesting shapes by the use of recursion. Some of these — shapes have very strange properties indeed,

as they were’ the inventions of

mathematicians intent on demonstrating

Our first program is acacia to ane ee alaues: To start with, we can simply draw a trunk with a right and a left branch. The branches may then be formed in exactly the same way (although they will _ besmaller), with a central main stem and right and left twigs. If this process is continued, a tree shape will gradually be built up. This is a good example ' of the way recursion can be used in LOGo.

Our procedure for drawing such a ‘binary’ tree requires two inputs: one for the length of the trunk and the other the ‘level’ number. The length of the branches is halved at each level away from the trunk.

TO BRANCH :LENGTH :LEVEL IF-LEVEL = 0 THEN STOP FD :LENGTH ys By eens : | - BRANCH (:LENGTH / 2) (:LEVEL = 1) RT 90: | : BRANCH (:LENGTH /2) (LEVEL - 1) LT 45 | | BK ‘LENGTH END.

Notice that the procedure is ‘state transparent. This is important as otherwise the ‘state’ of the turtle (its position and heading) would be changed each time the procedure calls itself, making it impossible to continue the drawing. : It must be admitted that this procedure produces an unrealistic tree — to make it more interesting, the procedure can be modified in various ways. Here’s a version that draws three branches, each of a different length, at each level:

TO BRANCH 1:LENGTH ‘LEVEL -IF:LEVEL = 0 THEN STOP — FD:LENGTH ‘ies LT 30 | veo BRANCH 1(:LENGTH /3) (:LEVEL-1) SPR AO: ee eee ee ea BRANCH1 (:LENGTH /2) (:LEVEL - 1) RT 50 os | BRANCH1 (:LENGTH / 1.5) (:LEVEL - 1) LT 60 BK :LENGTH END

: Try other modifications to produce 1 more lifelike

us ees.

CHEQUERED POLYGONS

The following procedure draws a square, divides it

into J then divides each part into four, and so on: 5 :

TO BOARD :LENGTH :LEVEL IF :LEVEL=0 THEN REPEAT 4 [FD LENGTH RT 90] STOP BOARD ( :LENGTH / 2) (:LEVEL - 1) FD (:LENGTH/2) | BOARD (:LENGTH / 2) (:LEVEL - 1) RT.90 ae FD (:LENGTH /2) 00 ee BOARD (:LENGTH / 2) (:LEVEL - 1) ~ BK (:LENGTH / 2) BOARD (:LENGTH / 2) (:LEVEL - 1 ) LT 90 : FD (:LENGTH /2) RT 90 | END

Write a similar procedure that splits a Handle up

~ into four smaller triangles, then splits each of these oh

up into four, and so on.

, SNOWFLAKES |

First draw. an equilateral triangle — one whose sides are all of the same length. Now divide each side into three equal parts and draw a new equilateral triangle on the centre section. Rub out the common lines, then repeat this sequence for

THE HOME COMPUTER ADVANCED COURSE 623

each side of the new shape, and continue the

‘process. The resultant shape is often called the ‘snowflake curve’ because of its anpeance: |

TO SNOW :SIZE:LEVEL . _ REPEAT 3 [SIDE :SIZE :LEVEL RT iia END ,

TO SIDE :SIZE :LEVEL IF LEVEL = 0 THEN FD :SIZE STOP SIDE (:SIZE/3)(:LEVEL-1) LT 60 tas SIDE (:SIZE/3) (:LEVEL- 1) RT d20"= | | SIDE (:SIZE/3) (:LEVEL- 1) LT 60 | | : SIDE (:SIZE/3) (:LEVEL- 1)

END

624 THE HOME COMPUTER ADVANCED COURSE.

Notice that SIDE is not state transparent, but

instead has been constructed so as to leave the

turtle in the correct place for drawing the next side. If this process of division is continued

_ indefinitely (mathematicians use the phrase ‘in the limit’), the result is a curve that has infinitelength

and yet surrounds a finite (fixed) area! It is possible to prove that this curve is neither one-dimensional nor two-dimensional, but is instead somewhere

between the two.

A similar curve may be built up by starting with

a square, dividing each side into three equal parts,

constructing squares on the middle sections, and so on. ‘ry writing a procedure that does this.

SPACE-FILLING CURVES

The series of curves shown here was invented bya mathematician called Sierpinski. If the process is continued in the limit, the result is a curve (a one-

dimensional line) that passes through every point

of the surrounding square (a two-dimensional

shape). There are many other ‘space-filling —

curves’ that exhibit this strange behaviour.

_ The procedure used for drawing this curve is

fairly complex. The level 1 curve is made up of four sides (shown in blue) that are joined by four

diagonals (shown in red). So the main procedure, - SIERP, just divides the process into four sections for

the procedure ONE.SIDE to handle one at a time. Consider just one of the sides. This is made up of three lines — a diagonal, a horizontal or vertical line, then another diagonal. At level 2, each diagonal is replaced by another, smaller, set of three lines, and the horizontal or vertical line is replaced by two similar sets of three joined by a line. The same process is carried out to move from level to level. : Here are the procedures for drawing the curves. Notice how the Loco command MAKE is used to

initialise DIAG:

“STOESIERP <SIDE /LEVEL.”» MAKE “DIAG :SIDE / SQRT (2 on REPEAT 4 [ONE.SIDE :LEVEL RT 45 FD :DIAG RT 45]

ENDS ,

TO ONE.SIDE :LEVEL IF:LEVEL=Q STOP ONE.SIDE (:LEVEL — 1 ) RT 45 © FD :DIAG © RT 45 ee ‘ONE. SIDE (LEVEL = 1°) LT 90 FD :SIDE a0! FD:SIDE EOD +” | ONE.SIDE ( :LEVEL - 1) RT 45 FD :DIAG a oe : TONE SIDE(:LEVEL= 113 END

s,: * aeaeeieanan

Sierpinski’s Curve -

| replace the ascending line by a broken line with six ©

| the task into two parts — one concerned with going

| to write two separate procedures to deal with the two

Procedure Problems 4

Our illustration shows a series of shapes that in their limit define a curve that has no gradient at any point. The first level consists of two lines — one going up, the other down. To proceed to the next level, we

parts. This rises to half the height of the original line, then drops all the way down; it again rises to half the height, continues to full height, drops back to half height and finally rises to full height again. The descending line is divided into six sections ina ~ similar way. LEVEL 1

Try writing a set of procedures that will draw this series of curves. You should use SETXY instead of FD and RT. Your top-level procedure should divide

up, the other with coming down. You will then need

parts of the top-level procedure. Remember that — these procedures may call each other as well as — themselves!

THE HOME COMPUTER ADVANCED COURSE 625

To Be Reviewed. .

ie arene iitearatad packages to

be reviewed in this series are Lotus 1-2-3, Open Access, Symphony and Framework

_ techniques have begun to filter down to home —

enable the programmer to switch between

SUM OF THE PARTS

‘Integrated software? has become o one e of the fashionable expressions of the software business. In this article we discuss exactly what is meant by integration and look at the advantages and disadvantages such a system offers. In future articles in this series we will look at specitic packages. re |

Integration represents one of the at | xaane

trends in software ever. And while for the moment

it applies mainly to business systems, its

micros. An example of this is the Sinclair QL,

whose four software packages encompass the

main principles of integration (see page 502). The main achievement of integration is to

different packages quickly and simply. In an ideal system it should not be necessary to quit one program, return to the operating system, swap disks and then start up another program. To be

_ effective, the change of application has to be

almost at the push of a key and some programs, such as the Lotus 1-2-3 and Ashton Tate's Framework achieve this.

It is also useful to be able to transfer data

between packages easily. For example, you might create a column of yearly sales figures for your

_ business in the spreadsheet program, then transfer

COURTESY OF MICROSCOPE

IAN McKINNELL

that whole column to the word processing

program where you might be writing the annual report. You could use the names and addresses in a database file with the word processor to write a oid ane ae to all the eee on file. On the

626 THE HOME COMPUTER ADVANCED COURSE

Lisa and Macintosh, this facility is extended to the

point where you can create a freehand drawing with the graphics program and then move it straight to a word-processed document.

In addition, all,the different programs should work in the same way and feel the same in use. Screen layouts, command keys, prompts, error

messages -— all the aspects of the ‘user interface’ —

should be identical or comparable. If they are not,

the user cannot confidently go from one area to.

another without having to stop and adjust to the change in operating procedures. This interrupts the flow with which the software can be used and does not allow it to be exploited to the full.

A handy side effect is that the package becomes easier to learn. Having to learn to use five new application. programs — some menu-driven, others command-driven, all with different command formats — is a daunting task for

anyone. But if they all work in the same way, you

need to learn one only. This feature is known as ‘commonality’ and is often referred to when integrated software is discussed.

We have established then that integrated

software involves three design principles: the ease

of switching from one application to another; freely interchangable data; and commonality of format. This contributes to making the computer

more accessible to the average user whose needs

can be met with two or three software applications.

‘It will also undoubtedly increase the popularity of

the personal computer as it becomes more efficient and easier to use.

However, integrated software also has. its disadvantages. Primary among these is the fact —

that integrated software packages need large

~amounts of RAM to operate. Imagine trying to fit

a word processor, spreadsheet, and database — the three applications that are most commonly integrated — into 16 or 32 Kbytes. It can probably be done, but there would not be much, if any, room left to store data. It is this problem. that restricts integrated software to machines with large

memories: in general, to computers with 128 Kbytes or more. Of course, programs that are ©

integrated can share some routines, so disk storage operations and other housekeeping activities need only to be written once. Nevertheless, each

application has its own special requirements, and

these take up space in RAM.

A second weakness of integrated software is an _ offshoot of the same problem of storage. To save

on the amount of memory a program requires,

_— software writers take shortcuts with the individual

applications. A word processor that is built into an integrated package with two or three other

—— +) ae =

applications cannot be as thorough and as complete as a word processor that stands on its own. The main reason for this is that a stand-alone program can take up as much memory by itself as the integrated programs take together.

One example can be found with two programs that run on the IBM PC and similar computers. Multimate is a program designed around the software that is used in Wang dedicated word processing equipment. It has many options for creating and formatting text that are not found in smaller programs, which makes creating very large and complex documents fairly simple. Multimate by itself requires 192 Kbytes of RAM to operate. Lotus 1-2-3, an integrated word processor, spreadsheet and database program, also requires 192 Kbytes. Yet the same space used by Multimate for one application must now handle all the software required for three complete programs in Lotus. As a result, the word processor in Lotus 1-2-3 is limited to simple memo writing.

A third disadvantage of integration also arises from one of its strengths. It is important, as we have seen, for integrated programs to look alike, so they are easy to learn and _ operate. Unfortunately, certain compromises have to be made by the software writers for this to be possible. The best way to operate a spreadsheet

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Fully Featured Integrationis the operating © .:#::s:nfiii: principle at the heart of Apple's Macintosh on which these illustrations were produced. Multiplan (the spreadsheet), MacWrite (the word processor) and MacPaint (the graphics program) communicate directly through the operating system so that three separate applications - . become effectively one ‘hal L lt 1 p ] i | mn Common Code Commonality of format between —_ - integrated applications is clearly : demonstrated in these

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ie mo EE he > R : ; S60 Umate you a 25th 3 .o ji: might not be the best way to use a database or | 20m ,Ms TM estog MEM. en, A ° F 26 TOR ete ee word processor, so elements of the optimum | = Piaget —r—“—iC—rSH : 3 420 “PC goto _ B+caort design for each tend to be blended into a usable | oe ee a ; S00 Ge-wg \SRLiR) asSGie ee j 2 he i Bul |

mixture. Microsoft found this to be a problem —“—iS—Sé« UE eats oe 6gi;5 Gu cin Right when designing a stand-alone word processor, | — SUD Totp eo, OSER ay Dow

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Microsoft Word. The company wanted the screen “mmc.

display and the program’s operation to be oe «si(<‘ ‘és * Me: OM compatible with their hugely — successful spreadsheet, Multiplan, so it would be easy to integrate the two programs. Microsoft included the same on-screen menu in Word that Multiplan users found so helpful, only to discover that the writers who needed a program like Word disliked having a menu on the screen all the time.

The point to remember about all software is that

it has to do what the user wants. If a person has

several tasks to do, like letter writing, simple accounts, and mailing lists, integrated software can make the job much easier. But there are sacrifices, and someone who wants to write a novel or very lengthy company reports on_ his microcomputer may have to continue to use

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separate, stand-alone word processors, ni fas neen moved directly. ie irectly spreadsheets, and database programs. from the spreadsheet into the

Nevertheless, as software writers learn more about compressing computer instructions into smaller and smaller spaces in memory, and the memory in home machines starts to inch its way upward, integrated software will become more and more important to the home user as well as the business user. As a hint of things to come, two under-£500

computers are being sold with integrated software:

Sinclair’s QL, and the Commodore Plus/4.

In future instalments of this series,.we willtakea _

closer look at some of the integrated programs that are having a large effect on software development.

word processor, demonstrating . the importance of data compatibility and transfer in integrated software

We will examine two distinct approaches to integration: one exemplified by Lotus 1-2-3 and similar programs, in which the applications, although working together, look very much like typical computer programs have always looked

_and the second type found on machines like Lisa

and Macintosh, where the whole operating environment is designed for integration. —

THE HOME COMPUTER ADVANCED COURSE 627

IAN McKINNELL

FLOPPY DISK

-In considering the development of RE in

the post-war period it’s easy to assume that the

techniques of Large-Scale and Very Large-Scale Integration (LSI and VLSI) have been the fuel,

- and the cheap mass-produced microprocessor -

chip the engine, of the microcomputer revolution. Of equal importance, however, has been the

_ development of cheap, fast, dependable back-up storage — the floppy disk. Single-drive units

costing about the same as a microcomputer that are capable of storing up to half a Megabyte of data or programs on a disk costing a couple of pounds make an over-priced electronic toy into a credible data processing system.

Inevitably in the growth of microcomputing, several disk formats have developed: the 8in

diameter floppy, the 54in mini-floppy, and the 3in

and 33in microfloppy. All consist of an oxide- coated thin (less than 0.5mm) flexible plastic disk,

ima protective jacket. This jacket is never removed,

and is pierced with a radial slot or window that gives access to the disk surface. In use the disk

_ spins inside the jacket at around 300 rpm, while the drive read-write head moves backwards and >

forwards in the window. This allows reasonably

fast access to every spot on the surface. Another slot in the jacket is called the write-protect notch: the drive unit checks that this notch i is open before

writing on the disk.

The read-write head is similar to a tape- recorder’s; it writes by changing the alignment of

the magnetic domains in the disk’s surface, and

reads by sensing those alignments. Information is written along concentric circles on the disk surface called tracks, divided into 20 to 40 sectors. High- quality, or double-density, disks can have up to 80

tracks, while single-density disks have 40. The boundaries between track sectors can be marked _byphotoelectrically-sensed holes — one per sector

Demo Disk The illustration shows a 53in

floppy disk, with the read-write — window, the sector index hole and

the write-protect notch en, visible

_— in the inside rim of the disk, in which case the disk is hard-sectored. The alternative — soft- sectored — disks have just one index hole, which

DAVID WEEKS

marks the start of the first sector on every track, Sa thea sector boundaries being aay De se AYO RE URI TANASE RETNA

_ 628 THE HOME COMPUTER ADVANCED COURSE

structure ©

FLOWCHART

Any graphical method of representing the

interactions of control or information in a program or system tends to be called a flowchart, though

_ many such diagrams represent relationships rather

than flows, and so should really be called “process diagrams’, or ‘data graphs’. The commonest form of flowchart comprises boxes of various shapes representing program processes such as input/ output, decision-making and data processing; these boxes are linked by arrowed lines that show how control passes from one process to another in the program. Most self-taught programmers use

‘this type of flowchart, but in recent years it has been discarded by professionals and academics

because it allegedly distorts or conceals program when used as __ descriptive documentation, and because it encourages badly structured program design. In general, the graphic approach is more and more discarded in favour of

precise requirements specifications, data

definition statements and function description _ languages.

FLOW CONTROL

Tn any real-time data communications application the receiver and the sender are likely to have different optimum transmit/receive rates; in order that slow receiving devices are overwhelmed by fast transmissions, some flow control strategy must be employed, either by the transmitter, or by the transmission network controller. The simplest strategy is known as end- to-end control, in which the amount of data sent is limited to the capacity of the receiver. Another strategy is called hop-by-hop — the amount of data sent is limited at every step through the

transmission network by the capacity of the node

or link smb the information.

FORMAT

Any pre-defined structure is a format, but in computing the term usually refers to disk or instruction formats. The former refers to the way

in which the pattern of tracks and sectors is

physically distributed on the surface of a magnetic disk (see page 124). Disks used by one

manufacturer’s disk drives may well be unreadable ~

to other drives precisely because of this. This

_ problem is eased somewhat by the emergence of

common operating systems such as CP/M and MS-DOS: software packages written for these systems often share a common disk format. To format a disk is to prepare it for a particular disk

_ drive; this destroys any information on the disk.

Instruction formats describe the syntax of instructions or commands, For example:

RENUMBER startno [,endno [,inc] ]

indicates the spelling of the command word, the nature of its possible arguments (first line number,

_ end line number and increment), the delimiters (the comma), and the variant forms of the —

command (anyihine in square brackets is

: Setenn)

not —

“P

CUDDLY TOY

For Commodore 64 artists the Koala-pad offers an easy way to produce high-quality graphic displays and overcomes the problems associated with graphics generation on this machine. Light, compact and easy to use, this peripheral device enables complex screen pictures to be built up at the touch of a finger.

Despite its ability to produce excellent graphic effects, the Commodore 64 has so far not been provided with the high-quality graphics peripherals

that have been produced for the BBC Micro. This is probably a result of the difficulties associated with the

production of high resolution graphics on the 64 - difficulties that have deterred manufacturers from producing such a_ device. Now, however, Audiogenic has begun importing a graphics pad made by the American company Koala Technology that allows Commodore 64 owners easier access to the machine's high resolution graphics capabilities. Unlike similar touch tablets such as the Grafpad

(see page 169), the Koala-pad is light and compact,

measuring only 20.5 by 16 cm (8 by 6in). In the centre is an 11 by 11 cm (4 by 4in) carbon fibre square that covers a touch-sensitive membrane that is similar to the Spectrum keyboard. By simply pressing a finger or a pen onto the membrane, the user can guide a cursor around the screen. This is in contrast to other graphics tablets, which require a special ‘stylus’ to complete the circuit.

The membrane consists of two sheets of

conducting wires — one in the horizontal and one in. .

the vertical axis. When the membrane is pressed, the pad detects which wires are in contact and sends the resulting co-ordinates to the computer. Above the touch membrane are two buttons, one of which must be pressed when the user wishes to colour in a point on the screen or select one of the various paint options that are available. Either button may be used — presumably this is to cater for both left- and right- hand users.

COLOURING IN

The Koala-pad connects to the computer via the joystick port, and Koala Painter (the software needed to work the pad) is loaded from disk. Once loaded, a display of the various options available appears on screen. At the bottom of the screen is the ‘palette’ containing 16 ‘true’ and 16 ‘tinted’ colours. The tints are built up by colouring alternate pixels in different hues, giving the effect of shading. Above the palette are eight boxes containing the ‘brushes’. These simply consist of various shapes that may be plotted on the screen, and range from a single pixel to combinations of pixels and lines. Surrounding the brushes are the

various options for drawing lines or shapes on the screen. These are selected by the user pressing on the pad membrane and thus directing a cursor arrow. When the arrow points to the desired option, pressing one of the Select buttons on the pad brings it into effect. The option flashes to remind the user

which mode is currently in use. The Koala-pad

provides the facilities to produce single lines, rays (lines drawn from a single point), frames and circles. Blocks of colour may be added by using the ‘box’ (coloured squares) or “disc’ option (coloured circles). Further colouring is achieved by using the FILL command to fill an enclosed area with a chosen colour. Colours are altered by using the X-COLOUR command. oe | : Identical figures may be drawn simultaneously by using the MIRROR command. This divides the screen into four sections, with the cursor restricted to the top left-hand quarter. Anything plotted within that

THE HOME COMPUTER ADVANCED COURSE 629

Hand-Held Complex and sophisticated

graphics screens can be built up ©

in a few hours using the easily understandable menu-driven software. Unlike many similar graphics tablets, the Koala-pad can be held in the hand while being operated

_ KOALA-PAD/HARDWARE ()}

IAN McKINNELL

Take Your Pick The main menu of the Koala- pad consists of boxes containing both the name and an explanatory icon. Although the icons are intended to aid understanding, some of the illustrations are perhaps.

confusing. The cursor arrow is.

moved into a box and the ‘Select’ button is pressed. The ‘name of the selected box will flash to remind the user which ‘mode is in use

IAN McKINNELL

Highly detailed pixel plotting is achieved by using the ZOOM command. The user may choose any part of the screen, and this is then displayed as an expanded ‘window’ at the bottom of the screen. The individual pixels are displayed as eight by eight pixel, character-sized squares. This feature makes the production of fonts and sprite-sized figures quick and simple. These figures are then placed anywhere on the screen by using the COPY command; this allows

the user to define an area of the screen, the contents

of which are copied to any other position.

After these options have been selected, the cursor

arrow is moved off the screen and the Select button is

pressed. The screen then changes to the ‘canvas’ on

which the required picture is built up. By moving the

cursor arrow and pressing the Select button, lines and

shapes may be drawn anywhere on the normal Commodore 64 graphics screen. |

MINOR NIGGLES | The quality of the graphics produced by this

device is excellent, rivalling high resolution screens

produced with commercial software. However, one disappointment — also shared by other graphics packages — is the quality of the freehand DRAW command. The membrane matrix resolution does not match that of the 64’s high resolution screen, so the user’s stylus (or fingertip) will often not be directly over a grid intersection and may in fact be triggering two points at the same time. The computer, in trying to interpret this, will plot a point, which unfortunately is not always at the position intended. This can result in what was planned as a

630 THE HOME COMPUTER ADVANCED COURSE

quadrant will automatically be copied to the corresponding area of the other three quarters.

straight line appearing as a messy scrawl. Another criticism is that, apart from the ZOOM command, there is no option to change the colour without returning to the main menu. But these are minor complaints that are more than offset by the speed at which the LINE and FILL commands are executed.

A further software limitation that could have been better thought out is the method used to erase mistakes. When a mistake is made, it is removed from the screen by use of the OOPS command, which is accessed from the main menu. However, using OOPS will erase all the work that has been performed since the user last exited from the main menu. This means that perhaps half an hour’s work may be erased, simply because of a single error. The alternative is to

erase a mistake by using the ZOOM command and

correcting the error pixel by pixel, which in the case of a badly placed disc or box could take some time. A welcome amendment would be to restrict the extent of the OOPS command to the last press of the Select button rather than the last exit from the main menu.

As may be suspected from a device that plugs into

the joystick port, it is possible to use the Koala-pad as

a joystick, thus allowing users to access the Koala- pad from their own programs. The position oi the cursor can be obtained from Basic by PEEKing locations 54297 and 54298 for the co-ordinates of X and Y respectively.

Screens may be saved onto disk and can easily be transferred to the user’s own programs, allowing the development of ‘Hobbit’-style adventures with text at the bottom of the screen and a picture above. By using the Koala-pad it is possible to save and recall up

to 16 different eight-Kbyte screens on a disk.

Although it is not possible to load a screen from disk

directly into screen memory, Koala Technology has provided a program in the Koala Painter user guide that enables screens to be transferred from the area in which they are loaded to the screen memory.

The maximum resolution of a saved Koala Painter screen is 255 by 255 pixels if it is included in a BASIC program, although higher resolutions may be gained by using machine code. This limitation may cause problems as the Commodore 64 high resolution screen is actually 320 by 200 pixels in size. The Koala-pad is restricted to a maximum of 255 pixels because this is the largest number that can be addressed by one byte — to address a full screen would require 16-bit addresses. Unfortunately this may result in the user losing a portion of the display.

This is because, when the screen is not actually being displayed, it will be stored somewhere within user RAM. Therefore the eight-Kbyte screen, together with the colour information, must be transferred from its location in user memory to the high resolution screen memory commencing at location 55296. , :

However, any minor niggles apart, it must be said that the Koala-pad is a very useful peripheral for anyone wishing to produce high resolution graphics screens on the Commodore 64. It is a pity that the software is supplied on disk only, as this will inevitably limit the device's appeal. But for disk owners, this would be a worthwhile investment —

particularly for the artist or the adventure-writer.

5 THE HOME COMPUTER ADVANCED COURSE

IAN McKINNELL

Though most successful commercial games

are fairly lengthy, and written in machine

code for speed, it is possible to write an entertaining game in BASIC. The game that follows is fairly simple, and indeed takes up only 35 lines of Basic, but is still good fun to play. Furthermore, it is a two-player game, so is less anti-social than most!

The game is called ‘On Your Bike’ and is based on

a scene from the classic Walt Disney film Tron. It is

a contest between two opponents that requires

skill and fast reactions and takes place in an

enclosed arena. You each have a bicycle that

travels at an incredible speed and cannot be stopped. Your only control allows you to turn through 90 degrees at top speed.These bicycles leave solid walls of light in their trail, and the object of the game is to force your opponent to crash in

the ever-tightening maze you create as you Zoom around the arena.

The game has been implemented on the ZX Spectrum, which is not known for the speed of its pasic. As this is an action game, the program has been designed for speed rather than elegance,so much of the listing may seem a little unstructured.

632 THE HOME COMPUTER ADVANCED COURSE |

ON YOUR BIKE

Subroutine calls; and other structured devices have been avoided as they would have sacrificed execution speed.

The first stage is the design of the arena and score display. As you can see this is fairly simple, which contributes to the shortness of the final program.The only point to note is that the border of the arena is now one character in from the usable screen area. This is to ensure that the graphics resulting from a collision with the arena’ wall do not go off screen:

10 LET p=0: LET q=0

100 BORDER 0: PAPER 0: CLS

110 PRINT AT 0.1; INK 6; “Bike One=”; 120 PRINT AT 0,19; INK 5; “Bike Two=”; p 130 INK 2 |

140 PLOT 8,8: DRAW 239,0: DRAW 0,159 150 DRAW -239,0: DRAW 0,-159

The arena has been drawn in red, and we have chosen. yellow to represent bike one, and cyan

(blue) for bike two. The variables p and q hold the

current score for the two contestants.

The next stage is to initialise all variables, and here we have to start thinking about how we are to implement the main action of the game. The action for a single bike is fairly straightforward, and is shown in the flowchart. Using POINT we check if the bike’s current position is occupied, and move to the collision routine if it is. If it is not, we move into that position using PLOT, and then read

_ the keyboard to check for any change in direction.

Our position is then incremented by one in our current direction, and the cycle begins again. We therefore need four variables: two for our current x and y co-ordinates, and two for our current direction along the x and y axes.

However, we are dealing with two bikes moving at the same time. An elegant solution would be to use four two-element arrays, x(2) and y(2), for the positions for example, but this would slow the game down so we have to use eight separate variables:

200 LET x=40: LET y=88

210 LET m=215: LET n=88

220 LET a=1: LET b=0 ©

230 LET i= -1: LET j=0 This sets the initial positions of the bikes, and sets them moving towards each other one pixel at a time. The basic action of the game is then fairly simple to implement:

400 IF POINT (x,y)=1 THEN LET col=6: GOTO 700

410 IF POINT (m,n)=1 THEN LET col=o: LET x=m LET y=n: GOTO 700

420 PLOT INK 6;x,y:PLOT INK5;m,n

eS SSS SSS SSS SESS

4 SS ee en ane

— % Se \

The only confusing point is perhaps line 410, where the variables for bike one are set to those for bike two, and a new variable,col, is introduced. This is so that a single routine can be used for the collision action, where x and y are simply used to indicate the point at which the collision takes place, and col sets the colour.

The routine for checking the keyboard has to be fast, but we regretfully have to use IF... THEN statements that are fairly slow. However, we can

use the fast IN command to read the keys. The

control keys chosen are Q and A, which control upward and downward movement for bike one, and P and ENTER for bike two. Left and right are X and C for bike one, and N and M for bike two. (See page 366 for a full explanation of how blocks

of keys relate to bytes in memory.)

All that remains is the collision routine, and the updating of the scores. An expanding series of concentric circles, centred on the point of impact was chosen, with radii of four, six and eight pixels:

This finishes the game, with the last statement looping back to the initialising procedures at the beginning. The game could, however, do with a starting procedure to make it more friendly to use:

300 PRINT AT 10,5; INK 7; “PRESS ANY KEY TO START”

310 IF INKEYS= “” THEN GOTO 310

320 PRINT AT 10,5; “ j

This gives you a break between consecutive rounds. All that remains is to save the game to cassette, preferably using SAVE “OnYerBike” LINE 10 so that the game automatically runs as soon as it is loaded.

The game could obviously be made more exciting, with instruction screens, loading screens, a one-player option with a strategy routine controlling the other bike as we have suggested, sound and better graphics. But certainly the latter options would make the game unplayably slow. In a future instalment we will rewrite this game in

“machine code to demonstrate its full potental.

Fitting In

computer is Player 1. The © program section that checks

input is bypassed, andthe — algorithm in the code given allows Player 1 to continue

One Hand Clapping —

_ movingina straight line until that In the one-piayer version the

Player 1 $ command keys for

generates an illegal move, or

Player 2 moves. In the latter case, _this move is mirrored or duplicated, and then checked for __ legality. Whenever anillegal ~ _ Player 1 move is generated, the

program looks for legal moves at

tight-angles to the illegal move -

direction: if none is found then the position is hopeless —

_ to the end of lines 550 to 570 -~

There are many ways of implementing a one-player

__ version of this game. The changes _ we suggest make it possible fo | _ choose between the one- and -

two-player versions at the start of every game. Itisntdifficultto

__ invent satisfactory algorithms for -

playing this game, butit’s extremely difficult to implement |

them in BASIC without ne down the game considerably.

_ Make the following changes to

the program:

20 LET keybd=500:LET pt—1.LET umoved=0: - RANDOMIZE

260 PRINI Al 10,5; No. of Players (1/2)?

Z2/OLEL aS-iINKEYS IF 3 3=—t=w as=< > 1 ANDaS< ye THEN GO 10 270

280 IFINKEYS< >*2” THEN

_LET keybd=440 These lines give the user the choice of game types, and implement that choice by

_ accessing either the one-player

Stratagem between lines 440 and A460, or the standard two- ae version between lines 500 and - 5/0. Our strategy is contained | in these lines:

430 GOTO keybd : 440 LE] pt=SGN (RND-0.5): IF umoved=pt THEN LET a=pt*): LE] b=pt7i: LE] umoved=0_ © 450 |F POINT (x+a,y+b)< > 1 THEN GOIO 540 460 LEI pt-SGN (RND-.5). LET a-a PELE] b-b’ ot LEI d-a LET a=b: LET b=<d: IFPOINT (x+a,y+b)< 7) THEN GOTO oC | |

“ABO LET aa: LET b=b: Goro.

0 Ct 540 IF IN 57342=190 THEN eT

_ i-0: LEI j=1: LET umoved= 7 _and similarly add:

- LE] umoved=1

One-Player Option

This simplified flowchart shows the program structure with only one player. Each process is repeated in the full two-player game

LIZ DIXON

THE HOME COMPUTER ADVANCED COURSE 633

SANOP NIAIY

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634 THE

Rn I

the joystick. We can summarise the actions of the joystick’s switches on the contents of the joystick memory location as s follows:

You may also have noticed that moving the joystick diagonally can cause two switches to close simultaneously. Although we do not require detection of diagonal movement to control, our vehicle, the results of such movements on the joystick memory location are as follows:

The following program uses a joystick to control the movements of the twin motor vehicle. The vehicle should be connected to the output box in the same way as on page 586 and the joystick should be plugged into games port 2, located on the right-hand side of the Commodore 64.

1@ REM CBM 64 JOYSTICK 20 DOR=56579:DATREG=56577

3@ POKEDOR,255:REM ALL GUTPUT

4@ JOY=PEEK(56320):REM JOY PORT 2 5@ GOSUBI@@@:REM TEST JOYSTICK

60 POKEDATREG,@:GOTO4a@

98 :

100@ REM TEST JOYSTICK S/R

19095 IFJOY=127THEN POKEDATREG ,@ 191@ IFJOY=126THEN POKEDATREG,5S 1920 IFJOY*#125THEN POKEDATREG, 10 1030 IFJOY=123THEN POKEDATREG,6 1049 IFJOY=119STHEN POKEDATREG,92 105@ IFJOY=111THEN POKEDATREG,@:END 1960 RETURN

BBC MICRO

The BBC joystick i is an a device relying on n 3

two potentiometers to provide information about the up/down and left/right movements made. The essential difference between a digital joystick and the analogue type used by the BBC Micro is

that the latter gives information about ‘position’ _

within given limits, whereas the former gives information about direction of travel only. The joystick potentiometers work in the following manner.

A potentiometer iS essentially a resistance : across which a voltage is applied. A third — connector to the potentiometer can move along - the resistance taking a fraction of the supply — voltage. This fraction depends on the position of the moving connection. On a linear type of — potentiometer, if the moving connection were

positioned halfway across the resistance, then the

voltages tapped off would be half the supply

voltage. Thus, by moving the central connection,

any voltage between zero and the supply voltage

Sid AO sey the ch OO

cH2 ov GROUND CHa ov

~ -FB2

can be obtained. Turning the volume control on a radio or record player is essentially moving the middle connection across the resistance of the volume control potentiometer. In an analogue joystick the movement of the middle connection is made by moving the joystick handle.

BBC Micro joysticks are generally supplied in.

pairs. The BBC Micro’s analogue port

connections for joysticks 1 and 2 are shown in the

above diagram. A reference voltage is provided by the micro

“across each potentiometer and the middle

connection’s tap-off voltage is input through two channel inputs. Channel 1 is used for the input from the left/right potentiometer : and channel 2 is

‘used to accept input from the up/down

potentiometer. The fire button i is a simple make or

: break switch.

Once the polediomene sits: have ‘bec

accepted they must be converted from analogue to

| 5 digital form by an_ ‘internal converter. This ~ conversion is done by comparing the input voltage with the reference voltage and conversion time is

around 10 milliseconds for each channel read.

Once the joystick input is in digital form we can.

use the values to control our vehicle. Input to the analogue port can be read from

BASIC using the BBC Basic command ADVAL. The

value returned by ADVAL is in the: range 0 to 65520,

the upper limit corresponding to an input equal to , the reference voltage. Reduced input voltages will

determined by the number in a bracket after the keyword. Thus ADVAL (1) will read channel 1 and return a value in the range 0 to 65520.

THE HOME COMPUTER ADVANCED COURSE 635

Analogue Reflections

BBC Micro joysticks are usually - supplied in pairs, going into a single connector. The analogue . port pin-outs as seen from outside the machine show that joystick 2 is connected in an — exactly similar way to joystick 1 Single joysticks may be connected in the place of either

of zero returned by, ADVAL. For our simple = application we are only really interested in the two. limiting values. The channel read by ADVAL is

rans

oe,

- ADVAL(0) performs two different functions. The = 2 = . | eas | least significant two bits correspond to the fire, | ites - buttons on joystick 1 and joystick 2. X=ADVAL i R | ahs | AND 3 will return a value of one if joystick 1’s fire ‘2: ©. ..-- A eT ee alee

BEF FPROCtest Jorstick

channe |= Lf gah

| | button is pressed. X=ADVAL(0) DIV 256 will give the 148 UNTIL channel <>@:REM KAIT FOR CONVERT 1S8 TF channel= HEM PROCtett right

: | number of the channel that last completed an A- ise 1F channei=2 LEN PRUCWO dow ho. oS

? yn 7@ ENDPROC 3 aes |

a i to-D conversion. a | | . 7 _ As conversion of each analogue input channel

me, eR takes about 10 milliseconds, then to processeach 277 TSS og, 7 | | |

ne | : | , of the four channels will take 40 milliseconds. In 228 iF joywai<iaa THEN ?DATREG=7 7 ) icati we ) 238° IF joyval?440@8 THEN ?DATREG=4

ome _ our application we use channels 1 and 2 only. 225 ¢;,.2=anuaice> AnD 3

| a _We can cut down on wasted conversion time by 258 FRINT?DATREG, channel, joyval

“ “fs : ° 248 UNTIL¢ jorva | OND joyval 64408) ORF fire= o , _ specifying the channels that require conversion. 37, er oe ies ae | This can be done by using *FX16,2, which enables 280 ENDPROC ie | , age channels 1 and 2 but disables channels 3 and 4. a6 ber pRocup down | ee Bip

? ) | The following program combines all this 21% FEFEAT | | - aks | information to control a twin-motor Lego car.

328 jorval=ADVAL( 2) a a1<¢18@@ THEN FDATREG=14 £4688 THEN PDATREG=3 ey

a ae ; i@ REM BEC JOYSTICK CONTROL ADVAL(a? AND = a S 2 ODR=&PES?T: DATREG=4-FE4a | - 348 PRINT ?POATREG channel, als <h 34 ?DDR=255:REM ALL OUTPUT S76 UNTIL¢ govval 188 AND J > OR fire= | 7. % 46 REM EMGGL SiGe : 328 PDATREG=6 | Sa #Fx¥14,2— 358 EMDPROC |

: : , 4a REPEAT fa P

636 THE HOME COMPUTER ADVANCED COURSE | |

ee eee ee

——————————aaa——aoooaeEeEeEeEeEer

JUMP LEADS

“indexed addressing on the 6809 processor, we now consider how indirect addressing is used, and illustrate this by describing routines to write characters to a screen display. a

Following our examination of

| First of all it should be stated that ia anect iaddaccite

is not a separate addressing mode in its own right but

is an additional feature that may be used in

combination with most other modes; it is really a further stage in calculating the effective address (the address from which the data is actually to be fetched). The effective address is calculated in any of the ways we have described (by direct access, by indexed addressing, or by effective address calculation), but if indirection is specified then the contents of the address so calculated and the next consecutive memory location are treated as an address. It is this address that becomes the final effective address, from which data is loaded. For example, if the following values are stored:

Address Contents — 3000 40 3001 0A 400A F2

then the instruction LDA $3000 will load the value $40 into accumulator A, the effective address being $3000. Indirection is always specified by placing square brackets around the operand, so LDA [$3000] will load the value $F2 into A, the effective address being the value stored in the address that is in turn stored in $3000 and $3001 — in this case, $400A. The contents of $3000 and $3001 form a pointer or vector to the effective address, $400A. Notice the 6809 convention that addresses are stored with hi-byte before lo-byte: thus $40 is stored in $3000 and SOA is stored in $3001. This is called the hi-lo convention. The Zilog Z80 and MOS Tech 6502 processors use the opposite convention — they would require SOA

(the lo-byte of the address) to be stored in $3000 and

$40 (the hi-byte) to be stored in $3001.

_Indirection can often be most effectively used in combination with indexed addressing. The instruction LDA [A,X], which is in indirect indexed

es addressing mode, will calculate an address by first

adding the contents of A and X and then using the 16- bit value that is stored at this and the next location as the effective address whose contents will be loaded into A.

The 6809 has, if fact, me use for indirect addressing than most processors (both 6502 and Z80 programs use it frequently) because of its wealth of indexed addressing modes. There are, however, situations in which indirection can be very useful —

one of these, which we will deal with at greater length. in a future instalment, is the use of peripheral

interface devices. Motorola processors, unlike Intel’s 8080 and 8086 families, have memory-mapped I/O (Input/Output). The communications registers in the interface devices appear in the system’s main memory map, and values can be stored to or loaded from them as if they were any other memory

locations instead of being, effectively, a channel to | ~ the interface device. A routine to controlone ofthese _ devices — for example, a print routine — needs the ©

address of the device’s interface register. If the device is relocated in the memory map, or if there is more than one device of that type, then itis much simpler to deal with this by changing one memory location that contains the address of the device communication register (a pointer to the device) rather than having to find and change every occurrence of the device address. The routine refers to the device indirectly, using the pointer.

This example illustrates the general usage of indirect addressing — when addresses that a program refers to may be changed, it is more convenient to use

fixed-address pointers to refer to these locations. In -

this way, changes in the actual locations require only changes in the pointer contents: the program refers to

the addresses indirectly.

Indirect Indexed Addressing

The argument [A,X] of the LDA. instruction is in brackets, meaning that the contents of X (SC019 here) are to be added to the contents of A (S23), givinga — 16-bit address (SC03C). This byte and the next (SCO3D) are to be treated as apointertothe — effective load address (SF84B) whose contents (S7E) are finally - loaded into A. Because X is added to A before the indirect access, this is known as pre-indexed indirection; the alternative, post- indexing, requires that the indirect address be calculated before the indexing takes place —

The most common use of this technique is in a

structure known as a jump table, which is simply a table of pointers. Any operating system contains a large number of useful routines that carry out the elemental functions of the machine — for example, reading a character from the keyboard or displaying a

‘THE HOME COMPUTER ADVANCED COURSE 637

COURTESY OF AIR CALL

Vital though indirect addressing is to computer operations, it is difficult to find examples of the technique in real life. A reasonable analogy, however, is a radio paging service. When

- someone wants to talk to a

subscriber, he doesn't call them directly, because they could be anywhere. Instead, he calls the paging service, who then page the subscriber. This is a simple, flexible system in which the paging service provides indirect accessing (or addressing) of its subscribers

character on the screen. Most machine code programs will need to use these routines at some time. In most cases, these routines will be accessed by using a jump table, which means that routines pointed to by the jump table vectors may be changed, or relocated in memory, without changes being needed in the programs that use them. In other words, such routines are always accessed indirectly,

through the appropriate pointers in the jump table.

When a new version of an operating system is designed, or an updated ROM produced, itis rare for

these primitive OS routines to remain in their original

positions; but if the jump table remains in position, with its pointers altered so that they reflect the new OS routine addresses, any software written for the old operating system that uses the jump table will run unchanged on the new system.

A common technique used in many operating systems is to have one entry point and to make all subroutine calls to this one address. One of the CPU registers is used in addition to pass a function code that is used to determine which subroutine will be called. This function code is used as an index or offset into the appropriate vector of the jump table, and control is transferred through this pointer to the desired routine.

_As an example, let us suppose that we have four Kbytes of ROM, located at SF000, the first 256 bytes of which ($F000 to SFOFF) contain a table of up to 128 addresses of subroutines stored in the ROM. The entry routine (the address by which all the OS

routines are addressed) is located at $F100, and this

expects a function code in the range 0 to 127 to be

638 THE HOME COMPUTER ADVANCED COURSE

Indirect Access |

stored in accumulator B; this code is used by the entry routine to pass control to the appropriate subroutines

and thence back to the calling program once

execution is complete. The calling routine for function number 1 is:

put the function code in B call the entry subroutine

_ The entry routine itself is:

Start address of the jump table

shift B one place to the left (equivalent to multiplying the contents of B by two) since each entry in the table is two bytes long. Thus the pointer appropriate to function code 1 is stored at SFO02 and SF003, while the pointer for code 2 is at SF004 and SF005, and so on

transfer control to the address found at the Bth position in the table

Note that the transfer to the routine is handled by a BRA (or JMP) rather than a BSR (or JSR); this is so that the RTS at the end of the OS routine will return control directly to the calling program instead of back to this entry routine.

Our next example shows a further possible use of indirect addressing in dealing with a memory mapped display screen; on many micros the screen memory occupies.a block of the main memory and

IAN McKINNELL

For simplicity, let us assume that the screen occupies a block of memory from SE000 to SE3FF, representing 16 lines of 64 characters. The position of the cursor is a 16-bit value in this range, and is located at $E400. The first subroutine clears the screen by writing a space character (ASCII code 32) at each character position. The second subroutine will write the

eS aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaamaaaaaaaacamcascacmacmascacmmmcccmccccccccssslll

EE Eee : - gc ’

character passed in A to the screen at the current cursor position, unless that character is a carriage return (ASCII 13), in which case it will clear the rest of the line and position the cursor at the beginning of the next line. The cursor is represented by the underline character (‘“__’) in this example. :

CURSOR

CURCHR

~ EQU 32 ASCII code for space EQU 13 ASCII code for carriage return EQU $E000 Start of screen memory EQU 1024 Size of.screen memory (16 lines x 64

characters = 1024)

EQU $E400 SE400 and SE401 together point to

the current address of the cursor in screen memory area

ORG $1000

FCB 95 Underline character (ASCII 95)

-s “Subroutine to clear screen***

LOOP

LDA #SPACE Space character in A

LOX #HOME Point cursor to beginning of screen

STX CURSOR Store current cursor position at CURSOR (i.e. SE400, SE401)

LOB #LENGTH = Size of screeninB

STA (CURSOR) Store a space in current cursor

3 position INC CURSOR Increment cursor position DECB Decrement amount of screen

memory remaining between cursor position and end of screen memory BGT LOOP1 Next space, until no more screen -* memory remains - STX CURSOR Cursor back to home position LDA CURSOR ASCII code of cursor character in A STA [CURSOR] Store cursor character in current Cursor position RIS =.

***Subroutine to display character in A, if displayable*** ,

CHKEOS

~ ***§Subroutine to scroll screen***

SCROLL

LOOP2

NOTP

CMPA SPACE Space is the first printable character —

in ASCII BLT NOTP lf accumulator contains ASCII value | less than 32, this is non- printable, SO GOTONOTP ~ | STA [CURSOR] Store in current cursor position INC CURSOR Increment cursor position

LOX #HOME Check for end of screen

LEAY #LENGTH,X End of screen in Y

CMPY CURSOR If cursor position exceeds end of ~ §creen then...

BGT FINISH we have reached the end of the

screen, SO GOTO FINISH

LEAY64,X Y is one line length from X (end of screen memory) LOB #LENGTH Calculate amount to scroll

SUBB #64 Subtract 64 from length LDA ,Y+ Move characters back one line (notice gh auto-increment — see page 618) STA ,X+ DECB BGT LOOP2 Loop until scrolling complete LOD CURSOR Cursor to start of last line SUBD #64 : STD CURSOR BRA FINISH **“Subroutine to check for carriage return ** . COMPA #CR Is this non- cen character a carriage return? _ BNE FINISH Ignore if not

_ FINISH

LDD CURSOR ‘You can work out how this gives the -

ANDB #%11100000 start of the next line Ueiee binary AND-mask)

ADDD #64

STD CURSOR

BRA CHKEOS — Check if end of screen

LDA CURSOR Cursor character in A

STA[CURSOR] = Store in current cursor position

_ THE HOME COMPUTER ADVANCED COURSE 639

ompute its copyright jea ous! y and these | rave all & o differed, at least cosmetically, from the original. Atarisolt has now agen ‘official’

pot—tl — ter of tl exide of ts At: | ND whereas the rahe are twice as mi as

ie alae

the original arcade maze game, and set ie as that on nthe Moai version :

| e formula that has since been follow cf by home an gives fuller instructions, 4 computer s« mes like Atic Atac and Jet Set Willy. In uch ga entral character must travel through } + a network 0 spain or rooms, all the while picking ersions. There is little. ‘. One | rie * -_ up treasure and avoiding the various hazards along ers could have done about the way.

nder certain conditions, it is epost for the playe atta — in Sab orchids me the jungl games, 1 similar ite 7

In Pacman, the centre of a maze, é

: e |Or iginal, the ering be playing t e game Bae like im. The V version’ S apres |

Kes the pier invisible to the anes in is idea was borrowed from adventure

| ich the posse IC SV

e player an advantage.

tle character is ee at the

en Pacman first made an appearance in 1980, quickly became a aso but today a

al software ———— pe games, Lees the

ination to trap the Pacman in the various ae ne | company has ¢ STiClal Versio machines S

_ scoring extra points, at e Te es and add to ae, score if Sick: : parison different versions of the same

doce oon of the ms original.

2 Este the most of that micro’s aeits __ particularly true for games designed s Atari machines, yaa re an enviable re

eatures ¢ arge maze with excellent sound i graphics smooth sprite movement anc various skill levels ¢ for the fact that the game is not played a faithful

% wah

rene

LIZ DIXON

640 THE HOME COMPUTER ADVANCED COURSE ee a3 € c

-— SACHA CalOuD

Symbolic

DESTINATION

bee 8

SOURCE

HL points to source

DE points to destination

BC is byte counter

DATABASE

Here, courtesy of Zilog Inc., we produce another part of the Z80 programmers’ reference card.

Block Search Group

LOCATION HL points to location in mamory

to be compared with accumulator contents BC is byte counter

| Ipod eVouele(-¥ 1s) (ole a Mb det stss(-1g9 onelelrel-lotyel ol Give) hy ors

(@lerorele) =

No. of

No.of M No.of 7

Comments

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