This article was scanned from DAInamics issue 4, April 1981. The color/colour pictures were of too bad quality and replaced by various sources.
EARLY personal computers, say around 1976, were very limited in their graphics capabilities. A video screen capable of displaying 16 lines of 24 characters was considered normal. There was, therefore, little possibility of using a video screen to present other than basic alphanumeric information.
The introduction of memory-mapping. techniques, where the individual character blocks on the screen corresponded to single memory locations, allowed more flexibility and machines like the Pet introduced reasonably-sophisticated graphics at a low price.
Since those days, two developments have occurred which could revolutionise the approach to the presentation of information on a screen, especially in the educational field. The first development was high-resolution graphics. Instead of the more usual 25 x 40 picture element (pixel) resolution, high-resolution graphics have up to 256 x 392 pixels. That allows much higher definition of diagrams on the video screen. For educational use, the advantages of high-resolution graphics are obvious.
The second development is inexpen- sive colour graphics. That development is so new that there are still many arguments about the uses and advantages of colour in graphics.
Most graphics systems work in similar
Inexpensive aids to
ways. Whatever form of output is used,
nearly every video display employs
similar methods of generating the
characters. A screen which can display
25 lines of 40 characters requires 1,000
memory locations to store the
information. In the so-called memory-
mapped systems, each character position
on the screen corresponds to a particular
memory location in the video memory.
All the computer does is to transfer the contents of the memory on to the
by Robin Bradbeer
screen. As each memory location can store one byte of eight bits, that means that 2’ or 256 characters are available.
The most common method of encoding characters is the ASCII system. As it has only seven bits of information, only 128 characters are available. Most personal computer systems add another 128 to make up the
number and they do so by creating
There is no standard for them so that Pet graphic symbols, for example, have different codes from Sorceror graphics. The codes are interpreted into letters and graphics symbols by a ROM called a character generator. It is clear that all this memory detracts from the amount of memory available for use in the system by the user.
Another method of generating video information is to split the screen into a series of discrete points, say 312 x 210. That requires a good deal more memory. Each point on the screen then corresponds to one bit in the memory, hence eight points require one byte of memory; 312 x 210 points therefore need 8K bytes.
The high-density graphics capability is available on a few computers, like the Apple II. It is a useful facility for educational users but unless complicated graphical analyses are needed, it is not ideal for business use.
To use graphics to the optimum effect involves adapting to the machine language of the computer. It is possible to do so from Basic with systems which have a character generator ROM but that limits the user to the characters available on the computer.
On the Pet, for example, it limits a user to 128 characters within the 25 x 40 format. Other systems have similar restraints.
Those with high-resolution graphics,
however, like the Apple II or 380-Z,
offer certain facilities which allow users
not only to create their own characters
but also to vary the number of pixels
available for use. The more memory
needed for graphics, the less is required
for program memory. Consequently
high-resolution graphics requires a
system with a good deal of RAM.
graphics are available on a number of
computer systems. The favourite
method is to have a board which plugs in
to the main computer system. That is
how Research Machines, Acorn and
most bus-structured systems work.
picture borrowed from www.backntime.net
19 - EDUCATIONAL COMPUTING March, 1981
(continued from previous page)
Other systems have a built-in high-resolution capability. They include the Texas Instruments TI 99/4, DAI, PC1, Atari 400 and 800, Apple II and ITT 2020. Coincidentally, they are also machines which use colour.
Historically, colour. graphics was available only as an expensive extra to mainframe or minicomputers. They needed high-quality colour monitors to give the 'definition required. The latest colour computers provide the video output as a normal TV signal and use normal colour television sets. Although this is cheaper, it restricts the resolution available.
With colour graphics, in its simplest form, at least two bytes are needed to store the information in each pixel. The first byte will define the character, the second the colour. In practice that is achieved by using less memory, with four bits - a nibble - being used to store the colour information. So most systems have the capacity to work with 16 colours.
Of the six systems generally available on the U.K. market, three have been . around for some time. The Apple II - and its later ITT equivalent - was the first low-cost colour system. The version available in the U.K. has the ability to split the screen into graphics or text or to partition the screen so that the lower four lines are text and the upper 21 are graphics.
Its maximum resolution is 280 x 192 points. Each point is addressable and the graphics generation requires each point to be programmed, although some
commands from Basic permit that to be achieved easily. Lines can be drawn and individual dots specified with single commands.
The Apple has many programs available and is a favourite with many people. It is beginnirig to show its age, however. and is now more expensive than some of its newer competitors offering better graphics facilities.
The 380-Z has a plug-in graphics board which gives a resolution of up to 319 x 191 points. with a choice of four grey levels or colours for each point.
Sixteen grey levels or colours are possible with a resolution of 159 x 95 points, It is possible to mix graphics and text and the plug-in board requires 16K bytes of RAM to function properly.
In the 319 x 191 mode, the user can program four options per point. as there are two bits per point. Normally one would use black and three grey levels or background colour and three colours. Thus it is possible to plot three graphs of different shades or different colours.
The relationship between the choice programmed at a point - 0, 1, 2 or 3 -
and the output is defined by a high-speed RAM which acts as a look-up table, producing eight outputs. In black-and- white systems those outputs drive an eight-bit digital-to-analogue converter. In colour systems the eight hits drive separate two-bit D-A channels for the different primary colours.
The contents of the look-up table RAM are under software control. so the user can program a particular set of grey scales or colours. and can change that set in a short time between successive television scans.
The board can be programmed to provide a 159(X) x 95(Y) array of pixels. In that mode the area of the screen covered by graphics is identical to that in the 319 x 191 mode. so that four lines of scrolling text can also be obtained if the graphics and 380-Z VDU boards are coupled.
In the 159 x 95 mode, four bits are used to define each point, resulting in 16 choices of output, usually grey levels or colours. In addition, the graphics board stores two separate 159 x 95 mode pictures, so that one can be displayed while another is being generated.
The use of a look-up table, as well as allowing the user to select four or 16 grey scales or colours from a very wide range of choices, also results in features which may at times be useful. For example, in the 319 x 191 mode one can arrange for the two hits per pixel to correspond to two separate pictures. Either of the two pictures can be displayed, or both superimposed, or one masked by the other or one masked by the inverse of the other.
In the 159 x 95 mode, each of the two stored pictures can be arranged to be one picture with four bits per pixel two pictures with two bits per pixel. or four pictures with one bit per pixel, with
(continued on next page)
picture borrowed from www.zoo.co.uk
20 - EDUCATIONAL COMPUTING March, 1981
(continued from previous page)
superimpositions or maskings as described for the 319 x 191 mode. The look-up table features may be of use to scientists and mathematicians in allowing them two separate super- imposable 319 x 191 pictures, or to producers of computer-aided learning material who want selective masking, or changing foregrounds with constant backgrounds.
The Texas Instruments TI 99/4 was conceived originally as a home computer but with its integral sound generator and speech synthesiser, Americans are using it in schools extensively. Its colour graphics capability is impressive - up to 192 x 256 points in 16 colours. You can also program your own characters or graphics shape called sprites, and even give them priorities so that they know which sprite is on "top" when two overlap.
Unfortunately, it does not work on the European colour standard and requires an expensive American-standard TV set TI will launch a PAL version summer but is keeping a low profile at the moment.
The Acorn Atom has reached the market only recently and its colour graphics plug-in module has not really been used widely enough to generate much comment, It has a resolution of 256 x 192 in black and white and generates three-colour graphics up to 128 x 192 pixels.
The Atari 800 has been on the market in the States for about a year and has been the subject of very favourable comment. At the moment it suffers from the same problem as the TI 99/4 but PAL versions are beginning to make their way to Europe and should be available generally shortly.
The Atari 800 and its cheaper brother, the 400, typify the way that colour graphics is progressing. Many American
picture borrowed from Apple //e manual
and Japanese companies are now supplying colour as standard and at a reasonable price.
The Atari 800 costs around £800 but sells for $800 in the States It should be available here for around £500 if enough are sold. The Atari 800 has a resolution of 320 x l 92 pixels in its highest graphics mode, with up to four colours available.
Like the Atari, the DAI and DC1 are two of the latest machines on the U.K. market, so it is worth looking at them in greater detail. Like the: TI 99/4 and Exidy Sorcerer. the Atari 800 has a plug-in ROM-pac which allows the user to configure the machine easily. An 8IL Microsoft Basic pack is standard, although other language options are becoming available.
RAM is in 16K modules which plug into slots in the body of the machine. The operating system of the computer is also in a plug-in ROM, a 10K module; The potential for future upgrade therefore is clear. The video output plugs into a normal colour, TV and it has a socket for a video recorder. Other peripherals
available are disc drives. printers. an RS 32C interface. games paddles and a light pen.
There is an on-screen editing facility and cursor control is possible directly from the keyboard; clear screen and tab controls are also provided. All keys repeat if held down for more than one second.
The screen format can be varied from 12 x 20 to 320 x 192. Table one indicates the options available. In normal text mode, 24 lines of 38 characters is the 'dafault' situation. It is also possible to have line deletion and insertion. as well as the more usual character deletion and insertion.
The cassette recorder uses two channels for recording One is digital. the other audio. Consequently a special recorder is required. Digital data transfer is at 600 baud The audio. channel allows speech to be interlinked with data. the sound emitting from the TV loudspeaker. The potential for educational use of the facility is enormous.
The Atari 800 has four integral sound generators for musical tones or game sounds They cover four octaves and have variable volume and tone. There is a small internal loudspeaker.
The software available within the Basic for graphics programming is fairly limited. Although it is possible to select from 128 colour/luminance combina- tions, only five colours can be displayed at one time. The more successful graphics commands include DRAWTO. which draws a line from the previously-plotted location to the
(continued on page 23)
Table 2 -- Mode definition table, DAI PC1
21 - EDUCATIONAL COMPUTING March, 1981
(continued from page 21)
defined co-ordinates and SETCOLOR. which is used to define one of the five colours available.
Although the DAI personal computer is only now beginning to appear on the market it has a. long and interesting history. Developed originally as a contender in the TI personal computer stakes, only to be jettisoned in favour of the 99/4. the DAI machine has a real multi-national flavour - it was designed by two British graduates and built in Belgium.
Data Applications is an industrial control company with many' years experience of designing and building systems and that is evident in the personal computer. It has obviously been designed by engineers definite application as a home computer.
Easiest to program
The basic computer has a 24K ROM containing the Basic interpreter, machine language utility and general housekeeping routines. and 48K of RAM. There is an optional 9511 math chip which provides hard-wired arithmetical operatiori.
The DAI personal computer obvi- ously has been designed to provide not only the best colour graphics capability for less than £l.000 but also the easiest programmability of those functions.
Output ports at the rear consist of two analogue paddle inputs: RS232C 25-pin connector; two cassette interfaces; a 34-pin DCE-BUS, an industrial control bus; and. internally, a 50-pin 8080 bus which bears a resemblance to the S-100 bus and a UHF video connector.
The Basic resides in 24k ROM and is one of the most powerful available at this level. With the utility program and colour control options the 24K ROM is well-used. The system is supplied with two manuals and connecting leads for video and tape recorder. The first manual is a 72-page introductory text aimed at the complete novice.
Some could complain that the manual is somewhat trite -. it starts by telling
you how to plug-in and switch-on - but it is better to under-estimate the potential users' knowledge than vice-versa.
From that simple beginning the user is introduced to the major attribute of the DAI, the colour graphics which are ridiculously easy to operate. There are 16 colours which. with the colour control on a TV turned down, are a perfect grey scale between black and white. The colours are labelled 0 to 15.
The screen can be defined in 13 ways - from 24 x 60 characters, through a mixture of graphics and text to 336 x 236 dots. Table two shows the configurations available. In each case the background and foreground can be defined precisely using the COLORT and COLORCG commands. Each configuration needs different RAM requirements.
A detailed look at the graphics generator is useful. although most other machines use a similar system. At the 'start of each line on the TV screen, two bytes are taken from memory which define the mode for that line and may update the colour RAM by two bytes. They are the Control and Colour bytes. The rest of the line is colour or character information and the number of bytes used for it is a characteristic of the particular mode.
The screen can operate at a number of different definitions horizontally - e.g., dots scan. In the highest-definition graphics mode there are 352 visible dots across the screen. The two lower definitions have respectively one-half and one-quarter of that numher. There are about 520 scans visible on 625 -line television set and the screen hardware can draw only. at minimum. two scans per line, due to the inter-lacing. That gives a maximum definition of 260 by 352, which is close to the 3:4 ratio of the screen sides. Thus circles are round.
Characters are fitted on to the grid by using eight columns of dots per character, the dot positions being defined for each character by a ROM. That allows 44 characters per line maximum - 22 or 11 in lower definition
modes. A fourth horizontal definition provides for a high-density character mode with 66 characters per line.
Sixteen colours. including white and black, can be displayed by the system. Whenever a four-bit code is used to describe a colour, it selects from that range of possibilities. In some modes - characters and or four-colour graphics - a set of four of the colours. not necessarily distinct, is loaded into a set of colour registers.
Any two-bit code describing a colour selects an entry from the registers. Vertical definition is set by a four-bit field in the control byte. In graphics mode that permits repetition of the information to fill any even number at scans from 2 to 32.
In character mode it defines the number of scans occupied by each line of characters: thus the vertical spacing on the screen can be changed to allow anything between an 8 x 7 - the sensible minimum - and 8 x 16 character matrix, giving between 35 and 15 lines of characters on the screen.
Background! or letter colour may be changed from the Basic. When operating from within Basic, three drawing facilities can be used. The screen is defined in X and Y co-ordinates, XMAX and YMAX being defined by the mode chosen. A dot may be placed at co-ordinates X, Y by programming DOT X, Y, C, where C is the defined colour of the dot.
A line is drawn by defining the beginning and end co-ordinates, e.g.. DRAW X1, Y1, X2, Y2 C will draw a line of colour C between Xl, Y1 and X2, Y2. FILL Xl, Y1, X2, Y2 draws a rectangle with opposite corners X1, Y1, X2, Y2.
Eight other colours are available as well as the 16 mentioned. The others act on the machine code directly and allow the graphics to he changed very simply under program control. The user effectively is implementing complicated machine code instructions from simple Basic statements.
Allied to the graphics capability are the three programmable sound synth- esisers, They allow the volume and frequency of each sound channel to be defined from Basic and eien provicle the option of tremelo and glissando. Volume is also set from the Basic commands. The SOUND command specifics a channel to which it applies, an envelope to be used, the required volume end frequency. A simple sound command would be:
SOUND 0 1 15 0 FREQ ( 1000)
(continued on next page)
23 - EDUCATIONAL COMPUTING March, 1981
(continued from previous page)
That would set channel 0, using envelope number 1, at a volume of 15 and frequency 1,000Hz. The ENVELOPE statement allows the volume of a note to be changed rapidly, in the same way as a musical instrument. Thus the rise and fall in volume for a note can be specified. The command specifies a set of pairs of volume and time. The volume constants are in the range 0 to 15 and the time is in units of 3.2 milliseconds.
For example, the command ENVELOPE 0 10, 2; 15, 2; 14, 4; 12, 5; 8. 10; 0 sets a volume envelope as shown in the diagram. A white noise generator is also provided.
Within Basic is an interesting edit facility. called EDIT; a very powerful feature it lists the program for editing. Screen editing, using cursor control, is possible only in EDIT mode. Individual lines or 4-line blocks can be edited. All control characters. e.g., CR, are shown.
Lines are presented in page mode. the width being determined by the length of the line. A window of 24 lines by 60
columns is presented, the windows being moved by cursor control keys. The EDIT mode can be used to merge Basic language programs or routines as well.
The system also has a comprehensive Error message capability with clear error types being displayed in plain English - there is no Error No. N problem.
The second instruction manual is probably one of the most comprehensive available but could be better laid-out and in a more logical order. It contains all the information needed - it is somewhat frustrating flipping back- wards and forwards all the time. Numerous program examples are given and the demonstration cassette supplied whets the appetite; it is a pity some loading errors were encountered.
It is also possible to plug-in a 9511
maths chip which speeds the execution of trigonometric functions by around a factor of 10.
Think of future
Future products provided by the Americans and Japanese promise to make up our minds for us. Tandy recently has introduced a colour system selling for less than $400. Commodore has a Pet-compatible system on the stocks for even less. Sharp and other Japanese companies are close behind.
With those systems selling in the U.K. for less than £200, colour will become the usual method for displaying information. That should be taken into account when thinking about future purchases of computer equipment.