ZX Spectrum 16K

Sinclair User

Stephen Adams

TAKING THE LID OFF THE SPECTRUM Stephen Adams looks inside Sinclair's latest machine and find it has a lot on common with the ZX-81. The ZX Spectrum is very similar in shape and style to the ZX-81 and there are many similarities. Sinclair has kept the original keyboard to save space but has provided a rubber sheet with moulded keys on it which fits over the top. The sheet is suspended over the flat keys - which on the ZX Spectrum are bigger than the ZX-81 - so that when a key is pressed it bends to give some feel to the keyboard. That and the fact that the keys repeat if held down for longer than one second, even when using SHIFT, makes the keyboard much easier to use. The single keyword system has been retained and that saves memory, as all the Basic words can be stored as one byte. It also means that two SHIFT keys are required to reach all the functions; one is called CAPS SHIFT and the other SYMBOL SHIFT. They are at opposite ends of the keyboard and as they are often used one after another. It tends to slow the input speed as you are constantly swapping hands. For instance. RUBOUT and the cursor movements use CAPS SHIFT and +-* are SYMBOL SHIFT. It would have been a better idea to put both on the left-hand side, as they often need to be used together and could be pressed with one hand while the other searches for the appropriate key. The keyboard is an input-output mapped device, as on the ZX-81, and along with the the ZX printer, which is the same for the ZX-81 and the Spectrum network/R3232 interface, discs, loudspeaker, tape interface and border colours require only one address line to work. That means that you must make all of the lower five address lines a binary 1 to use your own devices. The input-output map access has been improved greatly, however, with the addition to the Basic commands of IN and OUT. They give an instruction IN A(c) or OUT A(c) where registers BC give an address from 0 to 65535. The memory-mapped addressing of the RAM/ROM occupies 0-16K and the RAM 16K-32K on the basic 16K model. There is provision for an extra 32K board to be plugged into IC sockets at the back of the printed circuit board. The 48K version will have the board fitted but to add it later it will cost £60, which I think is expensive. There would be no difficulty in adding extra ports to the memory map, as on the ZX-81, above 32K - on the basic version - but for two things. There is no RAM CS line, so that the extra RAM can be turned off if required on the edge connector and the edge connector address lines have been moved to the outer edges so that it is incompatible with the ZX-81. The Spectrum has a 28-way double-sided edge connector of the same style as the ZX-81, with the keyway on pin 5. That makes any input-output device compatible with the ZX-81 but any memory-mapped devices would have to be rearranged. The edge connector also has a number of new signals on it which are not explained in the manual, plus a video output and colour outputs for VDUs. All the voltages used on the Spectrum are also brought out, namely + 5V. -5V. + 12V and -12V. They are obtained from the same buzzing transformer as is used in the 16K RAM pack and most of that RAM pack seems to have been transplanted on to the Spectrum. The obvious additions to the circuitry are the PAL colour mixer under the metal can which contains the video modulator and the fact that two crystal-controlled oscillators are used, one for the ULA, which controls the screen among other things, and the other for the colour mixer. The 14mHz clock for the ULA is also used to drive the Z-80A microprocessor after it has been reduced to 3.5mHz. That is 0.25mHz faster than the ZX-81. The Z-80A has also been freed of the job of putting-out the screen - by the ULA - and so no longer requires the commands FAST or SLOW, as it works at top speed all the time except when BEEP or PAUSE is used. PAUSE and BEEP both cause the ZBOA to stop for a time determined by the programmer and so it will do nothing else while those commands are being done. BEEP commands should be kept short in a program for that reason; 0.01 seconds is a good speed to PRINT AT and BEEP at the same time. As for programming the Spectrum. It can be considered as an extension of the ZX-81 Basic. The PAPER, INK, BRIGHT and FLASH commands for each character square are stored in a memory map above the dots for each character. They are all stored in one byte per character and can be read by the Basic word ATTR and altered either by Basic commands or POKEs. The dot screen is a different matter, however, and cannot be altered so easily, as the dots are stored in peculiar order, so you have to use the graphics commands - which can define all the dots on the 22 line by 32 character screen available to the user - or the SCREEN command. There can be only two colours for each character square, one for the foreground (INK) and one for the background (PAPER), but they can be any one of eight colours. They can also be inverted at a rate of one per second continuously. square by square (FLASH) or have two intensities of colour (BRIGHT). The screen takes up 6,912 bytes of the 16K memory and the system variables take up another 738 bytes. The rest of the memory is not free for the user to use as 11 other areas float above location 23733 and can expand and contract as required by the Spectrum. The program and variables are sandwiched in the middle of those, so REM statements cannot be used for machine code. There is an area, however, which can be used for machine code programming above RAMTOP which is ignored by the Basic and its length can be defined by the user. The user-definable characters area is stored above that so they can be kept from program to program. There are many tape arrangements which can be made with the Spectrum. The program, strings or machine code can all be SAVEd, LOADed and VERIFYd separately. The variables and screen can also be stored on tape but cannot be VERIFYd. As each is SAVEd, a message to start the tape recorder will appear and wait for you to press a key. That is a very good example of the user-friendliness of the machine and most of the errors appear with similar messages. SAVEing or LOADing causes the border to flash red and green or red and blue, depending at which part of the tape you are looking. All of my tape programs LOADed correctly and I was surprised with the difference in speed between it and the ZX-81. I have been able to deal with only a few subjects. There is so much more to learn about the Spectrum from the manual that it would become a series if I did not stop now. The Spectrum is real value for money and easy to use. It has some peculiarities but they do not seem important when you look at what it can offer. With disc, networking facilities and RS232 interface it is a great improvement on the ZX-81 but it cannot replace it, as the price of £129.95 will still be a little daunting to those who want to try a computer for the first time.

'The Spectrum is real value for money and easy to use. It has some peculiarities but they do not seem important when you look at what it can offer'.

Sinclair User

Robin Bradbeer

EARLY PROMISE IS SHOWN BY COLOURFUL SPECTRUM Robin Bradbeer looks at Sinclair Research's new ZX Spectrum and finds it 'incredible' value at £125 The Spectrum is a colour and sound computer for the incredibly low price of £125. At that price it undercuts the BBC Microcomputer Model A, its direct competitor, by around £175. In designing the ZX-82 it is clear that the rejection of Sinclair's offer to build the BBC Micro was foremost in the company's mind. The specification is very similar and will certainly affect sales of the Acorn-based machine. It is as if Clive Sinclair has turned to the powers that be in the Government and BBC and said "I told you so". The Spectrum is a small computer, measuring 233 x 144 x 30mm, or slightly wider but not so deep as the ZX-81. The basic model has 16KB of RAM and 16KB of ROM. That compares to most other common computers for ROM but it is more RAM than most models in the less-than £300 price range. RAM indicates the amount of memory available to the user for working data and programs and ROM is an indication of the power of the computer to run that program and manipulate the data. Another 32KB of memory is available at around £50 and that plugs into some sockets already built into the main board. The 48KB RAM model, therefore, is potentially as powerful as the very common Apple II computer costing around three times the price. The main difference between the ZX-82 and the ZX-81 is immediately obvious. The keyboard has 40 keys, the same as the ZX-81. Some of the keys, notably the enter key, are slightly larger than the others. Whereas the ZX-81 had a touch-sensitive keyboard, the ZX-82 has a keyboard with keys which are about half the size of typewriter keys. They move downwards firmly and feel like the keys on desk-top calculators. The other noticeable feature is the number of functions on the keyboard. More than 250 functions are available and some of them require three shifting operations. The colour coding of the keys and their functions makes life very easy, however, and with the single-key access to all functions, which is similar to the ZX-81, you soon become familiar with the conventions and could easily prefer them to the more normal way of doing things. For example, to obtain the square brackets symbol involves going into extended mode, accessed by pressing the symbols shift and capitals shift keys at the same time. Then you have to press either shift and the Y key. Although that may seem rather long-winded it is still shorter than typing VERIFY, which is achieved in a similar way to the square brackets but is on the 'R' key as a single function. Colour coding is effective in this case and the designers are to be complimented on the ergonomic strength, as well as for the design of the whole package, which feels very comfortable to hold and looks very presentable on a desk. The Spectrum plugs into a normal UHF TV tuned to channel 36 and all characters are shown lower-case unless specified by using the capitals shift. There is a capitals lock, which is very useful. The Basic is based on that of the ZX-81 and some of the features lacking on the original model have been included in the latest one. The screen can display 24 lines of 32 characters, although the individual pixels which make up each character - 64 in all - can be accessed and changed at will. That means that 256 x 192 pixels is available for graphics. The screen format is very similar to the Commodore, Vic, with a border area and the active screen within the border. At switch-on, the system automatically enters a mode where border and screen area, or paper as it is called, are white and the letters, or ink colour, black. That overcomes the strange effect noticeable on some colour computers where the border area is different from the working area, which makes the screen look smaller than it is. The colours of the border, paper and ink can be changed easily with commands of the same name. Eight colours are available, although judicious use of the graphics characters available make intermediate colours, like orange, possible. It is also possible to have 21 user-defined graphics characters, which will allow Greek or other alphabets to be used. Unlike some computers built in the States, the Spectrum also has a £ sign on the keyboard; everything can be printed on the ZX printer, which can be used for the ZX-82 as well as the ZX-81, although the ZX-81 expansion memory pack cannot be used with the Spectrum. Many commonly-used routines in the graphics are available automatically. For example, a circle can be drawn with the 'circle' command by specifying the centre and diameter. Lines can be drawn with the 'draw' command by specifying start and end locations. The colour control commands are very simple to use. Brightness can be at two levels, and the character can be steady or flashing. The attributes of the character are stored as a single byte and can be accessed and changed from the Basic. In addition to the ink and paper commands, the Basic has brightness and flashing commands. Other useful graphics functions include an over command which allows characters to be super-imposed at any point. The six colour control commands can be used over the whole active screen area or locally within each individual 8 x 8 pixel group which makes up each character. Like the ZX-81 the plot command accesses one pixel at a time and the attributes of each block can be used to control the characteristics of that pixel. Colour control codes, which can be accessed directly from the keyboard, can be inserted into text or program listings and, when displayed, will over-ride the globally-set colours until another control code is encountered. All control commands can also be used within strings and it is entertaining to define a string which has different-coloured characters and background colours in it. A simple print command using that string causes it to be printed to screen just as stored. Another powerful use of colour in listing is that certain sections of the listing can be picked out in different colour̶both the colour of the character and its background. It is also possible to have flashing REM statements. Any colour used in the listing is not used when running the program. The eight colours are given numbers from 0 to 7 and they are shown clearly above the keyboard numbers. The order gives a graded grey scale on a monochrome TV display. They are black, blue, red, magenta, green, cyan, yellow and white. All eight colours may be on the screen at the same time, with some areas flashing, some steady, some normal brightness and some extra bright. Editing is the same as for the ZX-81 but the addition of auto-repeat on every key makes editing easy, especially when moving the cursor around a long line. Some additions to the Basic include the means to enter a binary number directly. That is the method of generating the user-defined characters, of which there can be 21 directly-attributable to some of the keys on the keyboard. The 8 x 8 matrix is made up by defining the character as a series of eight bytes, each byte being one line of the character. A 1 indicates a pixel and a 0 the absence of one. Other new functions include READ, DATA and RESTORE, something which was sorely missing on the ZX-81. FN and DEF FN are also there. One of the best new additions to the specification is the ability to type in lower-case. That certainly makes reading and writing programs easier, especially as the keywords are still capitals. So strings, variables and arrays can be specified in a way which is simpler to use. Unlike the ZX-81, the Spectrum uses true ASCII codes for its alphanumeric characters and control codes. That means that ZX-81 cassettes cannot be read into the Spectrum. Other than the absence of SLOW, FAST and SCROLL, however, the Basic is identical. There is no need for FAST or SLOW, as the memory-mapped screen overcomes the need for screen writing during interlacing, as on the ZX-81. The ZX-82 has the speed of the ZX-81 in fast mode with the screen characteristics of the ZX-81 in the slow mode. One of the problems with both the ZX-81 and its predecessor, the ZX-80, was the rather idiosyncratic working of the cassette storage. The Spectrum incorporates a new cassette interface which is incompatible with the ZX-81. A tone leader is recorded before the information to overcome the automatic record level fluctuations on some recorders. An electronic circuit, called a Schmitt trigger, is used to remove noise on playback. All saved information is started with a header as to its type, title, length and address information. A number of types of information can be stored on cassette - programs, blocks of memory and arrays. The MERGE function allows programs to be merged and the VERIFY function allows stored data to be checked before being erased from the computer memory, so programs and arrays may be merged from tape to combine them with the existing contents of memory; where two-line numbers or variable names coincide, the old one is erased. Programs can also be saved with a line number to allow execution to start anywhere in the program on successful loading. By storing the screen memory, it is possible to load a screen image without having to run any program required to generate that image. The ZX-82 has an expansion port similar to the ZX-81, with the addition of the colour video information. Thus a colour monitor could be attached to give a high-quality display. Full data, address and control buses for the Z-80 processor are available and the ZX printer can be plugged-in directly. The LPRINT, LLIST and COPY commands work with Spectrum Basic, with the additional bonus that any user-defined graphics will also be printed. It is also possible to run a number of other peripherals at which Sinclair has only hinted. There will be an RS232C interface, so that standard printers can be attached. There will also be a network with an interface which fits on the expansion port, as will the ZX-Microdrive, to be launched later this year. It is possible to access all I/O ports by using the IN and OUT commands in the Basic. The Spectrum has a very basic sound capability. The internal speaker emits a 'raspberry'-like sound, set normally at a frequency of middle C. The pitch and duration of the note can be defined in the Basic with the BEEP command. The centre frequency being middle C, any other note can be defined by the number of semitones above or below that frequency. It is also possible to have fractional intervals so that unusual scales can be generated. In summary, the ZX Spectrum is a very fine computer and at the price will give Commodore. Acorn/BBC and Atari a run for their money.

Crowds trying to see the ZX Spectrum at the Earls Court Computer Fair.

'The 48KB RAM is potentially as powerful as the Apple II costing around three times the price'.

'The ZX Spectrum is a very fine computer and will give Commodore, Acorn/BBC and Atari a run for their money'.