1981 - the Sinclair ZX81

Sinclair ZX81 1K computer - 1981
Photo from Wikipedia
You are looking at the first popular and affordable home computer in the UK. In fact, this machine represents a milestone in early computer history. The ZX81 was a development of the ZX80, which was released a year earlier, but was much more basic even than the ZX81 and which had a tendency to overheat. As was standard practice in those days, the ZX81 would be connected to a television.
It’s perhaps true to say that the ZX81 is somewhere along the transition from pocket calculator to a modern home computer – it is certainly more computer than calculator, and it certainly wouldn’t fit in your pocket, but the graphics were very basic and the basic model came with just 1K of RAM!
A ZX81 cost £70 when it was released in 1981 – or, for £50 you could buy a ‘kit’ where the computer was supplied as bits – keyboard membrane, circuit board, numerous resistors and capacitors etc. – which required you to use a soldering iron in order to put it together. Imagine trying to solder together your own Windows 7 PC nowadays...
When you switched on a ZX81, you would get a screen as below:
Sinclair ZX81 startup screen

Not exactly as friendly as Windows, is it? But at least the computer is ready for you straight away without having to wait a minute or two for it to go through the process of initialising itself.

The ‘K’ in the bottom left is the cursor, and the computer is expecting you to type in a command or start writing a program. To do this, you select keywords (the ‘K’ is short for ‘keyword’) from the keyboard. For example, if you want to print the words “PLEASE PUT ME IN THE 21ST CENTURY” on the screen, you would do the following:

• Firstly press P on the keyboard which brings up the word PRINT followed by a space. Looking closely at the photograph of the keyboard, you may wonder how you can be sure that you won’t get TAB, P or ". The answer is: you just won’t.
• The cursor changes to an inverse ‘L’. press SHIFT + P and you get the opening quotation marks (") that you require.
• Type in PLEASE PUT ME ... Not too fast, though, or the computer won’t keep up with your typing speed.
• Press SHIFT + P again for the closing quotation mark (").
• Press the NEWLINE key.
The result is as below.

Example text display on Sinclair ZX81

The “0/0” in the bottom left means that the command was successful and there are no errors to report. So in order to use the computer effectively, you would have to be familiar with all the keywords as printed on the keyboard. You could run games or other software with a minimum of knowledge, but everything was very basic.

The keyboard has much fewer keys than a modern equivalent. Instead of the ‘delete’ key there is RUBOUT (SHIFT + 0). Even the cursor or arrow keys can only be accessed through SHIFT 3 to 6. And the punctuation marks are in different places – so a colon (:) for example is SHIFT + Z.

The entire character set for the Sinclair ZX81 is shown on the next page. The 64 symbols represent EVERYTHING that you could type – so no exclamation marks or apostrophes for example. Nor is there any lower-case alphabet! Note the block characters on the top two rows. These formed the basis for any graphics output from the computer – pictures would be made up of little black blobs.

Any software was loaded from cassette tape. This was the standard recording medium in the early 1980s for both computer software and for recording songs off the radio. There is more detail about computer software on cassette tape in the Acorn Electron section. An example game, a version of Pacman, is shown below.

There was also a simple printer available for the Sinclair ZX81. To look at, it was not too dissimilar to a roll of paper coming out of a financial calculator.

Incredibly, this computer only had 1KB of RAM! This meant that even storing the simple text display of 32 x 24 characters was a drain on memory space, and the ZX81 would often resort to only a partial display – rather like looking at a sheet of paper in pencil with the bottom half rubbed out. Any computer program that you might write would also have to be quite short. Not surprisingly, many users took advantage of some of the memory expansion units on offer. The most common of these was the 16K unit, which plugged into the back of the main machine; however, the connections were poor and users would often use Blu-Tack or similar to hold it in place! Nor was it cheap – £50 was quite dear in 1981.

Sinclair ZX81 character set
Sinclair ZX81 specification
PacMan game on Sinclair ZX81
Pac-Man, © 1982 Artic Computing Ltd

The ZX81 looks very basic of course nowadays, but was a classic computer of its time. Although it was released only year after the ZX80 came out in 1980, it was a significant development. The ZX80 could only deal with whole numbers for example – arithmetic such as 3.9 x 2.9 was beyond it!

My "ZX81 VDU" font

Here you can download for free a TrueType font that I have created which replicates the letters, numbers etc. that were displayed on the screen of the ZX81. To download click here

Other ZX81 fonts exist on the Internet, but in my version I have stuck to just those characters that existed on the original computer. So no ampserand (&), no 'at' sign (@) and not even an exclamation mark (!). Also, I have tried to ensure that the inverse symbols (white text on a black background) are true to the original. The spacing should also reflect the original appearance on the screen. I feel pleased with the result, but perhaps you should be the real judge!

I have also tried to make some of the inverse symbols and graphics characters accessible using the keys available on the normal modern computer keyboard. So some of the key mappings are as below. On my keyboard, the result is that all of the inverse symbols 1 to 8 are all in the same area. It is difficult to map the ZX81's original character set to the modern computer keyboard, as the two are very different, but hopefully the end result will work well.

Acorn Electron Amstrad CPC 6128 Main 1980s computing page

© Matthew Eagles 2008. Last updated 27th July 2010.