ZX 81 - BASIC Programming

Chapter 2.2 - Telling the computer what to do

5. Now - and you must always remember this - press NEWLINE, the key . This means 'message complete', or 'all right, computer, lets see some action'. The computer will now read the message, work out what has to be done, and do it. In this case, the screen will change to

4 is the answer - but of course you do not need to buy a computer to work that out. 0/0 (Note how zero is written with a slash to distinguish it from capital O. This is fairly common in computing circles.) is the report in which the computer tells you how it got on. The first 0 means 'OK, no problems'. (In appendix B there is a list of the other report codes that can arise, for instance if something goes wrong.) The second 0 means 'the last thing I did was line 0'. You will see later - when you come to write programs - that a statement can be given a number & stored away for execution later: it is then a program line. Commands do not actually have numbers, but for the sake of reports the computer pretends that they are line 0. You should imagine a report as hiding a cursor - if you press P for PRINT now, the report will disappear and the screen will change to

The cursor can also be used for correcting mistakes: type ++2, to get PRINT ++2 on the bottom line, Pretty incomprehensible stuff, and when you press NEWLINE you get

The is the syntax error marker (the syntax is the grammar of the message, saying which are allowed and which are not), and shows that the computer got as far as 'PRINT', but after that decided that it was not a proper message. What you want to do of course is rub out the first +, and replace it by - let us say - 3. First you have to move the cursor so that it is just to the right of the first +; there are two keys, and (shifted 5 and shifted 8), that move the cursor left and right. Holding SHIFT down, press the key twice. This moves the cursor left two places to give you PRINT ++2 Now press the RUBOUT key (shifted 0), and you will get PRINT +2 RUBOUT rubs out the character (or keyword) immediately to the left of the cursor. If you now press 3 this will insert a '3', again immediately to the left of the cursor, giving PRINT 3+2 and pressing NEWLINE gives the answer (5). The key (shifted 8) works just like the key, except that it moves the cursor right instead of left. Summary This chapter has covered how to type messages in for the ZX81, explaining the single keystroke system for words, the & cursors, reports, the syntax error marker, & how to correct mistakes using , and RUBOUT. The keyboard Here is a picture of the keyboard.

Remember that to use SHIFT, you have to hold it down at the same time as you press another key. Do not confuse digit 0 with letter O.