First Steps

by Mark Hutchinson


Issue 19

Jan/Feb 86

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As I mentioned in the last column, this one has to be prepared early so while all of you are busy working on your suggestions for the game outlined in issue 18 I will take this opportunity to go back over a few items I have touched on before.


Back in issue 11 I discussed Boolean algebra and I have been asked if this can be utilised in any other subroutines, so what follows will be additional to issue 11 which you may need to peruse again.

Probably one of the most powerful, and possibly one of the least used, commands is the ON .. GOTO/GOSUB statement. If you have never used this before then I shall explain. Instead of using several lines such as

10 IF X=1 THEN GOTO 100

20 IF X=2 THEN GOTO 200

we can use a single line of ON/GOTO. We must get a value for the variable X, for example by using INPUT X, and then use this in the ON/GOTO statement. In our example X must equal (say) 1 to 4 or else be re-input. When we have a value we can use it to choose a subroutine as follows

10 ? "PRINT CHOICE (1-4)"
20 INPUT X : IF X<0 OR X>4 THEN 10

30 ON X GOTO 100, 200, 300, 400
40 REM - Subroutines in lines 100,200,300,400

If the limitations did not exist in line 20, then if X fell outside the range 1- 4 it would be ignored and the program would continue to line 40. X must be in the range of 1 to (number of choices) for this to work. The subroutine is chosen from the value of X so that if X equals 10 then the tenth routine will be chosen. You must be sure to have enough subroutines to cover the maximum value of X.

What would happen if you used X=PEEK(764) when looking at a keyboard entry? As this means you could have several lines such as

10 IF X=250 THEN GOTO 100

you would then need 250 choices for the ON/GOTO routine to work. Instead we can set the limitation by Boolean algebra. For example, if we wanted X to equal 10, 20, 30,40 or 50 only we would have to use five single lines of IF/THEN statements. A Boolean equivalent would be as follows

10 Y=(X=10)+2*(X=20)+3*(X=30)+4*(X=40) +5*(X=50)
20 ON Y GOTO 100, 200, 300, 400, 500

To use the subroutine on line 200, X would need to equal 20. If it did then this is the only true statement in line 10. So (X=20) would be the only true statement and the line would evaluate as Y=(0)+2*(1)+3*(0)+4*(0)+5*(0), or 2. So on Y=2 the computer will choose the second subroutine at line 200. The values of the subroutines need not be in order, nor need the values of X, just as long as they correspond numerically to the subroutines.


I was asked by Stephen Plunkett to explain about the LOCATE statement. This is an excellent way of testing a location on the screen to see what is stored at location X,Y. It can be used in the same way as the PMG collision registers.

The statement takes the form LOCATE X, Y, Z. You just position the cursor (even if switched off) at position X,Y and the data under the cursor is stored in Z. So if the target is in COLOR 1, you know you have hit it when Z=1. Z will equal 0-3 in four colour modes, 0 or 1 in two colour mode - and 0-255 in the text modes.

Now for the bad news. A PRINT or PUT directly after the LOCATE will move the cursor one point to the right and may modify the character under the cursor. Here is the solution to this little problem. Locations 85 and 86 (cursor horizontal position) are updated by the LOCATE statement by adding 1 to the number stored here, thus repositioning the cursor. You can store the data from 85 and 86 first, use LOCATE, and then restore the data. For example

P1=PEEK(85): P2=PEEK(86): LOCATE X,Y,Z: POKE 85,P1: POKE 86,P2

Location 93 holds the data found under the cursor, so you would do exactly the same here to stop corruption of the data i.e. PEEK 93 first, use LOCATE and then POKE 93. Another handy set of locations is 94, 95 which holds the current position of the cursor. By the way, location 84 holds the vertical position of the cursor and you may need to use this after rollover when the cursor moves down a line.

The LOCATE statement is just like using


I have included a simple little program to illustrate LOCATE in action. It should be well enough documented not to need an explanation.

AtariLister - requires Java


I was interested by the letters in issue 12. John Dimmer (a regular to PAGE 6) is quite right about paying someone to push in a few chips. If your machine is outside the warranty you could try this yourself but be careful to take anti-static precautions before touching chips, and if the IC legs are gold-plated do not rub it off. Reader B. Sutcliffe must have had a heart attack when he watched the service engineer open the cartridge with a screwdriver. If you try this then use something soft or plastic. Never, never use an eraser to clean anything - they contain emery or pumice dust and this is lethal. Use a proprietary cleaner/degreaser and cotton wool. Make sure that cotton buds are not glued on as the cleaner will dissolve the glue and leave you with a sorry mess. As a matter of fact, you should clean your cartridge edge connectors regularly before use, but never with anything abrasive.

I would like to wish all readers a very Happy Christmas and New Year and I am looking forward to a full post bag prior to my next column!