First Steps

Mark Hutchinson


Issue 11

Sep/Oct 84

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Have you ever written a program in Graphics 1 with a text window and wished that you could have the space of Graphics 0 but still use the window? Well, you can. Normally changing the screen means altering the display list but as the window itself is in Graphics 0 I would consider this an impossibility. You may be able to use the vertical blank interrupt but you would need to know machine language. The answer is really very simple. When you call a Graphics mode the machine must store either 24 lines of Graphics 0, 4 lines for the window of another mode or 0 lines for a Graphics mode + 16 (i.e. no text window). Location 703 is used for this purpose. All you need to do is call GR.0 and POKE 703,4. When the display handle reads 703 it is forced to open device #6 as with any other mode with a window. This means you PRINT#6; to the top 20 lines and PRINT to the bottom 4 lines. Didn't I say it was simple?

If you have little memory to spare and want to try out Player Missile Graphics in single-line resolution do you realise that you can only use five players (four plus the combined missiles) maximum which means 5 x 256 bytes? As you need to start on a 2K boundary, you may think that there are 3 x 256 bytes wasted. Not at all. You can still store other bits of data here if you wish but, be warned, BASIC A+ uses this technique so be careful if you use this language.

Have you ever wished to add a utility to your program but cannot as both the utility and the program use the page 6 storage area? The answer is simple. Set one or both routines up as a string. For example


20 X=USR(1536)

uses page 6. Try the following


20 X=USR(ADR(A$))

Although you have used extra memory through the DIM statement the routine itself has now become relocatable, i.e. the computer protects the string itself and will move it around in memory to do so. Alternatively you could use


The FOR/NEXT- READ statements will take up a lot of space, so I suggest that once you have the string defined, delete the above lines and use the following after having RUN line 10 only.

PRINT A$ (in direct mode)

Use the editing features to make 19 spaces before the printed string and add 

10 DIM A$(31): A$"

After the printed A$ add - ":X=USR(ADR(A$))

You can now delete all the DATA and save yourself some memory.

By taking this one stage further you could use X=USR(ADR('" assembly codes "")) and save eight bytes by not using a string. By coincidence you can also save eight bytes each time you use POKE instead of SETCOLOR and, finally, using cursor control characters takes one byte each instead of the massive 15 for a POSITION statement.

I am now finding this column hard to compile. Not because I have nothing to write about but mainly because I am limited to one page and the articles I would like to include would take more room. What I need now is readers suggestions, either to me or the Editor, for page size articles. With your help I can explore the areas YOU wish to read about.

You will be reading this in September and, hopefully, I should be in sunny Florida in October. If you want any questions answered directly please write as soon as possible, otherwise you may have to wait many weeks for a reply.

Finally, I hope that by the time you read this I will have my last(?) FIRST STEPS TUTORIAL tape out. I hope that they have been of some benefit and, although they were designed for the beginner, I hope that the tapes included something for the more advanced.

Write to Mark at BAUG SOFTWARE, P.O. BOX 10, BELFAST, BT10 0DB