M/C SCREEN$?

Back in June Peter Jones (Hi Peter, hope you’re still reading?) asked after a machine-code version of the BASIC SCREEN$ command, to help him convert a BASIC program.  At the time I briefly replied that it wasn’t the best way to do things in m/c, and now I’ll explain why, and what a better alternative is.

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When graphics (visually) collide!

We now have the knowledge to draw sprites anywhere we want on the screen, and could try and draw as many of them as we like, but there’s a problem…

If we try to draw two of our ‘smiley faces’ next to each other, then one will overwrite the other, as below:

sprite-overlaid Continue reading

Smoothly Horizontal – Part 3 – Shifting as we go

So pre-shifted graphics are great to use in certain circumstances – objects which are constantly moving horizontally whilst animating, for example (think Manic Miner) – and are fast,  but they also have their drawbacks – not the best use of memory for one.  We could fix the memory issue by not having all those individual graphics, and just shift one copy into the required position when we need to. Continue reading

Smoothly Horizontal – Part 1

Drawing graphics at horizontal pixel, rather than character, positions isn’t quite as straightforward as doing it vertically.  Vertically, as each pixel row has it’s own address on the screen, we just had to find the correct address to start from, and work down from there.  Horizontally however, groups of 8 pixels are held within a single byte, using a single bit each, which makes things more complicated to know which bits to set. Continue reading

Smoothly vertical

Now that we’ve done drawing graphics at character positions, ala BASIC, lets start drawing them at pixel positions – starting with placing them vertically at any pixel row.  This is very similar to the character position code, with a couple of changes…

Firstly we need to make a pixel row address look-up table, which is simply a version of the character table, but for all 192 rows: Continue reading