A Tale Of Two Banners: ZX

For the 40th Anniversary of the ZX Spectrum's announcement on April 23rd, 1982, I decided to write a little banner program.

Here's an example of what it can do:

ZX Spectrum BASIC is blessed with a large number of useful commands to access graphics, so the program was quite easy to write and relatively short:

As you can see, there's very little to this program. How does it work?

The main part of the program, obviously is the mechanism for enlarging the characters. This is surprisingly easy. The ZX Spectrum has 16 graphics characters arranged as Battenberg (as in the cake) graphics which occupy character codes 128 to 143.

The ZX Spectrum also has a POINT(x,y) function, which returns the pixel value at coordinates (x,y). By POINT ing at (x,y), (x+1,y), (x, y+1), (x+1, y+1) we can simply multiply each point by the appropriate Battenberg pixel value and add 128 to generate the correct graphics character. This is why you can see a tiny version of the '!' character in the bottom right-hand corner: the standard character is displayed and then it is scanned and the appropriate Battenberg characters are generated.

The program allows for the banner to be edited by simply supporting a carriage return feature and by wrapping the bottom line back to the top line.

The harder part is to generate the stripy coloured lines at the bottom right-hand corner. The difficulty with the ZX Spectrum is that it's not possible for more than 2 different colours (an INK colour and a PAPER colour) to occupy the same character square. So, to make the stripes we have to carefully make sure that each stripe is exactly 1 character wide and map the colours so that the stripe never clashes.

To do this we define a graphic character, UDG "a", whose address in RAM is returned by USR "a" as a diagonal triangle filling the bottom half of the character. This gives us the diagonal stripes by using the previous ink colour as the next paper colour on the next stripe - a classic ZX Spectrum colour trick! (You can see how it works from the image below where alternate diagonal rows are brighter ).

To make the program a little bit more fun, each character is assigned a random colour and the cursor flashes.  Because the ZX Spectrum came out in a 16kB form, and one of my friends had one at the time, I thought it would be considerate to make sure the program would run on either model (it's so short, I could hardly fail 😉 !).

You can type in the program by going to the ZX Spectrum Javascript website! Here's the keyboard to help you! https://jsspeccy.zxdemo.org .

Following this I thought I'd do the same, for the Commodore VIC-20. It turns out that, despite arguably superior graphics capabilities (well, it can do 2 bits per pixel graphics!), it's far harder (see Part 2).