80-Bus News

And what sort of instructions are there? Well, lots. For a start, there are two screens of 640 by 288 dots, and any dot can be individually set to any of the colours the board produces. So there is a routine to plot a point, given its coordinates; there’s another one to plot a point at a given x-y displacement from the current position. This latter routine has two forms, one for distances that can be expressed in a single byte, and one for longer distances. Then there’s the line drawing routines, also in three forms. There are routines to move to a given point, or by a given displacement. It is possible to read the colour of a pixel, again using absolute or relative coordinates. After all that, you could be forgiven for thinking I have finished, but not a bit of it! Rectangles can be filled, or even copied to another location. Part of one screen can even be copied onto the other screen.

And each of the operations described above is influenced by two system variables called WPROT and STYLE. WPROT is used to write protect one or more colour planes, while STYLE determines whether the new information replaces the old, or is combined with it in several other possible ways. For instance, the colour being printed can be exclusive OR’ed with the existing colour of a pixel. This sort of thing can be used to produce red/​green flashing text on a green/​red flashing background, if that is what turns you on. Text? But of course! The standard ASCII character set is catered for, and an 80 column display can be produced. It must be possible to amend CP/M to use this board as its display device, somewhat like an amazing IVC, but I may not even try. [Ed. – I know of someone who did this some time ago, and commented on its amazing slowness due to all the characters being held as dots, and the consequent amount of memory shifting that has to be done when the screen is scrolled. The AVC suffers similarly (but even worse I believe). He also commented that backspace was not supported, and so ‘cursor left, space, cursor left’ has to be issued by the CP/M BIOS. It’s an obvious case of horses for courses – text cards for text, and colour graphics cards for ....] You see, there is a marvellous moment when the ZX81 owner first notices that your computer has two screens. More to the point, really, is the fact that I have a lot of software that uses the old Nascom screen, or rather, its relocated equivalent. I don’t really think I want to edit these articles in colour, unless the magazine is going to change over to using a colour plotter instead of the daisy wheel, and start looking like OZ…

As part of the Taunton Computer Club’s latest theme program writing game, I also wrote a thing that draws a picture of a winter scene. In this program, I made use of the Pluto facility for defining new characters, of any kind and colour whatsoever; these are referred to as symbols. Once a symbol has been defined, it can be printed at whatever location is current, just by sending a single byte out through the data port. This function is impressively fast, as is the other very useful one I left out above. Referred to as “polyfill”, this fills complex shapes in with colour. I saw someone using their BBC computer the other day, and their polyfill routine (in BASIC, naturally) took ages, as well as not always getting into all the corners. Having this sort of facility readily available will make the rapid development of programs much more easy. I should be able to do a really nice space invaders in no time…

It is a tremendous shame that such a superb lump of hardware is being let down by the way IO run their business. They take a long time to send things out, which can be forgiven when a firm is building up its business, and may be running with less staff than it will have when things level off abit. But they could easily send out a card to say they have had your order. They don’t even bother to answer all the letters they are sent, and this is unforgivable, when they are being asked technical questions about the product they have sold you. The only way to get answers to this sort of question seems to be to ring them up. Their phone is more or less permanently engaged, of course, presumably by people who want to know why their letters are ignored! I wanted to write about the additional ROM, which contains even more amazing facilities like the marvellous “wallpaper” routine, but after three weeks I am still waiting. Still, it will be something to write about next time.