80-Bus News

So the ports have been set up. The U command in NAS-SYS, or the LST vector in RP/M should be set to point to the output routine, and study of the manuals should reveal what is required to be done. Like the serial routine the first thing that happens is that the character to be printed is PUSHed on the stack whilst the BUSY line is examined until it is free. The character is then POPped back and sent to the data port. Bit one is then toggled to cause a strobe pulse thus latching the data into the printer.

And that is all there is to it. Of course, there will be problems with both the serial and the parallel routines, but ‘thinking down’ to the bit level where it is all happening will help. I can offer little practical advise at this stage. Playing around with software is fun, and if you don’t have a glimmer of insight after all I’ve written, I’m afraid you will be forever doomed to high level languages and their restrictions. Playing at the very heart of the machine is my idea of fun. But of course it requires a thorough working knowledge of both the hardware and the affects of the software.

In drawing this series to an end, I like to think I’ve helped someone along the way. I know what I have written is incomplete and inaccurate in places but that is probably not so important as the overview of systems I have presented. To encapsulate all that should be written on the subject would require several very fat books, and falls into the province of the professional writer. Unfortunately the most prolific writers do not necessarily have a deep understanding of what it is they are trying to convey. They are quite adapt at disguising this fact, usually with jargon, and the indadequacies of this sort of book is often overlooked. There is nothing to beat hands on experience.