Micro­power

The SAVE command is used to transfer a program or data from the computers memory to disc. This command has to be told how much of the memory you want it to save, in 256 byte pages. The area saved starts at 0100H, and a typical example of the command is SAVE 55 SPLAT.COM which would copy the section of RAM from 0100H to 05FFH onto the disc and give it the file name SPLAT.COM in the directory.

The final built-in command is TYPE which types out a file. It will put it on the screen and if CP/M has been told that the printer is on, it will send it there as well.

So, how do we tell the system to send its output ta the printer? This is acomplished using one of the Control key commands mentioned earlier, Control P. There are several of these commands besides Control P. Control C will (usually) reboot the system, Control H works like the backspace key Control M is the same as the enter key and Control S stops and starts the console display so that you can read all of the directory, for example, when it would otherwise be scrolled off screen. All these commands are covered in the two introductory books that I suggest new CP/M users buy, in great detail. The books are “The CP/M Handbook” by Rodney Zaks and “Using CP/M” by Judi Fernandez and Ruth Ashley. Of the two, I prefer the first but this a matter of taste. If you like the kind of book where you are expected to fill in the answers to the questions as you go along, then the latter book is excellent too. What neither of them tells you, much to my disappointment, is how the machine code addict can make use of CP/M in his programs. The information is in the CP/M documentation. Sadly, it is terse in the extreme and does not make several important points at all clearly. In future articles I hope to give an insight into the ways in which the routines in CP/M can be used by your programs.

The description of CP/M that I have given so far does not make any mention of how to run a program. It could not be simpler, as all you do is type the name of the program and press the enter key. The system checks what you have typed and if it is not a built-in command, it looks on the disc for a file of that name with a .COM extension. Suppose you type Invaders and press enter; there is no built-in command with this name, so CP/M looks on the disc for a file called INVADERS.COM. If it finds it, it loads the file into memory at address 0100H onwards and jumps to the start of it. A program of the kind described is called a transient program because it is not in memory all the time. Several transient programs are supplied with CP/M when you buy it, all intended to make the system easier to use. The transient programs supplied with the Gemini implementation of CP/M that I use, and doubtless supplied with all the other implementations of the system are as follows: