80-Bus News


November–December 1982 · Volume 1 · Issue 4

Page 17 of 51

WHAT IS CP/M? An independent look.

D. R. Hunt

This question is often asked when the purchase of new computer systems is considered. What advantages has CP/M got to offer, is it very complicated, even what does CP/M stand for ? In many ways all these questions may be answered glibly by saying that these things are not seen by the user, and therefore the user need not concern himself with them. However, this answer is not entirely true, as the choice of computer operating system is of great importance when it comes to system support and choice of available software.

CP/M stands for Control Program for Microcomputers, and the original version was designed in the late 1970’s by Digital Research Inc., to allow a standard form of interchange between one computer system and another. CP/M as such has been a story of growing success, and has been progressively improved with time. What it is, is a standard control program which takes over the computer disk system and the way in which the computer talks to the outside world, through the keyboard, display monitor, printer, etc. This offers one great advantage, in that a program written obeying the CP/M rules on one type of machine will be entirely compatible when run on another type of machine. This of course opens up the availabilty of software to a far wider market leading to greater choice and lower prices.

In use CP/M is a fairly simple concept, and only has five straight forward commands. These are at the system level and are rarely used by computer operators concerned with business systems, who are primarily concerned with the operation of a program, stock control, financial, etc. The program is working under CP/M, where CP/M is the controlling executive handling the transfer of data and programs to and from the disks, and handling the communication of the program through the keyboard, printer and display monitor. In this respect, CP/M is transparent to the user, and it is the program in use which appears to be in control. At the system development level, where the user is concerned with the design and development of programs, CP/M offers a number of useful utilities giving access to the computer at the machine program level. Most of the CP/M utilities are useful to the machine programmer, except the context editor utility (ED.COM) which is awkward and inconvenient to use. This utility is easily replaced with others of considerably greater usefulness.

CP/M has had much adverse comment unfairly levelled at it. Unfair because the criticism has been concerned with the CP/M input and output facilities, which are not the concern of CP/M, but of the manufacturer of the machine fitted with CP/M. This has come about because, whilst CP/M itself is standardized, the machines to which it is fitted vary widely. To allow CP/M to be adapted to any machine, the machine manufacturer has to write an interface program between CP/M and the machine, this is know as the Basic Input/​Output System, or BIOS. It is in this area where criticism is called for, many machine BIOSes are simplistic indeed, and lead to what can only be described as unfriendly operation, giving only simple error messages and the minimum of help. It is rare to find a CP/M machine with useful and friendly performance through the BIOS, although such machines exist, the Gemini range being a shining example.

So what has CP/M got to offer, this may be summed up in one word, compatibility! As mentioned above a program written using the CP/M rules can be run on almost any CP/M based machine. The word ‘almost’ was used advisedly as the Tandy range of computers have adapted CP/M for their own purposes and in the process made their implementation of CP/M incompatible with all other CP/M machines, a self defeating sales gimmick to ensure that software for their machines can only be purchased through themselves, thereby limiting choice and competition. Having said that CP/M programs are compatible, which is true, there are of course some incompatiblities, usually mechanical. The most obvious is the

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