CP/M operating system

Helping orphans the way you would do it

CP/M (Control Program/Monitor or -/Microcomputer) was an operating system for Intel 8080/85 and Zilog Z80 based microcomputers. It was created by Digital Research, Inc founded by Gary Kildall. The combination of CP/M and S-100 bus computers patterned on the MITS Altair was the first "industry standard", and was widely used through the late 1970s and into the mid-80s.

Table of contents

1 Description
2 History

2.1 The beginning and CP/M's heyday
2.2 The 16-bit world
2.3 MS-DOS takes over

3 See also
4 External links

Description

CP/M's command line interface, implemented in the CCP command control processor, was patterned after the operating systems from Digital Equipment, such as RSTS/E for the PDP-11. Commands generally took the form of a keyword followed by a list of parameters separated by spaces or special characters. The commands themselves would sometimes be somewhat obscure; for instance, the command to duplicate files was named PIP (Peripheral-Interchange-Program).

One key innovation in CP/M was the use of an abstraction layer that separated the operating system into two main parts. The CCP translated user commands into a series of high-level instructions. These instructions were then fed into the BDOS (Basic Disk Operating System), which provided functionality like "open file". The BDOS then translated these commands into a new series of lower-level instructions. These were then fed into the BIOS (Basic I/O System), which contained the hardware-specific code that carried out the instructions from BDOS.

To illustrate the flow of commands, consider the PIP command mentioned earlier. When a PIP command was entered into the CCP it was broken down into a series of instructions for the BDOS, which would be similar to "locate the file named foo.txt, open it, create a new file named bar.txt...". The BDOS commands in turn were sent to the BIOS as a string of even simpler instructions, like "move the disk head to this sector, read raw data from sector..." etc.

The vast majority of the complexity in CP/M was isolated in the BDOS, and to a lesser extent, the CCP. This meant that by porting the limited number of simple commands in the BIOS to a particular hardware platform, the entire OS would work. This significantly reduced the development time needed to support new machines, and was one of the main reasons for CP/M's widespread use. Today this sort of abstraction is common to most OSs, but at the time of CP/M's birth, OSs were typically intended to run on only one machine platform, and multilayer designs were considered unnecessary.

History

The beginning and CP/M's heyday

CP/M was originally distributed on 8 inch floppy disks, and ran on the Intel 8080 CPU (as well as the compatible and very popular Zilog Z80). Eventually, the industry moved to the 5¼ inch disk format, and CP/M followed. Programs written for CP/M were typically completely portable amongst different machines with the same CPU; this made it popular, and much more software was written for CP/M than for operating systems that only ran on one brand of hardware.

Hundreds of different brands of machines ran CP/M, some notable examples being the above-mentioned Altair, the IMSAI 8080, the Osborne 1 and Kaypro portables, and even the Apple II when an extra Z80-card was installed. WordStar, one of the first widely used word processors, and dBASE, the first widely-popular database program for small computers, were originally written for CP/M.

The 16-bit world

Versions of CP/M were later completed for some 16-bit CPUs as well, although they required the programs to be re-compiled for the new CPUs. One of the first was CP/M-86 for the Intel 8086, which was soon followed by CP/M-68k for the Motorola 68000. At this point the original 8-bit CP/M became known as CP/M-80 to avoid confusion.

CP/M-68k was widely used only in one application, it formed the basis of the Atari ST computer. CP/M-86 had the potential of becoming the standard operating system of the new IBM PCs, but minor legal issues made IBM turn to Microsoft instead. They purchased a CP/M clone known as QDOS, and used it to create PC-DOS/MS-DOS which went on to become the "official" PC operating system. CP/M-86 never became popular.

MS-DOS takes over

Many of the basic concepts and internal mechanisms of early versions of MS-DOS were copied from those of CP/M. Internals like file-handling data structures were identical, and both referred to disk drives with a letter (A:, B:, etc.). This was done on purpose, to make it easy to port popular CP/M software like WordStar and dBase.

The user interface of MS-DOS, however, was a bit more friendly. Compare for example CP/M's copy command, PIP:

PIP <destination filename>=<source filename>

to the more intuitive syntax of DOS's COPY (roughly patterned after that of UNIX's cp):

COPY <source filename> <destination filename>

CP/M would slowly lose market share as the microcomputing world moved to the PC platform, and it never regained the popularity it once had. Later versions of CP/M-86 made significant strides in terms of performance and usability however, and for some time in the 1980s was considered to be a better MS-DOS than MS-DOS. To reflect this compatibility the name was changed, and CP/M-86 became DR-DOS.