A Brief History of CP/M

CP/M, originally standing for Control Program/Monitor and later Control Program for Microcomputers, is a mass-market operating system created in 1974 for Intel 8080/85-based microcomputers by Gary Kildall of Digital Research, Inc. CP/M is a disk operating system and its purpose is to organize files on a magnetic storage medium, and to load and run programs stored on a disk. Initially confined to single-tasking on 8-bit processors and no more than 64 kilobytes of memory, later versions of CP/M added multi-user variations and were migrated to 16-bit processors.

Gary Kildall originally developed CP/M as an operating system to run on an Intel Intellec-8 development system, equipped with a Shugart Associates 8-inch floppy-disk drive interfaced via a custom floppy-disk controller. It was written in Kildall's own PL/M (Programming Language for Microcomputers). Various aspects of CP/M were influenced by the TOPS-10 operating system of the DECsystem-10 mainframe computer, which Kildall had used as a development environment.

The CP/M name follows a prevailing naming scheme of the time, as in Kildall's PL/M language, and Prime Computer's PL/P (Programming Language for Prime), both suggesting IBM's PL/I; and IBM's CP/CMS operating system, which Kildall had used when working at the NPS.

CP/M supported a wide range of computers based on the 8080 and Z80 CPUs.  An early outside licensee of CP/M was Gnat Computers, an early microcomputer developer out of San Diego, California. In 1977, the company was granted the license to use CP/M 1.0 for any micro they desired for $90. Within the year, demand for CP/M was so high that Digital Research was able to increase the license to tens of thousands of dollars.

CP/M 2.0

Under Kildall's direction, the development of CP/M 2.0 was mostly carried out by John Pierce in 1978. Kathryn Strutynski, a friend of Kildall from Naval Postgraduate School (NPS), became the fourth employee of Digital Research Inc. in early 1979. She started by debugging CP/M 2.0, and later became influential as key developer for CP/M 2.2 and CP/M Plus. Other early developers of the CP/M base included Robert "Bob" Silberstein and David "Dave" K. Brown.

CP/M eventually became the de facto standard and the dominant operating system for microcomputers, in combination with the S-100 bus computers. This computer platform was widely used in business through the late 1970s and into the mid-1980s. 

The operating system was described as a "software bus", allowing multiple programs to interact with different hardware in a standardized way. Programs written for CP/M were typically portable among different machines, usually requiring only the specification of the escape sequences for control of the screen and printer. This portability made CP/M popular, and much more software was written for CP/M than for operating systems that ran on only one brand of hardware. 

One restriction on portability was that certain programs used the extended instruction set of the Z80 processor and would not operate on an 8080 or 8085 processor. Another was graphics routines, especially in games and graphics programs, which were generally machine-specific as they used direct hardware access for speed, bypassing the OS and BIOS (this was also a common problem in early DOS machines).

CP/M increased the market size for both hardware and software by greatly reducing the amount of programming required to port an application to a new manufacturer's computer. An important driver of software innovation was the advent of (comparatively) low-cost microcomputers running CP/M, as independent programmers and hackers bought them and shared their creations in user groups. 

MP/M

MP/M (Multi-Programming Monitor Control Program) is a multi-user version of the CP/M operating system, created by Digital Research developer Tom Rolander in 1979.It allowed multiple users to connect to a single computer, each using a separate terminal.

MP/M was a fairly advanced operating system for its era, at least on microcomputers. It included a priority-scheduled multitasking kernel (before such a name was used, the kernel was referred to as the nucleus) with memory protection, concurrent input/output (XIOS) and support for spooling and queueing. It also allowed for each user to run multiple programs, and switch between them.

The 8-bit system required a 8080 (or Z80) CPU and a minimum of 32 KB of RAM to run, but this left little memory for user applications. In order to support reasonable setups, MP/M allowed for memory to be switched in and out of the machine's "real memory" area. So for instance a program might be loaded into a "bank" of RAM that was not addressable by the CPU, and when it was time for the program to run that bank of RAM would be "switched" to appear in low memory (typically the lower 32 or 48 KB) and thus become visible to the OS. This technique, known as bank switching, was subsequently added to the single user version of CP/M with version 3.0.

MP/M II 2.0 added file sharing capabilities in 1981, MP/M II 2.1 came with extended file locking in January 1982.

CP/M 3

The last 8-bit version of CP/M was version 3, often called CP/M Plus, released in 1983. Its BDOS was designed by David K. Brown. It incorporated the bank switching memory management of MP/M in a single-user single-task operating system compatible with CP/M 2.2 applications. CP/M 3 could therefore use more than 64 KB of memory on an 8080 or Z80 processor. The system could be configured to support date stamping of files. The operating system distribution software also included a relocating assembler and linker.  CP/M 3 was available for the last generation of 8-bit computers, notably the Amstrad PCW, the Amstrad CPC, the ZX Spectrum +3, the Commodore 128, MSX machines and the Radio Shack TRS-80 Model 4.

16 Bit Versions

There were versions of CP/M for some 16-bit CPUs as well.

The first version in the 16-bit family was CP/M-86 for the Intel 8086 in November 1981.  Kathryn Strutynski was the project manager for the evolving CP/M-86 line of operating systems.  At this point, the original 8-bit CP/M became known by the retronym CP/M-80 to avoid confusion.

Soon following CP/M-86, another 16-bit version of CP/M was CP/M-68K for the Motorola 68000. The original version of CP/M-68K in 1982 was written in Pascal/MT+68k, but it was ported to C later on. CP/M-68K, already running on the Motorola EXORmacs systems, was initially to be used in the Atari ST computer, but Atari decided to go with a newer disk operating system called GEMDOS. CP/M-68K was also used on the SORD M68 and M68MX computers.

In 1982, there was also a port from CP/M-68K to the 16-bit Zilog Z8000 for the Olivetti M20, written in C, named CP/M-8000.

These 16-bit versions of CP/M required application programs to be re-compiled for the new CPUs. Some programs written in assembly language could be automatically translated for a new processor. One tool for this was Digital Research's XLT86, which translated .ASM source code for the Intel 8080 processor into .A86 source code for the Intel 8086. The translator would also optimize the output for code size and take care of calling conventions, so that CP/M-80 and MP/M-80 programs could be ported to the CP/M-86 and MP/M-86 platforms automatically. XLT86 itself was written in PL/I-80 and was available for CP/M-80 platforms as well as for VAX/VMS.

Later versions of CP/M-86 made significant strides in performance and usability and were made compatible with MS-DOS. To reflect this compatibility the name was changed, and CP/M-86 became DOS Plus, which in turn became DR-DOS.

The Fall of CP/M

In the early 1980s, Digital Research had sold more than 250,000 CP/M licenses; InfoWorld stated that the actual market was likely larger because of sublicenses. Many different companies produced CP/M-based computers for many different markets; the magazine stated that "CP/M is well on its way to establishing itself as the small-computer operating system". Even companies with proprietary operating systems, such as Heath/Zenith (HDOS), offered CP/M as an alternative for their 8080/Z80-based systems. Also, an estimated 2000 CP/M applications existed. Many expected that it would be the standard operating system for 16-bit computers. 

In 1980 IBM approached Digital Research, at Bill Gates' suggestion, to license a forthcoming version of CP/M for its new product, the IBM Personal Computer. Upon the failure to obtain a signed non-disclosure agreement, the talks failed, and IBM instead contracted with Microsoft to provide an operating system.

Many of the basic concepts and mechanisms of early versions of MS-DOS resemble those of CP/M. Internals like file-handling data structures are identical, and both refer to disk drives with a letter (A:, B:, etc.). MS-DOS's main innovation was its FAT file system. This similarity made it easier to port popular CP/M software like WordStar and dBase. However, CP/M's concept of separate user areas for files on the same disk was never ported to MS-DOS. Since MS-DOS has access to more memory (as few IBM PCs were sold with less than 64 KB of memory, while CP/M can run in 16 KB if necessary), more commands are built into the command-line shell, making MS-DOS somewhat faster and easier to use on floppy-based computers.

BYTE in 1982 described MS-DOS and CP/M as David and Goliath, the magazine stated that MS-DOS was "much more user-friendly, faster, with many more advantages, and fewer disadvantages". InfoWorld stated in 1984 that efforts to introduce CP/M to the home market had been largely unsuccessful and most CP/M software was too expensive for home users. In 1986 the magazine stated that Kaypro had stopped production of 8-bit CP/M-based models to concentrate on sales of MS-DOS compatible systems, long after most other vendors had ceased production of new equipment and software for CP/M.

CP/M rapidly lost market share as the microcomputing market moved to the IBM-compatible platform, and never regained its former popularity. Byte magazine, one of the leading industry magazines for microcomputers, essentially ceased covering CP/M products within a few years of the introduction of the IBM PC. For example, in 1983 there were still a few advertisements for S-100 boards and articles on CP/M software, but by 1987 these were no longer found in the magazine.

References:

This article uses material from the Wikipedia article CP/M which is released under the Creative Commons Attribution-Share-Alike License 4.0,

This article uses material from the Wikipedia article MP/M which is released under the Creative Commons Attribution-Share-Alike License 4.0,