specially CP / M-86) in the direction of MS-DOS / PC DOS. In fact, through this relationship, the genesis of some (only a few) routines at the source code level can be traced back to 1976, i.e. long before the introduction of the IBM PC, even if the development of DR-DOS as such did not begin until much later has been.
The development of MS-DOS / PC DOS-compatible operating systems began at Digital Research in 1983 with the development of the DOS emulator PCMODE, an additional module for their own operating system Concurrent CP / M-86 3.1 (BDOS- Kernel 3.1). The DR Concurrent CP / M-86 was a multitasking version of the CP / M-86. With the next version of the operating system in 1984, the emulator became an integral part of the system, which was accordingly renamed DR Concurrent DOS 3.2 (with BDOS kernel 3.2). The CDOS OEM adaptation for IBM PC compatible computers was expressed by the additional designation "PC" in the name of DR Concurrent PC DOS (alias CPCDOS). This Digital Research operating system could run CP / M-86 and MS-DOS / PC-DOS-1.1 applications at the same time. Despite the similarity of names, the system must be strictly distinguished from IBM PC DOS, which as an MS-DOS OEM version offered neither CP / M-86 compatibility nor multitasking. Later versions of CDOS also supported EEMS bank switching (CDOS 86 / XM, 1986) and used the 286 or 386 protected mode (CDOS 286, 1985, or CDOS 386, 1987) for memory management and multitasking. The DOS emulator was also increasingly developed and offered compatibility with DOS 2.x / 3.x / 5.x applications. Further developments of Concurrent DOS were FlexOS and Multiuser DOS (MDOS), from which Datapac System Manager and IMS REAL / 32 developed much later. CDOS, FlexOS and MDOS were mainly used in professional environments and for industrial control tasks because of their high reliability and were not widely used on normal desktop computers.
Digital Research made several attempts to gain a foothold in the desktop market with its own MS-DOS / PC-DOS-compatible operating systems.
The first of these attempts consisted in DOS Plus 1.2-2.1 (1986–1988), a single-user descendant of DR Concurrent PC DOS 4.1-5.0, which runs PC-DOS 2.11 applications in addition to CP / M-86 applications could and was available for a handful of OEM platforms. As an emulation based on a CP / M kernel, however, DOS compatibility was still limited. a. since many commands were still similar to CP / M and z. B. no DOS device drivers could be loaded.
Digital Research started the next and ultimately successful attempt in 1988 with the presentation of DR DOS 3.31 (with BDOS kernel 6.0) in a phase when the further development of MS-DOS / PC DOS at Microsoft and IBM seemed to be stagnating and urgently needed adjustments were being made the possibilities of newer machines were dragged there over the years. This new single-user operating system line was a spin-off from Concurrent DOS 6.0, in which the XIOS was replaced by an IBM PC-compatible DOS BIOS in order to achieve maximum compatibility with the boot phase of DOS and with DOS device drivers . A CP / M-like BDOS kernel was still working inside, but it was no longer compatible with CP / M system calls and was pretending to be IBM PC DOS 3.31. The system offered no multitasking, but almost 100% compatibility with DOS applications, which with the MS-DOS editions common up to now, overdue support for the management (at that time) of "large" hard disks (based on the Compaq MS-DOS 3.31 model ), as well as a number of enhancements compared to the competition MS-DOS, such as password-based access control for files and directories, floating drives, subdirectories that can be nested as deeply as required, a command line and input history function that is effective everywhere, meaningful error messages and extended help functions.
DR DOS was able to survive several versions (DR DOS 3.31, 3.32, 3.33, 3.34, 3.35 with BDOS 6.0, DR DOS 3.40, DR DOS 3.41 with BDOS 6.3, DR DOS 5.0 with BDOS 6.5, DR DOS 6.0 with BDOS 6.7) powerful alternative to MS-DOS and PC DOS claim. From 