DOS Plus

DOS Plus
Company type	Operating system
Industry	Computer software
Founded	1985
Founder	Digital Research OEM Support Group, Newbury, UK
Products	DOS Plus operating system

DOS Plus (also referred to as DOS+) was the inaugural operating system created by Digital Research's OEM Support Group based in Newbury, UK, with its launch occurring in 1985. The initial DOS Plus 1.0 version merged CP/M-86 Plus with the PCMODE emulator derived from Concurrent PC DOS 4.11. While CP/M-86 Plus and Concurrent DOS 4.1 originated in the United States, Concurrent PC DOS 4.11 represented an internationalized and bug-corrected variant developed by Digital Research UK. Subsequent releases of DOS Plus 2.x were founded on Concurrent PC DOS 5.0. Functioning as a transitional product, DOS Plus bridged the gap between Concurrent CP/M-86 and DR DOS.

DOS Plus enabled execution of applications designed for both CP/M-86 and MS-DOS 2.11, maintaining compatibility with their respective floppy disk formats. It supported multitasking for up to four CP/M-86 programs simultaneously but restricted DOS applications to single-task foreground operation.

User interface

The user interface of DOS Plus resembles that of MS-DOS, providing a command-line environment managed by a command interpreter named COMMAND.COM (also known as DOSPLUS.COM). It accommodates an AUTOEXEC.BAT startup script but does not include support for a CONFIG.SYS file, except in some DOS Plus 2.1 editions where FIDDLOAD is implemented to enable loading of field-installable device drivers (FIDD). A distinctive element of the interface is a status bar positioned at the screen's bottom, which displays active processes, current keyboard layout, printer port assignments, status of lock keys, and the system time. When running DOS programs, this status line is concealed as these applications operate exclusively in the foreground.

Users can change keyboard layouts by pressing , , along with function keys –.

Commands

To facilitate multitasking, DOS Plus incorporates several additional commands:

ADDMEM: Provides allocation of extra memory for EXE format programs.
ALARM: Functions as a message-based alarm clock.
BACKG: Displays and terminates background processes.
COMSIZE: Configures memory allocation parameters for COM programs.
PRINT: Manages print spooling operations.
SLICE: Regulates CPU time distribution for the foreground application.
USER: Assigns user numbers for accessing CP/M media.

Moreover, it includes subsets of conventional DOS and CP/M commands such as COPY and PIP for file duplication tasks.

The CD command supports "floating drives," which map drives N:, O:, or P: to directories located on other drives—this enables older software that lacks subdirectory awareness to operate correctly within DOS Plus. For instance:

CD N:=C:\DATA\ACCOUNTS

This mechanism streamlines file organization and is also present in Concurrent DOS, Multiuser DOS, System Manager 7, REAL/32, and earlier versions of DR DOS (up to BDOS 6.7). These environments extend floating drive assignments across nearly all unused drive letters except L:. The BDOS kernel manages floating drives internally, allowing them to be accessed transparently by DOS applications through directory path specifications.

More advanced platforms like Concurrent DOS XM dynamically allocate floating drive L: to correspond with an application's load path, standardizing file references and simplifying software relocation without requiring path modifications. This contrasts with MS-DOS’s approach using environment block load paths, which demands explicit application support and consumes additional memory resources—whereas this floating drive method operates seamlessly with any program.

Additionally, these operating systems support virtually unlimited directory nesting depth due to BDOS handling directories as relative links internally—unlike MS-DOS/PC DOS and later DR DOS releases (post-1992), which restrict directory depth through the Current Directory Structure (CDS) limit of 66 characters.

These features arise from CP/M's original design lacking subdirectory concepts, necessitating sophisticated directory translation within DOS emulation layers.

Internal architecture

DOS Plus initiates from a single file, either DOSPLUS.SYS or NETPLUS.SYS, in contrast to the MS-DOS reliance on the IO.SYS/MSDOS.SYS pair. This modular file, formatted in CP/M-86 CMD style, consists of several components:

The BDOS kernel oversees multitasking and enforces the CP/M API standards.
The XIOS module manages hardware-specific low-level disk and character input/output operations.
NETPLUS.SYS integrates support for DR Net networking.
The DOS emulator (PCMODE) converts MS-DOS system calls into equivalent CP/M commands, such as translating directory change requests into multiple 'open directory' instructions.
A bootstrap stub is responsible for loading COMMAND.COM or DOSPLUS.COM as necessary, embedding the identifier string "OS=CPCDOS", which reflects its Concurrent PC DOS lineage.

Systems utilizing DOS Plus

DOS Plus served as the primary ROM-based operating system for the Philips :YES, its initial target platform. PC-compatible editions were bundled with the Amstrad PC1512 and France's Jasmin Turbo developed by T.R.A.N. S.A. The BBC Master 512, which was not fully PC-compatible, came with DOS Plus versions 1.2, 1.2a, or 2.1, frequently combined with Solidisk's PC Plus memory expansion hardware. Customized variants of DOS Plus 2.1e/g accommodated the Apricot ACT series’ unique 315 KB and 720 KB FAT12 floppy disk formats. Additionally, it supported Sperry IT PCs featuring logical sectored FAT hard drives (partition type 0xF2) and was compatible with 80186 co-processor cards used in Acorn Archimedes A300/A310 models and Acorn 286 prototypes.

Releases

The recognized releases include:

DOS Plus 1.0 (embedded in ROM on Philips :YES) incorporating BDOS version 4.1
DOS Plus 1.1 featuring BDOS 4.1
DOS Plus 1.2 also with BDOS 4.1
DOS Plus 2.1 equipped with BDOS version 5.0

External links

http://bbc.nvg.org/doc/Master%20512%20Technical%20Guide/