DOS Plus

Digital Research DOS Plus Version 1.2
Developer	Digital Research
OS family	CP/M-86
Working state	Discontinued
Source model	Closed source
Initial release	1985; 40 years ago (1985)
Latest release	DOS Plus 2.1 (withBDOS 5.0)
Available in	English
Platforms	Intel 8086,Intel 80186
Kernel type	Monolithic kernel
Default user interface	Command-line interface (COMMAND.COM)
License	Proprietary
Preceded by	CP/M-86 Plus
Succeeded by	DR DOS

DOS Plus (erroneously also known asDOS+) was the firstoperating system developed byDigital Research's OEM Support Group^[1] inNewbury, Berkshire, UK, first released in 1985. DOS Plus 1.0 was based onCP/M-86 Plus combined with thePCMODE emulator fromConcurrent PC DOS 4.11.^[1] While CP/M-86 Plus andConcurrent DOS 4.1 still had been developed in the United States, Concurrent PC DOS 4.11 was an internationalized and bug-fixed version brought forward by Digital Research UK.^[1] Later DOS Plus 2.x issues were based onConcurrent PC DOS 5.0 instead. In the broader picture, DOS Plus can be seen as an intermediate step betweenConcurrent CP/M-86 andDR DOS.

DOS Plus is able to run programs written for either CP/M-86 orMS-DOS 2.11, and can read and write the floppy formats used by both of these systems. Up to four CP/M-86 programs can bemultitasked, but only oneDOS program can be run at a time.

DOS Plus attempts to present the samecommand-line interface as MS-DOS. Like MS-DOS, it has acommand-line interpreter calledCOMMAND.COM (alternative nameDOSPLUS.COM). There is anAUTOEXEC.BAT file, but noCONFIG.SYS (except for FIDDLOAD, an extension to load some field-installabledevice drivers (FIDD) in some versions of DOS Plus 2.1). The major difference the user will notice is that the bottom line of the screen contains status information similar to:

DDT86    ALARM                  UK8 PRN=LPT1      Num    10:17:30

The left-hand side of the status bar shows runningprocesses. The leftmost one will be visible on the screen; the others (if any) are running in the background. The right-hand side shows the keyboard layout in use (UK8 in the above example), the printer port assignment, the keyboardCaps Lock andNum Lock status, and the current time. If a DOS program is running, the status line is not shown. DOS programs cannot be run in the background.

The keyboard layout in use can be changed by pressingCtrl,Alt and one of thefunction keysF1–F5.

DOS Plus contains a number of extracommands to support its multitasking features:

• ADDMEM: Sets the amount of extra memory to allocate toEXE programs.
• ALARM: A message alarm clock.
• BACKG: Allowsbackground processes to be listed and stopped.
• COMSIZE: Sets the amount of memory to allocate toCOM programs.
• PRINT:Print spooler.
• SLICE: Sets the amount of processor time to give to the foreground program. See also the%$SLICE% environment variable.^[2][3]
• USER: Sets the user number to use when accessing CP/M media.

It also contains subsets of the standardDOS commands and CP/M commands – for example, it has both a built-inCOPY command, and aPIP utility, both of which copy files.

TheCD command can assign one of the three drives N:, O: or P: to a directory on a different drive, in a similar manner to the MS-DOS commandSUBST. For example,

CD N:=C:\DATA\ACCOUNTS

will cause the directoryC:\DATA\ACCOUNTS to appear as drive N:. This so-calledfloating drive feature allows old programs which don't supportsubdirectories to work under DOS Plus and can be used to make file handling at thecommand prompt much easier.This feature is also present inConcurrent DOS,Multiuser DOS, System Manager 7, andREAL/32, however, these systems extend the concept to all unused drive letters from A: to Z:, except for drive letter L:.DR DOS 3.31 – 6.0 (up to the November 1992 updates withBDOS 6.7 only) also support this (including drive letter L:). Under the later systems, the4DOS/NDOS command processor supports this features as well.^[4] Floating drives are implemented in the BDOS kernel, not in the command line shell, thus this feature can also be used from within DOS applications when enteringdirectory paths for as long as the application does not parse and split the dirspec for further processing.

Using a similar feature, Concurrent DOS, Multiuser DOS, System Manager and REAL/32 will dynamically assign a floating drive L: to the load path of a loaded application, thereby allowing applications to refer to files residing in their load directory under a standardized drive letter instead of under a fixed absolute path. Thisload drive feature makes it much easier to move software installations on and across disks without having to adapt paths to overlays, configuration files or user data stored in the load directory or subsequent directories. (For similar reasons, the appendage to the environment block associated with loaded applications under MS-DOS/PC DOS 3.0 (and higher) contains a reference to the load path of the executable, however, this consumes more resident memory, and to take advantage of it, support for it must be coded into the executable, whereas DRI's solutions transparently works with any kind of application.)

Another feature resulting from the BDOS' internal organization of current working directories as relative links to parent directories is the theoretically unlimited directory depth supported by all those above mentioned operating systems – in contrast to MS-DOS/PC DOS (and DR DOS since 1992), where an MS-DOS compatible internal data structure namedCurrent Directory Structure (CDS) limits directory depths to a maximum of 66 characters.

These features can be attributed to the fact that CP/M itself did not have a concept of subdirectories, and DOS was emulated under these operating systems, so directories had to be translated to internal CP/M structures in some intelligent way.

DOS Plus boots from a single file called either DOSPLUS.SYS or NETPLUS.SYS (rather than theIO.SYS /MSDOS.SYS combination of MS-DOS). This file is in the CP/M-86CMD format, and is structured internally as a number of modules:

• TheBDOS basic disk operating systemkernel, which handles multitasking and implements the CP/MAPI.
• TheXIOS extended input/output system, which is themachine-dependent component that performs low-level disc and character I/O.
• In the case of NETPLUS.SYS with DR Net support.
• The DOSemulator (internally namedPCMODE), which converts each MS-DOS function call into one or more CP/M calls. In the case of a request to change the current directory, for example, this would translate to a series of 'open directory' calls, one for each directory in the provided path.
• A stub which loads COMMAND.COM (or DOSPLUS.COM) when required. This stub contains the string "OS=CPCDOS",^[5][6] suggesting that it is based on theConcurrent PC DOS codebase.

DOS Plus was the main operating system in ROM for thePhilips :YES, for which it was originally developed.^{[1][nb 1]}PC compatible versions were supplied with theAmstrad PC1512^[1] and the Jasmin Turbo from the French company T.R.A.N. S.A.^[7][8] The non-IBM PC compatibleBBC Master 512,^[9] came with DOS Plus 1.2, 1.2a or 2.1,^[10][11] also in conjunction with Solidisk's PC Plus memory expansion.^[10][11] There were DOS Plus 2.1e/g adaptations for theApricot ACT series' non-standard315 KB and720 KBFAT12 floppy formats. Thelogical sectored FAT hard disk format (partition type0xF2) ofSperry IT PCs, and the 80186co-processor boards for theAcorn Archimedes A300 andA310 as well asAcorn 286 prototypes were supported as well.

Known operating system versions include:

• DOS Plus 1.0 (ROMed inPhilips :YES) (with BDOS 4.1).^{[nb 1]}
• DOS Plus 1.1^[12] (with BDOS 4.1)
• DOS Plus 1.2 (with BDOS 4.1)
• DOS Plus 2.1 (with BDOS 5.0)^[9]

• Personal CP/M-86 2.0 (with BDOS 4.1)
• Personal CP/M-86 2.11 (with BDOS 4.1)
• CP/M-86 Plus
• MSX-DOS
• GEMDOS
• GEM XM

• Gilmour, Jean (1986).Amstrad Personal Computer PC1512 User Instructions.
• Morris, Stephen (1986).Using DOS Plus on the Amstrad PC. Digital Research books. Glentop Publishers Ltd.ISBN 1-85181-063-3.

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

• CP/M variants
• Microcomputer software
• Disk operating systems
• Floppy disk-based operating systems
• Embedded operating systems
• ROM-based operating systems
• DOS variants
• Digital Research operating systems
• Discontinued operating systems
• Assembly language software
• 1985 software

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• CS1 location test
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