DOS Plus

DOS Plus(erroneously also known asDOS+) was the firstoperating systemdeveloped byDigital Research's OEM Support GroupinNewbury, Berkshire,UK, first released in 1985. DOS Plus 1.0 was based onCP/M-86 Pluscombined with thePCMODEemulator fromConcurrent PC DOS 4.11.While CP/M-86 Plus andConcurrent DOS 4.1still 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.Later DOS Plus 2.x issues were based onConcurrent PC DOS 5.0instead. In the broader picture, DOS Plus can be seen as an intermediate step betweenConcurrent CP/M-86andDR DOS.

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(1985)
Latest release	DOS Plus 2.1 (withBDOS5.0)
Available in	English
Platforms	Intel 8086,Intel 80186
Kerneltype	Monolithic kernel
Default user interface	Command-line interface(COMMAND.COM)
License	Proprietary
Preceded by	CP/M-86 Plus
Succeeded by	DR 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 oneDOSprogram can be run at a time.

User interface

DOS Plus attempts to present the samecommand-line interfaceas MS-DOS. Like MS-DOS, it has acommand-line interpretercalledCOMMAND.COM(alternative nameDOSPLUS.COM). There is anAUTOEXEC.BATfile, 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 LockandNum Lockstatus, 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,Altand one of thefunction keysF1–F5.

Commands

DOS Plus contains a number of extracommandsto support its multitasking features:

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

It also contains subsets of the standardDOS commandsand CP/M commands – for example, it has both a built-inCOPYcommand, and aPIPutility, both of which copy files.

TheCDcommand 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\ACCOUNTSto appear as drive N:. This so-calledfloating drivefeature allows old programs which don't supportsubdirectoriesto work under DOS Plus and can be used to make file handling at thecommand promptmuch 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 DOS3.31 – 6.0 (up to the November 1992 updates withBDOS6.7 only) also support this (including drive letter L:). Under the later systems, the4DOS/NDOScommand processor supports this features as well.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 pathsfor 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 drivefeature 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.

Internal structure

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

• TheBDOSbasic disk operating systemkernel,which handles multitasking and implements the CP/MAPI.
• TheXIOSextended input/output system, which is themachine-dependentcomponent 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 ",suggesting that it is based on theConcurrent PC DOScodebase.

Computers that used DOS Plus

DOS Plus was the main operating system in ROM for thePhilips:YES,for which it was originally developed.PC compatibleversions were supplied with theAmstrad PC1512and the Jasmin Turbo from the French company T.R.A.N. S.A.The non-IBM PC compatibleBBC Master 512,came with DOS Plus 1.2, 1.2a or 2.1,also in conjunction with Solidisk's PC Plus memory expansion.There were DOS Plus 2.1e/g adaptations for theApricot ACTseries' non-standard315 KBand720 KBFAT12floppy formats. Thelogical sectored FAThard disk format (partition type0xF2) ofSperry ITPCs, and the 80186co-processorboards for theAcorn Archimedes A300andA310as well asAcorn 286prototypes were supported as well.

Versions

Known operating system versions include:

• DOS Plus 1.0 (ROMed inPhilips:YES) (with BDOS 4.1).
• DOS Plus 1.1(with BDOS 4.1)
• DOS Plus 1.2 (with BDOS 4.1)
• DOS Plus 2.1 (with BDOS 5.0)

See also

• 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

Further reading

• 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.ISBN1-85181-063-3.

External links

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