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It is able to run programs written for either CP/M-86 or MS-DOS 2.11, and can read and write the floppy formats used by both these systems. Up to four CP/M-86 programs can be Multitasked , but only one DOS program can be run at a time.


USER INTERFACE

DOS Plus attempts to present the same Command Line Interface as MS-DOS. Like MS-DOS, it has a Command Line Interpreter called COMMAND.COM . There is an AUTOEXEC.BAT file, but no CONFIG.SYS . 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 running Processes . 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 keyboard Caps Lock and Num 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 pressing Ctrl , Alt and one of the Function Keys F1-F5.


COMMANDS

DOS Plus contains a number of extra commands to support its limited multitasking features:

  • ADDMEM: Sets the amount of extra memory to allocate to EXE programs.

  • ALARM: A message alarm clock.

  • BACKG: Allows background processes to be listed and stopped.

  • COMSIZE: Sets the amount of memory to allocate to COM programs.

  • PRINT: Print spooler.

  • SLICE: Sets the amount of processor time to give to the foreground program.

  • USER: Sets the user number to use when accessing CP/M media.


It also contains subsets of the standard DOS Commands and CP/M commands - for example, it has both a built-in COPY command, and a PIP utility, both of which copy files.

The CD 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 command SUBST. For example,

CD N:=C:\DATA\ACCOUNTS

will cause the directory C:\DATA\ACCOUNTS to appear as drive N:.


INTERNAL STRUCTURE

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

  • The BDOS Kernel , which handles multitasking and implements the CP/M API.

  • The XIOS, which is the machine-dependent component that performs low-level disc and character I/O.

  • The DOS emulator, 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 path.

  • A stub which loads COMMAND.COM when required. This stub contains the string "OS=CPCDOS", suggesting that it is based on the Concurrent DOS codebase.



COMPUTERS THAT USED DOS PLUS

DOS Plus was the main operating system for the Non-PC Compatible BBC Master 512 and the Philips Yes . PC Compatible versions were supplied with the Amstrad PC-1512 and the TRAN Jasmin Turbo .


REFERENCES

Gilmour, Jean (1986) ''Amstrad Personal Computer PC1512 User Instructions''