These were basically single-user multi-tasking operating systems, designed for developing and running Process control type applications.
It reused many of the parts of DOS, but added multi-user access, OS4000 JCL Command-line interpreter, Batch processing, OS4000 hierarchical filesystem (although on-disk format very similar to the non-hierarchical DOS filesystem).
The main customer for this was the central computing service of University College London (Euclid), where a multi-node system consisting of a Hub file server and multiple Rim multi-access compute server systems provided service for over 100 simultaneous users.
Linked-OS was also used to construct fail-over Process control systems with higher resilience.
OS4000 Rel 5 introduced a modified version of the OS4000 filesystem called CFSX, in order to allow easier use of larger disks.
The architecture of OS4000 is very heavily based around the features of the platform it runs on, the GEC 4000 series minicomputers, and these are rather unusual.
They include a feature called Nucleus,[2] which is a combination of a hardware- and firmware-based kernel, which cannot be altered under program control.
Nucleus supports up to 256 processes, and schedules these automatically using a fixed priority scheme.
Functions such as filesystems, store allocation, terminal drivers, timing services, etc.
OS4000 JCL limits characters in file path components to upper case letters and numbers only.
Each file path starts with a context pointer which is a name which refers to a position in a filesystem, followed by zero or more catalogues (equivalent to Unix directories), and ending with a filename.
File types PST, PSB, PRT also exist in theory, but have the same capabilities as PRB and are not generally used.
Additionally, there is a Logical Indexed Sequential (LIS) filetype, which is an ISAM file and always appears to be sorted on its key field, and a Byte stream (BYT) filetype, which was added in Rel 6.5 to better support the OS4000 NFS server.
Support for Unix style symlinks (arbitrary text stored in a catalogue) was added in Rel 6.5 to better support the OS4000 NFS server, but symlinks can only be created and are only visible from NFS clients.
This supports exactly the same types of file as permanent filesystems, except for CAT, REF, ODP, and symlinks.
The following shows a short Multi-access login session: In this case, user SMAN has logged in and issued the EXAMINE command.
A user normally also gets an AIDA process which is privileged and used to load only trusted debugging programs.
[5] This required the addition of a software Nucleus emulation, as this was not a feature of the GEC Series 63 hardware.