DNIX (original spelling: D-Nix) is a discontinued Unix-like real-time operating system from the Swedish company Dataindustrier AB (DIAB). A version named ABCenix was developed for the ABC 1600 computer from Luxor. Daisy Systems also had a system named Daisy DNIX on some of their computer-aided design (CAD) workstations. It was unrelated to DIAB's product.
History
Inception at DIAB in Sweden
Dataindustrier AB (literal translation: computer industries shareholding company) was started in 1970 by Lars Karlsson as a single-board computer manufacture in Sundsvall, Sweden, producing a Zilog Z80-based computer named Data Board 4680. In 1978, DIAB started to work with the Swedish television company Luxor AB to produce the home and office computer series ABC 80 and ABC 800.
In 1983 DIAB independently developed the first Unix-compatible machine, DIAB DS90 based on the Motorola 68000 CPU. D-NIX here made its appearance, based on a UNIX System V license from AT&T Corporation. DIAB was however an industrial control system (automation) company, and needed a real-time operating system, so the company replaced the AT&T-supplied UNIX kernel with their own in-house developed, yet compatible real-time variant. This kernel was inspired by a Z80 kernel named OS.8 created for Monroe Systems division of Litton Industries.
Over time, the company also replaced several of the UNIX standard userspace tools with their own implementations, to the point where no code was derived from UNIX, and their machines could be deployed independently of any AT&T UNIX license. Two years later and in cooperation with Luxor, a computer called ABC 1600 was developed for the office market, while in parallel, DIAB continue to produce enhanced versions of the DS90 computer using newer versions of the Motorola CPUs such as Motorola 68010, 68020, 68030 and eventually 68040. In 1990 DIAB was acquired by Groupe Bull who continued to produce and support the DS machines under the brand name DIAB, with names such as DIAB 2320, DIAB 2340 etc., still running DIABs version of DNIX.Historien om DIAB – Dataindustrier AB
Derivative at ISC Systems Corporation ISC Systems Corporation (ISC) purchased the right to use DNIX in the late 1980s for use in its line of Motorola 68k-based banking computers. (ISC was later bought by Olivetti, and was in turn resold to Wang, which was then bought by Getronics. This corporate entity, most often referred to as 'ISC', has answered to a bewildering array of names over the years.) This code branch was the SVR2 compatible version, and received extensive modification and development at their hands. Notable features of this operating system were its support of demand paging, diskless workstations, multiprocessing, asynchronous input/output (I/O), the ability to mount processes (handlers) on directories in the file system, and message passing. Its real-time support consisted largely of internal event-driven queues rather than list search mechanisms (no 'thundering herd'Accept() scalability on Linux), static process priorities in two classes (run to completion and timesliced), support for contiguous files (to avoid fragmentation of critical resources), and memory locking. The quality of the orthogonal asynchronous event implementation has yet to be equalled in current commercial operating systems, though some approach it. (The concept that has yet to be adopted is that the synchronous marshalling point of all the asynchronous activity could also be asynchronous, ad infinitum. DNIX handled this with aplomb.) The asynchronous I/O facility obviated the need for Berkeley sockets select or SVR4's STREAMS poll mechanism, though there was a socket emulation library that preserved the socket semantics for backward compatibility. Another feature of DNIX was that none of the standard utilities (such as ps, a frequent offender) rummaged around in the kernel's memory to do their job. System calls were used instead, and this meant the kernel's internal architecture was free to change as required. The handler concept allowed network protocol stacks to be outside the kernel, which greatly eased development and improved overall reliability, though at a performance cost. It also allowed for foreign file systems to be user-level processes, again for improved reliability. The main file system, though it could have been (and once was) an external process, was pulled into the kernel for performance reasons. Were it not for this DNIX could well have been considered a microkernel, though it was not formally developed as such. Handlers could appear as any type of 'native' Unix file, directory structure, or device, and file I/O requests that the handler could not process could be passed off to other handlers, including the underlying one on which the handler was mounted. Handler connections could also exist and be passed around independent of the file system, much like a pipe. One effect of this is that text terminal (TTY) like devices could be emulated without needing a kernel-based pseudo terminal facility.
An example of where a handler saved the day was in ISC's diskless workstation support, where a bug in the implementation meant that using named pipes on the workstation could induce undesirable resource locking on the fileserver. A handler was created on the workstation to field accesses to the afflicted named pipes until the appropriate kernel fixes could be developed. This handler required approximately 5 kilobytes of code to implement, an indication that a non-trivial handler did not need to be large.
ISC also received the right to manufacture DIAB's DS90-10 and DS90-20 machines as its file servers. The multiprocessor DS90-20's, however, were too expensive for the target market and ISC designed its own servers and ported DNIX to them. ISC designed its own GUI-based diskless workstations for use with these file servers, and ported DNIX again. (Though ISC used Daisy workstations running Daisy DNIX to design the machines that would run DIAB's DNIX, there was negligible confusion internally as the drafting and layout staff rarely talked to the software staff. Moreover, the hardware design staff didn't use either system! The running joke went something like: "At ISC we build computers, we don't use them.") The asynchronous I/O support of DNIX allowed for easy event-driven programming in the workstations, which performed well even though they had relatively limited resources. (The GUI diskless workstation had a 7 MHz 68010 processor and was usable with only 512K of memory, of which the kernel consumed approximately half. Most workstations had 1 MB of memory, though there were later 2 MB and 4 MB versions, along with 10 MHz processors.) A full-blown installation could consist of one server (16 MHz 68020, 8 MB of RAM, and a 200 MB hard disk) and up to 64 workstations. Though slow to boot up, such an array would perform acceptably in a bank teller application. Besides the innate efficiency of DNIX, the associated DIAB C compiler was key to high performance. It generated particularly good code for the 68010, especially after ISC got done with it. (ISC also retargeted it to the Texas Instruments TMS34010 graphics coprocessor used in its last workstation.) The DIAB C compiler was, of course, used to build DNIX, which was one of the factors contributing to its efficiency, and is still available, in some form, through Wind River Systems.
These systems are still in use as of this writing in 2006, in former Seattle-First National Bank branches now branded Bank of America. There may be, and probably are, other ISC customers still using DNIX in some capacity. Through ISC there was a considerable DNIX presence in Central and South America.
Asynchronous events DNIX's native system call was the dnix(2) library function, analogous to the standard Unix unix(2) or syscall(2) function. It took multiple arguments, the first of which was a function code. Semantically this single call provided all appropriate Unix functionality, though it was syntactically different from Unix and had, of course, numerous DNIX-only extensions.
DNIX function codes were organized into two classes: Type 1 and Type 2. Type 1 commands were those that were associated with I/O activity, or anything that could potentially cause the issuing process to block. Major examples were, F_OPEN, F_CLOSE, F_READ, F_WRITE, F_IOCR, F_IOCW, F_WAIT, and F_NAP. Type 2 were the remainder, such as, F_GETPID, F_GETTIME, etc. They could be satisfied by the kernel immediately.
To invoke asynchronicity, a special file descriptor called a trap queue had to have been created via the Type 2 opcode, F_OTQ. A Type 1 call would have the, F_NOWAIT bit OR-ed with its function value, and one of the additional parameters to, dnix(2) was the trap queue file descriptor. The return value from an asynchronous call was not the normal value but a kernel-assigned identifier. At such time as the asynchronous request completed, a, read(2) (or, F_READ) of the trap queue file descriptor would return a small kernel-defined structure containing the identifier and result status. The, F_CANCEL operation was available to cancel any asynchronous operation that hadn't yet been completed, one of its arguments was the kernel-assigned identifier. (A process could only cancel requests that were currently self-owned. The exact semantics of cancelling was up to each request's handler, fundamentally it only meant that any waiting was to be terminated. A partially completed operation could be returned.) In addition to the kernel-assigned identifier, one of the arguments given to any asynchronous operation was a 32-bit user-assigned identifier. This most often referenced a function pointer to the appropriate subroutine that would handle the I/O completion method, but this was merely convention. It was the entity that read the trap queue elements that was responsible for interpreting this value.
struct itrq { /* Structure of data read from trap queue. */ short it_stat; /* Status */ short it_rn; /* Request number */ long it_oid; /* Owner ID given on request */ long it_rpar; /* Returned parameter */
Of note is that the asynchronous events were gathered via normal file descriptor read operations, and that such reading was also able to be asynchronous. This had implications for semi-autonomous asynchronous event handling packages that could exist within one process. (DNIX 5.2 did not have lightweight processes or threads.) Also of note is that any potentially blocking operation could be issued asynchronously, so DNIX was well equipped to handle many clients with a single server process. A process was not restricted to having only one trap queue, so I/O requests could be grossly prioritized in this way.
Compatibility In addition to the native dnix(2) call, a complete set of 'standard' libc interface calls was available. open(2), close(2), read(2), write(2), etc. Besides being useful for backwards compatibility, these were implemented in a binary-compatible manner with the NCR Tower computer, so that binaries compiled for it would run unchanged under DNIX. The DNIX kernel had two trap dispatchers internally, one for the DNIX method and one for the Unix method. Choice of dispatcher was up to the programmer, and using both interchangeably was acceptable. Semantically they were identical wherever functionality overlapped. (In these machines the 68000 trap #0 instruction was used for the unix(2) calls, and the trap #4 instruction for dnix(2). The two trap handlers were very similar, though the [usually hidden] unix(2) call held the function code in the processor's D0 register, whereas dnix(2) held it on the stack with the rest of the parameters.)