Accent was an operating system kernel developed at Carnegie Mellon University (CMU). Accent was developed as a follow-on to the Aleph kernel developed at the University of Rochester, fixing several of its problems and re-targeting its hardware support for networks of workstation machines instead of minicomputers. Accent was part of the SPICE Project at CMU which ran from 1981 to 1985. Development of Accent led directly to the famous Mach kernel. // An operating system (OS) is the software that manages the sharing of the resources of a computer. ... A kernel connects the application software to the hardware of a computer. ... Carnegie Mellon University is a private research university in Pittsburgh, Pennsylvania, United States. ... Aleph was an operating system kernel developed at the University of Rochester as part of their RIG project in 1975. ... The University of Rochester (UR) is a private, coeducational and nonsectarian research university located in Rochester, New York. ... Sun SPARCstation 1+, 25 MHz RISC processor from early 1990s A workstation, such as a Unix workstation, RISC workstation or engineering workstation, is a high-end desktop or deskside microcomputer designed for technical applications. ... Minicomputer (colloquially, mini) is a largely obsolete term for a class of multi-user computers which make up the middle range of the computing spectrum, in between the largest multi-user systems (traditionally, mainframe computers) and the smallest single-user systems (microcomputers or personal computers). ... Year 1981 (MCMLXXXI) was a common year starting on Thursday (link displays the 1981 Gregorian calendar). ... Year 1985 (MCMLXXXV) was a common year starting on Tuesday (link displays 1985 Gregorian calendar). ... Mach is an operating system kernel developed at Carnegie Mellon University to support operating system research, primarily distributed and parallel computation. ...

The original Aleph project used data copying to allow programs to communicate. Applications could open ports, which would allow them to receive data sent to them by other programs. The idea was to write a number of servers that would control resources on the machine, passing data along until it reached an end user. In this respect it was similar in concept to Unix, although the implementation was much different, using messages instead of memory. Unfortunately this turned out to have a number of problems, notably that copying memory on their Data General Eclipse was very expensive. Filiation of Unix and Unix-like systems Unix (officially trademarked as UNIX®) is a computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs including Ken Thompson, Dennis Ritchie and Douglas McIlroy. ... The Data General Eclipse line of computers by Data General were 16-bit minicomputers released in early 1974. ...

In 1979 one of the Aleph engineers, Richard Rashid, left for CMU and started work on a new version of Aleph that avoided its problems. In particular, Accent targeted workstation machines featuring a MMU, using the MMU to "copy" large blocks of memory via mapping, making the memory appear to be in two different places. Only data that was changed by one program or another would have to be physically copied, using the copy-on-write algorithm. Richard Rashid currently oversees Microsoft Researchs worldwide operations. ... This 68451 MMU could be used with the Motorola 68010 MMU, short for memory management unit or sometimes called paged memory management unit as PMMU, is a class of computer hardware components responsible for handling memory accesses requested by the CPU. Among the functions of such devices are the translation... Copy-on-write (sometimes referred to as COW) is an optimization strategy used in computer programming. ...

To understand the difference, consider two interacting programs, one feeding a file to another. Under Aleph the data from the provider would have to be copied 2kB at a time (due to features of the Eclipse) into the user process. Under Accent the data simply "appeared" in the user process for the cost of a few instructions sent to the MMU unit. Only if the user process changed the data would anything need to be copied, and even then, only the portions of the data that actually changed.

Another problem in Aleph was that its ports were identified by unique ID's that were assigned sequentially. It was simple for a program to "guess" them, thereby gaining access to resources on the computer that it had not been granted. This made the Aleph system rather insecure.

To address this, Accent made the port ID's internal to the kernel only. Programs opening ports were handed back a different ID, stored in a mapping in the kernel. Whenever a message was sent to the kernel for delivery, it would first check that the program had access to the port in question by comparing with the mapping table for that program. Guessing port numbers no longer worked, the program's port IDs gave no clue of the "real" IDs in the kernel, and any attempt to talk on one not explicitly handed out by the kernel was an error. Thus Accent's ports represented capabilities, granting rights to use resources as the result of being handed a valid port ID.

After a few years the Accent project started looking less and less interesting. In the early 1980s many felt that future gains in performance would be made by adding more CPUs to machines, something the Accent kernel was not really equipped to handle. Adding to the problem was that a new generation of more powerful workstations were appearing, meaning that Accent would likely have to be ported to them anyway. Likewise Unix had grown into the operating system of choice for experimental work, both on operating system design, as well as a development platform for user applications. “CPU” redirects here. ...

In order to address these changes, it was decided to end work on Accent and start again. The new system would use Accent's ports system within a Unix kernel, creating the famed Mach kernel. Mach is an operating system kernel developed at Carnegie Mellon University to support operating system research, primarily distributed and parallel computation. ...

Some features of Accent:

• Port capabilities
• Copy-on-write virtual memory management
• Distributed file management
• Distributed process management
• Protected message-based inter-process communication

Inter-Process Communication (IPC) is a set of techniques for the exchange of data between two or more threads in one or more processes. ...

Trivia

Accent was based on the concept of passing messages, often shortened to msg. Accent was also part of the SPICE project. The name "Accent" was used for a spice sold by Accent Inc., which consisted entirely of monosodium glutamate -- better known as "MSG". This article is about monosodium glutamate as a food additive. ...

External links

• Rick Rashid's page at Microsoft Research