The C.mmp was an early MIMD

MIMD

In computing, MIMD is a technique employed to achieve parallelism. Machines using MIMD have a number of processors that function asynchronously and independently. At any time, different processors may be executing different instructions on different pieces of data...

 multiprocessor system developed at Carnegie Mellon University

Carnegie Mellon University

Carnegie Mellon University is a private research university in Pittsburgh, Pennsylvania, United States....

 by William Wulf

William Wulf

William Allan Wulf is a computer scientist notable for his work in programming languages and compilers.Born in Chicago, Illinois, he attended the University of Illinois, receiving a BS in Engineering Physics and an MS in Electrical Engineering, then achieved the first Ph.D. in Computer Science...

 (1971). The notation C.mmp came from the PMS notation of Bell and Newell, where a CPU was designated as C and a variant was noted by the dot notation; mmp stood for Multi-Mini-Processor

Sixteen PDP-11

PDP-11

The PDP-11 was a series of 16-bit minicomputers sold by Digital Equipment Corporation from 1970 into the 1990s, one of a succession of products in the PDP series. The PDP-11 replaced the PDP-8 in many real-time applications, although both product lines lived in parallel for more than 10 years...

 minicomputers were used as the processing elements (named Compute Modules in the system).
Each CM had a local memory of 8K and a local set of peripheral devices. One of the challenges was that a device was only available through its unique connected processor, so the I/O system (designed by Roy Levin) hid the connectivity of the devices and routed the requests to the hosting processor. If a processor went down, the devices connected to its Unibus became unavailable, which became a problem in overall system reliability. Processor 0 (the boot processor) had the disk drives attached.

Each of the Compute Modules shared these communication pathways:

• An Interprocessor bus - used for distribution of system-wide clock, interrupt and process control messaging among the CMs
• A 16x16 crossbar switch
  Crossbar switch
  In electronics, a crossbar switch is a switch connecting multiple inputs to multiple outputs in a matrix manner....
  
   - used to connect the 16 CMs on one side and 16 banks of shared memory
  Shared memory
  In computing, shared memory is memory that may be simultaneously accessed by multiple programs with an intent to provide communication among them or avoid redundant copies. Depending on context, programs may run on a single processor or on multiple separate processors...
  
   on the other. If all 16 processors were accessing different banks of memory, the memory accesses would all be concurrent. If two or more processors were trying to access the same bank of memory, one of them would be granted access on one cycle and the remainder would be negotiated on subsequent memory cycles.

Since the PDP-11 only had an address space

Address space

In computing, an address space defines a range of discrete addresses, each of which may correspond to a network host, peripheral device, disk sector, a memory cell or other logical or physical entity.- Overview :...

 of 16-bits, an additional address translation unit was added to expand the address space to 25 bits for the shared memory space. The UniBus architecture provided 18 bits of address, and the two high-order bits were used to select one of four relocation registers which selected a bank of memory. Proper management of the relocation registers was one of the challenges of programming the operating system kernel.

The original C.mmp design used magnetic core memory, but during its lifetime, higher performance dynamic RAM

Ram

-Animals:*Ram, an uncastrated male sheep*Ram cichlid, a species of freshwater fish endemic to Colombia and Venezuela-Military:*Battering ram*Ramming, a military tactic in which one vehicle runs into another...

 became available and the system was upgraded.

The original processors were PDP-11/20 processors, but in the final system, only five of these were used; the remaining 11 were PDP-11/40 processors, which were modified by having additional writeable microcode space. All modifications to these machines were designed and built at CMU.

Most of the 11/20 modifications were custom changes to the motherboard, but because the PDP-11/40 was implemented in microcode, a separate "proc-mod" board was designed that intercepted certain instructions and implemented the protected operating system requirements. For example, it was necessary, for operating system integrity, that the stack pointer register never be odd. On the 11/20, this was accomplished by clipping the lead to the low-order bit of the stack register. On the 11/40, any access to the stack was intercepted by the proc-mod board and generated an illegal data access trap if the low-order bit was 1.

The operating system

Operating system

An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...

 was called HYDRA

Hydra (operating system)

HYDRA was an early capability-based, object-oriented, microkernel designed to support a wide range of possible operating systems to run on top of it...

. It was an capability

Capability-based security

Capability-based security is a concept in the design of secure computing systems, one of the existing security models. A capability is a communicable, unforgeable token of authority. It refers to a value that references an object along with an associated set of access rights...

-based object-oriented multi-user operating system. System resources were represented as objects and protected through capabilities.

Among the programming languages available on this system was a subset of ALGOL 68

ALGOL 68

ALGOL 68 isan imperative computerprogramming language that was conceived as a successor to theALGOL 60 programming language, designed with the goal of a...

. This language was in fact more a superset than a subset, as the features supporting parallelism were vastly improved, to make good use of the C.mmp. The operating system and most applications were written in the language Bliss-11, which required cross-compilation from a PDP-10. The Algol-68 compiler ran native on the Hydra operating system. There was very little assembly code used in the operating system.

Because overall system reliability depended on having all 16 CPUs running, there were serious problems with overall hardware reliability; if the MTBF of one processor was 24 hours, then the overall system reliability was 16/24 hours, or about 40 minutes. Many of these failures were due to timing glitches in the many custom circuits added to the processors. Considerable effort was expended in getting the hardware reliability improved, and when a processor was noticeably failing, it was partitioned out, and would run diagnostics for several hours. When it had passed a first set of diagnostics, it was partitioned back in as an "I/O processor" and would not run application code (but its peripheral devices were now available); it continued to run diagnostics. If it passed these after several more hours, it was reinstated as a full member of the processor set. Similarly, if a block of memory (one page) was detected as faulty, it was removed from the pool of available pages, and until otherwise notified, the operating system would ignore this page. Thus, the operating system became an early example of a fault-tolerant system, able to deal with hardware problems which would inevitably arise.

External links

• C.mmp - A multi-mini-processor, W. A. Wulf and C. G. Bell, C-MU 1972
• C.mmp: The CMU Multiminiprocessor Computer. Requirements and Overview of the Initial Design, Bell, C. G. ; Broadley, W. ; Wulf, W. ; Newell, A. ; Pierson, C. C-MU 1971

• Wulf, Wm. A. and Harbison, S. P. III, Reflections in a Pool of Processors/An Experience Report on C.mmp/Hydra
• http://www.abebooks.co.uk/servlet/BookDetailsPL?bi=640259300&tab=1&searchurl=an%3DWulf%252C%2BWilliam%2BAllan%253BHarbison%252C%2BSamuel%2BP.%253BLevin%252C%2BRoy%26ltrec%3Dt%26paratrk%3D16329%26sortby%3D3%26tn%3DHYDRA%252FC.Mmp%252C%2Ban%2BExperimental%2BComputer%2BSystem HYDRA/C.mmp, an Experimental Computer System, by Wulf, Wm. A, Harbison, Samuel P. III and Levin, Roy; McGraw-Hill, 1981, (ISBN 0070721203)]