C.mmp
The C.mmp was an early multiple instruction, multiple data multiprocessor system developed at Carnegie Mellon University by William Wulf. The notation C.mmp came from the PMS notation of Gordon Bell and Allen Newell, where a central processing unit was designated as C, a variant was noted by the dot notation, and mmp stood for Multi-Mini-Processor., the machine is on display at CMU, in Wean Hall, on the ninth floor.
Structure
Sixteen Digital Equipment Corporation PDP-11 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 input/output system 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 had the disk drives attached.Each of the Compute Modules shared these communication pathways:
- An Interprocessor bus – used to distribute system-wide clock, interrupt, and process control messaging among the CMs
- A 16x16 crossbar switch – used to connect the 16 CMs on one side and 16 banks of shared memory 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.
The original C.mmp design used magnetic-core memory, but during its lifetime, higher performance dynamic random-access memory 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 extra 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 wire-wrapped backplane, 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 done 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.
Operating system
The operating system was named Hydra. It was capability-based, object-oriented, multi-user, and a microkernel. System resources were represented as objects and protected through capabilities.The OS and most application software was written in the programming language BLISS-11, which required cross-compiling on a PDP-10. The OS used very little assembly language.
Among the programming languages available on the system was an ALGOL 68 variant which included extensions supporting parallel computing, to make good use of the C.mmp. The ALGOL compiler ran native on Hydra OS.