Asymmetric multiprocessing
An asymmetric multiprocessing system is a multiprocessor computer system where not all of the multiple interconnected central processing units are treated equally. For example, a system might allow only one CPU to execute operating system code or might allow only one CPU to perform I/O operations. Other AMP systems might allow any CPU to execute operating system code and perform I/O operations, so that they were symmetric with regard to processor roles, but attached some or all peripherals to particular CPUs, so that they were asymmetric with respect to the peripheral attachment.
Asymmetric multiprocessing was the only method for handling multiple CPUs before symmetric multiprocessing was available. It has also been used to provide less expensive options on systems where SMP was available.
[Image:Asmp 2.gif|250px|thumb|right|Asymmetric multiprocessing]
Background and history
For the room-size computers of the 1960s and 1970s, a cost-effective way to increase compute power was to add a second CPU. Since these computers were already close to the fastest available, two standard-speed CPUs were much less expensive than a CPU that ran twice as fast. Also, adding a second CPU was less expensive than a second complete computer, which would need its own peripherals, thus requiring much more floor space and an increased operations staff.Notable early AMP offerings by computer manufacturers were the Burroughs B5000, the DECsystem-1055, and the IBM System/360 model 65MP. There were also dual-CPU machines built at universities.
The problem with adding a second CPU to a computer system was that the operating system had been developed for single-CPU systems, and extending it to handle multiple CPUs efficiently and reliably took a long time. To fill this gap, operating systems intended for single CPUs were initially extended to provide minimal support for a second CPU. In this minimal support, the operating system ran on the “boot” processor, with the other only allowed to run user programs. In the case of the Burroughs B5000, the second processor's hardware was not capable of running "control state" code.
Other systems allowed the operating system to run on all processors, but either attached all the peripherals to one processor or attached particular peripherals to particular processors.
Burroughs B5000 and B5500
An option on the Burroughs B5000 was “Processor B”. This second processor, unlike “Processor A” had no connection to the peripherals, though the two processors shared main memory, and Processor B could not run in Control State. The operating system ran only on Processor A. When there was a user job to be executed, it might be run on Processor B, but when that job tried to access the operating system the processor halted and signaled Processor A. The requested operating system service was then run on Processor A.On the B5500, either Processor A or Processor B could be designated as Processor 1 by a switch on the engineer's panel, with the other processor being Processor 2; both processors shared main memory and had hardware access to the I/O processors hence the peripherals, but only Processor 1 could respond to peripheral interrupts. When a job on Processor 2 required an operating system service it would be rescheduled on Processor 1, which was responsible for both initiating I/O processor activity and responding to interrupts indicating completion. In practice, this meant that while user jobs could run on either Processor 1 or Processor 2 and could access intrinsic library routines that didn't require kernel support, the operating system would schedule them on the latter whenever possible.
CDC 6500 and 6700
Control Data Corporation offered two configurations of its CDC 6000 series that featured two central processors. The CDC 6500 was a CDC 6400 with two central processors. The CDC 6700 was a CDC 6600 with the CDC 6400 central processor added to it.These systems were organized quite differently from the other multiprocessors in this article. The operating system ran on the peripheral processors, while the user's application ran on the CPUs. Thus, the terms ASMP and SMP do not properly apply to these multiprocessors.