Asymmetric multiprocessing

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.

Since these computers were already close to the fastest available (near the peak of the price:performance ratio), 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.

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.

[5] Control Data Corporation offered two configurations of its CDC 6000 series that featured two central processors.