Barrel Processor

A barrel processor is a CPU that switches between threads of execution on every cycle. This CPU design technique is also known as "interleaved" or "fine-grained" temporal multithreading. Unlike simultaneous multithreading in modern superscalar architectures, it generally does not allow execution of multiple instructions in one cycle.

Like preemptive multitasking, each thread of execution is assigned its own program counter and other hardware registers (each thread's architectural state). A barrel processor can guarantee that each thread will execute 1 instruction every N cycles, unlike a preemptive multitasking machine, that typically runs one thread of execution for hundreds or thousands of cycles, while all other threads wait their turn.

A technique called C-slowing can automatically generate a corresponding barrel processor design from a single-tasking processor design. An n-way barrel processor generated this way acts much like n separate multiprocessing copies of the original single-tasking processor, each one running at roughly 1/n the original speed.