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Concept
Superscalar processor
Summary
A superscalar processor is a CPU that implements a form of parallelism called instruction-level parallelism within a single processor. In contrast to a scalar processor, which can execute at most one single instruction per clock cycle, a superscalar processor can execute more than one instruction during a clock cycle by simultaneously dispatching multiple instructions to different execution units on the processor. It therefore allows more throughput (the number of instructions that can be executed in a unit of time) than would otherwise be possible at a given clock rate. Each execution unit is not a separate processor (or a core if the processor is a multi-core processor), but an execution resource within a single CPU such as an arithmetic logic unit.
While a superscalar CPU is typically also pipelined, superscalar and pipelining execution are considered different performance enhancement techniques. The former executes multiple instructions in parallel by using multiple execution units, whereas the latter executes multiple instructions in the same execution unit in parallel by dividing the execution unit into different phases.
The superscalar technique is traditionally associated with several identifying characteristics (within a given CPU):
Instructions are issued from a sequential instruction stream
The CPU dynamically checks for data dependencies between instructions at run time (versus software checking at compile time)
The CPU can execute multiple instructions per clock cycle
Seymour Cray's CDC 6600 from 1964 is often mentioned as the first superscalar design. The 1967 IBM System/360 Model 91 was another superscalar mainframe. The Motorola MC88100 (1988), the Intel i960CA (1989) and the AMD 29000-series 29050 (1990) microprocessors were the first commercial single-chip superscalar microprocessors. RISC microprocessors like these were the first to have superscalar execution, because RISC architectures free transistors and die area which can be used to include multiple execution units (this was why RISC designs were faster than CISC designs through the 1980s and into the 1990s).
Official source
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