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3.5.2 Superscalar Processors

Figure 16: Vector Chaining.

The evolution of microprocessors has reached the point where architectural concepts pioneered in vector processors and mainframe computers of the 1970s (most notably the CDC-6600 and Cray-1) are starting to appear in RISC processors. Early RISC machines were very simple single-chip processors. As VLSI technology improved more room became available on the chip. Rather than increase the complexity of the architecture, most designers decided to use this room on techniques to improve the execution of their current architecture. The two principle techniques are on-chip caches and instruction pipelines.

The latest step in this evolutionary process is the superscalar processor. The name means these processors are scalar processors that are capable of executing more than one instruction in each cycle. The keys to superscalar execution are an instruction fetching unit that can fetch more than one instruction at a time from cache; instruction decoding logic that can decide when instructions are independent and thus executed simultaneously; and sufficient execution units to be able to process several instructions at one time. Note that the execution units may be pipelined, e.g. they may be floating point adders or multipliers, in which case the cycle time for each stage matches the cycle times on the fetching and decoding logic. In many systems the high level architecture is unchanged from earlier scalar designs. The superscalar designs use instruction level parallelism for improved implementation of these architectures.