Alternatives
Collectively, these limits drive investigation into alternative architectural changes such as Very Long Instruction Word (VLIW), Explicitly Parallel Instruction Computing (EPIC), simultaneous multithreading (SMT), and multi-core processors.
With VLIW, the burdensome task of dependency checking by hardware logic at run time is removed and delegated to the compiler. Explicitly Parallel Instruction Computing (EPIC) is like VLIW, with extra cache prefetching instructions.
Simultaneous multithreading, often abbreviated as SMT, is a technique for improving the overall efficiency of superscalar CPUs. SMT permits multiple independent threads of execution to better utilize the resources provided by modern processor architectures.
Superscalar processors differ from multi-core processors in that the redundant functional units are not entire processors. A single processor is composed of finer-grained functional units such as the ALU, integer multiplier, integer shifter, floating point unit, etc. There may be multiple versions of each functional unit to enable execution of many instructions in parallel. This differs from a multi-core processor that concurrently processes instructions from multiple threads, one thread per core. It also differs from a pipelined CPU, where the multiple instructions can concurrently be in various stages of execution, assembly-line fashion.
The various alternative techniques are not mutually exclusive—they can be (and frequently are) combined in a single processor. Thus a multicore CPU is possible where each core is an independent processor containing multiple parallel pipelines, each pipeline being superscalar. Some processors also include vector capability.
Read more about this topic: Superscalar
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