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| instruction processing | |
| central processing unit | |
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Before the first RISC processors were designed, many Computer Architects tried to bridge the " Semantic Gap " - to design instruction sets to support High-level Programming Language s by providing "high-level" instructions such as procedure call and return, loop instructions such as "decrement and branch if non-zero" and complex Addressing Mode s to allow data structure and array accesses to be combined into single instructions. Additionally, the compact nature of a CISC ISA results in smaller Program sizes and fewer calls to main memory, which at the time (the 1960s ) resulted in a tremendous savings on the cost of a computer. While they achieved their aim of allowing high-level language constructs to be expressed in fewer instructions, it was observed that they did not always result in improved performance. For example, on one processor it was discovered that it was possible to improve performance by ''not'' using the procedure call instruction but using a sequence of simpler instructions instead. Furthermore, the more complex the instruction set, the greater the overhead of decoding any given instruction, both in execution time and silicon area. This is particularly true for processors which used Microcode to decode the (macro)instructions. In other words, adding a large and complex instruction set to the processor even slowed down the execution of simple instructions. Implementing all these complex instructions also required a great deal of work on the part of the chip designer, and many transistors; this left less room on the processor to optimize performance in other ways. Examples of CISC processors are the CDC 6600 , System/360 , VAX , PDP-11 , Motorola 68000 family, and Intel and AMD X86 CPUs. The term, like its Antonym RISC , has become less meaningful with the continued evolution of both CISC and RISC designs and implementations. Modern "CISC" CPUs, such as recent x86 designs from both Intel and AMD usually support every instruction that their predecessors did, but are designed to work most efficiently with a subset of instructions more resembling a typical " RISC " instruction set. Indeed, many CISC CPUs (such as modern x86 processors from both Intel and AMD ) decode many x86 instructions into a series of smaller internal "micro-operations" that are then executed internally by the processor. SEE ALSO |
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