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RISC Versus CISC Processors

CISC is an acronym for Complex Instruction Set Computer. The primary goal of CISC architecture is to complete a task in as few lines of assembly as possible. This is achieved by building processor hardware that is capable of understanding and executing a series of complex operations. In this case, each instruction can execute several low-level instructions. One of the primary advantages of this system is that the compiler has to do very little work to translate a high-level language statement into assembly. Because the length of the code is relatively short, very little RAM is required to store instructions. In a nutshell, the emphasis is to build complex instructions directly into the hardware. Examples of CISC processors are the CDC 6600, System/360, VAX, PDP-11, the Motorola 68000 family, and Intel and AMD x86 CPUs.

RISC is an acronym for Reduced Instruction Set Computer. This type of microprocessor emphasizes simplicity and efficiency. RISC designs start with a necessary and sufficient instruction set. The objective of any RISC architecture is to maximize speed by reducing clock cycles per instruction. Almost all computations can be done from a few simple operations. The goal of RISC architecture is to maximize the effective speed of a design by performing infrequent operations in software and frequent functions in hardware, thus obtaining a net performance gain.

To understand this phenomenon, consider any assembly-level language program. It has been observed that it uses the MOV instruction much more frequently than the MUL instruction. Therefore, if the architectural design implements MOV in hardware and MUL in software, there will be a considerable gain in speed, which is the basic feature of RISC technology. Examples of RISC processors include Sun’s SPARC, IBM and Motorola’s PowerPCs, and ARM-based processors.

The salient features of a RISC processor are as follows:

1. The microprocessor is designed using hard-wired control. For example, one bit can be dedicated for one instruction. Generally, variable-length instruction formats require microcode design. All RISC instructions have fixed formats, so no microcode is required.

2. The RISC microprocessor executes most of the instructions in a single clock cycle. This is due to the fact that they are implemented in hardware.

3. The instruction set typically includes only register-to-register load and store.

4. The instructions have a simple format with few addressing modes.

5. The RISC microprocessor has several general-purpose registers and large cache memories, which support the very fast access of data.

6. The RISC microprocessor processes several instructions simultaneously and so includes pipelining.

7. The software can take advantage of more concurrency.


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