Difference between RISC and CISC Processors
👉CISC Processors
One of the earlier objectives of CPU designers was- to provide more and more instructions in the instruction set of a CPU to ensure that the CPU supports more functions directly. It makes it easier to translate high-level language programs to machine language. It ensures that the machine language programs run more effectively on the computer.
Of course, every extra added instruction in the instruction set of a CPU needs additional hardware circuity to handle that instruction, adding more complexity to the CPU’s hardware circuitry.
Another objective of CPU designers was to optimize the usage of expensive memory. To achieve this, designers tried to load more instructions in memory by presenting the idea of variable-length instructions such as half-word, one-and-half-word, etc.
For example, an operand in a direct instruction needs fewer bits; hence, a CPU designer can design it as a half-word instruction.
Designers originally designed CPUs to support various addressing modes. CPUs with an extensive instruction set, variable-length instructions, and a variety of addressing modes are called CPUs based on CISC (Complex Instruction Set Computer) architecture.
Since CISC processors have so many processing features, they make the job of machine language programmers more comfortable. However, they are complex and costly to produce. Most everyday personal computers use CISC processors.
👉RISC Processors
In the early 1980s, some CPU designers realized that many instructions supported by a CISC-based CPU are rarely used. Hence, they came up with the idea of significantly reducing the design complexity of a CPU by implementing in hardware circuitry only the bare minimum essential set of instructions and some of the more repeatedly used instructions.
The instruction set of the CPU need not support other complex instructions because a computer can implement them in software by using the basic set of instructions.
While working on a more straightforward CPU design, the designers also devised the idea of making all instructions of uniform length so that decoding and executing all instructions become simple and quick.
Similarly, to speed up computation and reduce the complexity of handling several addressing modes, they decided to design all instructions in such a way that they retrieve operands stored in registers in the CPU instead of from memory.
These design ideas resulted in making faster and low-cost processors. CPUs with small, fixed-length instructions and reduced references to memory to retrieve operands are called CPUs based on RISC (Reduced Instruction Set Computer) architecture.
Since RISC processors have a small instruction set, they place additional demand on programmers who must consider how to implement challenging computations by merging simple instructions.
However, their more straightforward design makes RISC processors faster for most applications, less complex, and less expensive to produce than CISC processors.
Popular RISC processors used in workstations are POWER (used in IBM workstations), SPARC (used in SUN workstations), and PA-RISC (used in HP workstations).
Supporters of RISC technology claim that increased processing speed and lower cost of RISC processors easily offset the limitations of a reduced instruction set.
However, critics of RISC technology believe that a RISC processor has to process more of these simple programmed instructions to complete a task, placing an additional burden on system software.
There seems to be no exact solution as to which technology is better. The answer may be that each technology lends itself best to specific applications so that both technologies will coexist.
RISC Processors Vs CISC Processors
RISC Processors
📌Some of the instructions refer to memory.
📌There are a few addressing modes. Most instructions have register-to-register addressing modes.
📌There are few instructions in RISC.
📌It can include simple instructions and carries one cycle.
📌Hardware executes the instructions in RISC.
📌There are fixed format instructions.
📌Decoding can be easier as instructions have a fixed format.
📌There are multiple register sets used.
📌RISC Processors are highly pipelined.
📌It can load and store functions found in different instructions.
CISC Processors
📌Most of the instructions refer to memory.
📌There are many addressing modes.
📌There are many instructions.
📌It can include complex instructions and carries multiple cycles.
📌Micro-program executes the instructions in CISC.
📌There are variable format instructions.
📌Decoding can be complex as instructions have variable formats.
📌A single register set is used.
📌CISC Processors are not pipelined or less pipelined.
📌It can load and store functions found in a single instruction.
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