Both RISC and CISC are the architectural designs of CPU based on instruction set variation. The two are differentiated on various factors such as programming unit, addressing mode, clock, implementation, application, etc. The significant difference between RISC and CISC is that RISC utilizes a hardwired programming unit, while, CISC has a microprogramming unit.
Not only this, as the name indicates RISC has a reduced instruction set, which means that only a few instructions are present in its instruction set. However, CISC has multiple different instructions present in its instruction set and these are used to perform complex operations.
As the two are associated with an instruction set, so let us first understand,
What is an instruction set?
The instruction set or instruction set architecture abbreviated as ISA is a combination of various instructions that command the processor to execute any desired operation. It has multiple instructions, addressing modes, some registers, memory architecture, etc. ISA defines the multiple instructions which the processor supports.
Some examples are Intel x86, AMD, etc.
Content: RISC Vs CISC
Comparison chart
| Basis for Comparison | RISC | CISC |
|---|---|---|
| Stands for | Reduced instruction set computing | Complex instruction set computing |
| Programming unit | Hardwired unit | Microprogramming |
| Nature of instruction set | Simple | Complex |
| Execution time | Very short | Comparatively long |
| Instruction set optimization | On software | On hardware |
| Instruction format | Fixed (32 bits) | Variable (16 – 64 bits) |
| Memory unit | Not present | Present |
| Offered addressing modes | Limited | Compound |
| Requirement of register set | Multiple | Single |
| Memory space required | Large | Comparatively small |
| Decoding offered | Simple | Complex |
| Data types | Less | More |
| Clock requirement per cycle for execution | Single | Multiple |
| Complexity associated with | Compiler | Microprogram |
| Need for external memory | Not exist | Exist |
| Arithmetic operations | Only register to register | Register to register, register to memory and memory to memory. |
| Cache | Individual for data and instruction. | Combined for data and instruction. |
| Examples | ARM, Alpha, Atmel’s AVR, RISC-V, etc. | Intel x86, AMD, VAX, etc. |
| Applications | Useful in video and image processing along with telecommunication industry. | Useful in security systems, home automation. |
Definition of RISC
RISC is a processor-based computing approach that offers the execution of computing tasks with multiple simple instructions. These were designed with the aim that computer processors perform simple instruction execution quite fastly.
Through this approach, the data processing can be fastened by reducing the instructions which are permanently stored in the processor from the beginning as it offers to rely on software programs more as compared to inbuilt stored ones.
Here basically, any complex code is first broken down into simple units and then each simple unit gets executed. It requires a greater number of general-purpose registers, as even a single instruction is divided into multiple commands whose storing requires multiple registers.
Definition of CISC
CISC is another processor-based computing approach that uses complex instruction sets or codes for the execution of any desired task. It puts the whole emphasis on the way to lessen the total instructions per program. Thus, it is associated with such processor hardware that can deal with the execution of various serially linked operations using a specific instruction.
It offers the instruction execution directly with the memory band of the system thus here no need for storing or loading functions exist. This operation is not much dependent on the compiler as complex instructions get built on the hardware itself thus, RAM requirement is also less.
Basically, it offers operation execution in memory as a complex instruction can be able to do so thus less number of registers are required.
Key Differences of RISC and CISC
- The reduced instruction set computing i.e., RISC is mainly regarded as simple due to the fact that here the instruction set involves instructions for loading, evaluating and storing operations. While, complex instruction set computing i.e., CISC regarded as complex because a single instruction performs, loading, evaluating and storing operations.
- RISC utilizes a hardwired programming unit whereas CISC possesses microprogramming.
- The instruction set optimization of RISC is software-based while that of CISC is hardware-based.
- The simple nature of the RISC instruction set, offers a short execution time than the CISC instruction set, as CISC possess complex nature.
- For the purpose of storing instructions, multiple register sets are needed by the RISC processor while the same requires a single register set in the case of a CISC processor.
- The size of the instruction format in the case of RISC is fixed and is 32 bits whereas CISC offers a variable instruction set format whose size ranges between 16 to 64 bits for each instruction.
- The program designed for RISC needs, large memory space for its storage. On the contrary, for CISC architecture, the designed program requires comparatively less space.
- The instruction decoding of RISC is comparatively simpler than that of CISC.
- In RISC, there is a limited number of addressing modes that support the operation whereas CISC offers compound addressing modes for the operation to take place.
- The average clock required per cycle for operation execution in RISC is generally single while CISC requires multiple clocks for operation execution i.e., around 2 to 15.
- In a RISC processor, both data and instruction require a separate cache. On the contrary, there is a combined cache for data and instruction in CISC.
- In RISC, the complexity is associated with the compiler whereas in CISC the complexity is associated with the microprogram.
- For the purpose of calculation, no external memory is used by RISC whereas CISC requires external memory for the same.
- At the time of code expansion, there may be some problem in RISC but the code expansion causes no issue in CISC.
Conclusion
Thus, this discussion concludes that RISC and CISC are two different approaches where the operation execution takes place in different ways.
If we consider subtraction of two binary numbers of 8-bit each then in the RISC approach a separate instruction is needed for loading the data in the register, further use of a suitable operator will perform the task and in the next instruction, the result will get stored in the accurate location. But for the same operation, when we talk about the CISC approach then a single instruction can perform subtraction of two binary digits.