RISC vs CISC | Computer Organization & Architecture

Summary

This educational video delves into the distinctions between CISC and RISC architectures, pivotal for competitive and university exams. It highlights the fundamental differences based on instruction set complexity, with CISC featuring more instructions and variable-length formats, while RISC boasts fewer instructions and fixed-length formats. The video also touches on addressing modes, cost implications, hardware requirements, and control units. Examples like Fugaku, a supercomputer using RISC, are provided. The script contrasts CISC's single-instruction complexity with RISC's multi-instruction simplicity, concluding with the observation that RISC's popularity has surged due to the decreasing cost of RAM.

Takeaways

• 🎓 **Importance of Risk vs. CISC**: The video emphasizes the significance of understanding the differences between Risk and CISC architectures for competitive and university-level exams.
• 📚 **Syllabus Coverage**: The channel has covered most of the computer organization and architecture syllabus, with only 2-3 topics remaining.
• 🔍 **Instruction Set Basis**: The categorization of CISC and RISC is based on the complexity of the instruction set, which refers to the number of instructions available.
• 📈 **Complex vs. Reduced Instruction Set**: CISC has a large number of instructions, making it complex, whereas RISC has a reduced set of instructions.
• 🛠️ **Instruction Format**: CISC uses variable-length instruction formats, while RISC uses a fixed-length format, typically 32 bits.
• 📊 **Program Counter Clarity**: Fixed-length instructions in RISC provide clarity for the program counter, simplifying the process of locating the start and end of each instruction.
• 🔄 **Addressing Modes**: CISC supports a large number of addressing modes, whereas RISC has a limited, more straightforward set of addressing modes.
• 💰 **Cost and Power**: CISC architectures are more powerful but costly due to the complexity and extensive instruction set, whereas RISC is less costly and more efficient.
• 🔄 **Cycles and CPI**: CISC instructions may require multiple cycles to complete, leading to a higher CPI (Cycles Per Instruction), while RISC aims for a CPI of 1 through pipelining.
• 💾 **Memory Utilization**: CISC tends to perform more memory-to-memory operations, whereas RISC relies more on register-to-register operations with load and store instructions.
• 🛠️ **Control Unit**: CISC architectures often use microprogrammed control units, which are flexible, whereas RISC uses hardwired control units, which are fixed and less flexible.

Q & A

What is the main difference between CISC and RISC architectures?

The main difference between CISC (Complex Instruction Set Computer) and RISC (Reduced Instruction Set Computer) architectures is the number of instructions. CISC has a large number of instructions, while RISC has a reduced number of instructions.

What is the significance of variable length instruction format in CISC?

In CISC, the variable length instruction format means that instructions can be of different sizes, such as 16-bit, 32-bit, or 64-bit, which is not of one fixed size.

How does fixed length instruction format benefit the program counter in RISC?

Fixed length instruction format in RISC benefits the program counter by providing a consistent size for each instruction, making it easier to determine the start and end of each instruction and the address of the next available instruction.

What is the impact of the number of addressing modes on the complexity of instructions in CISC and RISC?

CISC supports a large number of addressing modes, which can make the instructions more complex. In contrast, RISC supports fewer addressing modes, which simplifies the instructions.

Why is the cost of hardware higher in CISC compared to RISC?

The cost of hardware is higher in CISC because it requires more complex instructions, additional functionality, and more resources to support the large number of instructions.

How does the execution of instructions differ between CISC and RISC in terms of cycles?

CISC may require multiple cycles to execute complex instructions, while RISC aims to complete instructions in a single cycle, often using pipelining to maintain a CPI (Cycles Per Instruction) value of 1.

What is the typical memory manipulation strategy in CISC?

In CISC, instructions are mostly memory-to-memory, meaning that many operations are performed directly in RAM without the need for intermediate register operations.

How does RISC utilize memory compared to CISC?

RISC tends to use registers more often for operations and relies on load and store instructions to move data between memory and registers, rather than performing operations directly in memory.

What is the difference between micro programmed control unit and hardwired control unit in the context of CISC and RISC?

CISC typically uses a micro programmed control unit, which allows for more flexibility and the ability to make changes. RISC, on the other hand, uses a hardwired control unit, which is more fixed and less flexible but can be more efficient.

Can you provide an example of a RISC architecture mentioned in the script?

Yes, Fugaku, which was a supercomputer in 2020, is mentioned as an example of a RISC system.

How does the script illustrate the difference between CISC and RISC with an example involving multiplication?

The script provides an example where in CISC, a single MULT instruction can be used to multiply values X and Y directly in memory, whereas in RISC, this operation would require multiple instructions, such as loading the values into registers, performing the multiplication, and then storing the result back in memory.