6. OCR A Level (H046-H446) SLR2 - 1.1 CISC vs RISC

Summary

This video explores the differences between CISC and RISC processors. CISC processors use complex instructions to complete tasks in fewer lines of assembly, while RISC processors use simpler instructions executed in a single clock cycle. Despite requiring more lines of code, RISC's efficiency, lower power consumption, and use of pipelining make it dominant in modern devices. The video also discusses the performance equation and historical challenges RISC faced before becoming the industry standard.

Takeaways

• 💻 **Instruction Set Importance**: The script explains that at the core of all computers is the instruction set, which is the set of all machine code instructions recognized by the CPU.
• 🔍 **CISC vs RISC**: There are two main categories of instruction sets: Complex Instruction Set Computers (CISC) and Reduced Instruction Set Computers (RISC).
• 📚 **CISC Efficiency**: CISC processors aim to complete tasks with fewer lines of assembly code, which means the processor hardware and circuitry is more complex.
• 🔧 **CISC Example**: The script uses 'malt' as an example of a complex instruction in CISC that can multiply two numbers and store the result in one operation.
• 📉 **CISC Decline**: CISC architecture, once common in desktops and laptops, is now less common, with Intel's x86 processors being a notable exception.
• 🏃 **RISC Simplicity**: RISC processors use simple instructions that are executed within a single machine or clock cycle, which requires more lines of assembly code.
• 🚀 **RISC Performance**: RISC architecture can take advantage of pipelining to speed up the processor, as all instructions have uniform execution times.
• 🔋 **RISC Power Efficiency**: RISC processors are popular in low-power devices due to their lower energy requirements and simpler hardware.
• 📈 **RISC Dominance**: ARM and other RISC-based processors now make up over 90% of all processors in use, indicating their widespread adoption.
• 📊 **Performance Equation**: The script introduces a performance equation that contrasts CISC's focus on minimizing instructions per program with RISC's focus on reducing cycles per instruction.
• 📱 **RISC Adoption**: Despite early challenges, RISC has become dominant due to factors like the reduction in RAM prices and advancements in compiler technology.

Q & A

What is the core element of all computers?

The core element of all computers is the instruction set, which is the set of all instructions written in machine code that can be recognized and executed by a given processing unit or CPU.

What are the two main categories of instruction sets?

The two main categories of instruction sets are Complex Instruction Set Computer (CISC) and Reduced Instruction Set Computer (RISC).

What is the primary goal of CISC processors?

The primary goal of CISC processors is to complete tasks in as few lines of assembly as possible, which means the processor hardware and circuitry is more complicated to understand and execute a series of operations.

What is an example of a complex instruction in CISC?

An example of a complex instruction in CISC is 'malt', which would load two values into a register, multiply them together, and store the result back out.

Where is CISC architecture mainly found today?

CISC architecture is mainly found in desktop computers and laptops, with Intel's x86 processors being an example, although they now operate much like RISC with the use of microcode.

What is the aim of RISC processors?

The aim of RISC processors is to use simple instructions that will be executed within a single machine or clock cycle.

How does RISC handle complex commands that don't exist in its architecture?

RISC handles complex commands by separating them into a number of simpler commands, requiring more lines of assembly but each executable in a single clock cycle.

What are the benefits of RISC architecture?

RISC architecture benefits include lower energy requirements, simpler hardware, more room for general-purpose registers, and the ability to use pipelining to speed up the processor.

Why have RISC architectures become popular in low power and portable devices?

RISC architectures have become popular in low power and portable devices due to their efficiency, lower energy requirements, and the fact that they can execute instructions in a single clock cycle.

What is the performance equation and how does it relate to CISC and RISC?

The performance equation is used for expressing a computer's performance potential. CISC aims to minimize instructions per program but sacrifices cycles per instruction, while RISC aims to reduce cycles per instruction at the cost of increasing the number of instructions in the assembly program.

Why did it take RISC a long time to gain acceptance on a commercial scale?

It took RISC a long time to gain acceptance on a commercial scale due to a lack of software support compared to CISC, and the influence of companies like Intel which were producing powerful CISC processors.

How has the reduction in price of RAM and increased sophistication of compiler technology affected RISC?

The reduction in price of RAM and increased sophistication of compiler technology has made the RISC emphasis on software over hardware ideal, leading to the widespread adoption of RISC-based processors.