पहले CISC आया था, बाद में RISC आया है | लेकिन आज अब दोनों उपयोग होता है |

💡 WHY RISC vs CISC? (The Actual Need)

To understand the need, we must see the problem computer designers were trying to solve.

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🔴 BEFORE — The problem

Early computers used CISC because:

• Memory was expensive
• Compilers were weak
• Programmers needed powerful instructions to do more work with fewer lines of code

So CISC provided:

• Very large instruction sets
• Complex instructions that performed multiple operations

BUT… this created new problems:

• Instructions took many clock cycles
• Chips became complex
• Difficult to pipeline
• Slower performance

As technology advanced, these problems became significant.

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🟢 THEN — The need for RISC

Researchers noticed an important fact:

👉 Most programs only use about 20% of all instructions regularly.

Yet CISC processors were carrying hundreds of instructions that were rarely used.
This wasted chip space and reduced performance.

So RISC was developed with the idea:

“Make instructions simple, uniform, and fast.”

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✔ WHY RISC was needed

1. To increase execution speed
   Simple instructions → 1 cycle each → faster performance.
2. To make pipelining easier
   Uniform instruction size and format = efficient pipeline.
3. To reduce hardware complexity
   Hardwired control unit → faster & simpler than microprogramming.
4. To use transistors more efficiently
   Instead of storing hundreds of instructions, use chip space for:
   • More registers
   • Better pipeline
   • Cache
5. To match advancements in compiler technology
   Modern compilers can optimize code well, so hardware no longer needed extremely complex instructions.

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✔ WHY CISC was still needed

Even after RISC came, CISC didn’t disappear because:

1. Backward compatibility
   x86 architecture must support millions of old programs.
2. Smaller code size
   Complex instructions reduce number of lines → useful when memory was still costly.
3. High-level instructions simplify programming
   One complex instruction can perform multiple operations.

RISC was needed to improve speed and efficiency by simplifying hardware.

CISC was needed to reduce code size and maintain compatibility with older systems.

Both architectures exist because they solve different design needs.

🧠 One Line Summary

• RISC = 4 fast steps
• CISC = 1 big slow step

Similarly, a CPU splits instruction execution into stages:

5-Stage Pipeline

1. IF – Instruction Fetch
2. ID – Instruction Decode
3. EX – Execute
4. MEM – Memory Access
5. WB – Write Back

Each instruction moves one stage per cycle, and multiple instructions stay in the pipeline at the same time.

🟩 One-Line Answer (Exam Friendly)

Both RISC and CISC use pipelining, but RISC processors use pipelining more effectively due to their simple, uniform instructions.