STUDYORB

Computer architecture has evolved with two main approaches: CISC (Complex Instruction Set Computers) and RISC (Reduced Instruction Set Computers). CISC machines, like Intel Pentium, use complex instructions to do more in a single step, while RISC machines, like Sun SPARC, focus on simpler instructions to speed up common tasks. Although CISC aims to reduce the number of instructions, RISC designs are often faster because they optimize frequently used operations. Today, most modern processors follow the RISC approach for better performance.

• CISC (Complex Instruction Set Computers):

  • Philosophy: Focuses on creating machines with complex instructions and multiple addressing modes to reduce the number of instructions needed for a task. This approach aims to decrease memory read/write operations and bridge the gap between high-level and low-level languages.
  • Examples: Intel Pentium, Motorola MC68000, IBM & Macintosh PowerPC.
  • Issue: Adding complexity in instructions leads to difficulties in supporting high clock speeds due to more complex computations per clock cycle.

• RISC (Reduced Instruction Set Computers):

  • Philosophy: Aims to speed up the execution of common operations by simplifying instructions, reducing the number of addressing modes, and focusing on optimizing frequently used operations like assignment, branching, and procedure calls.
  • Examples: Sun SPARC, MIPS.
  • Benefits: Studies show RISC machines lead to faster execution of programs due to simpler instructions, and many modern microprocessors follow this design.

• Trade-Off Between CISC and RISC:

  • CISC machines focus on complex instructions to reduce the number of instructions, whereas RISC machines aim to increase performance by simplifying instructions, often at the cost of increased instruction count.
  • The debate on which architecture is better remains unresolved, with RISC generally offering faster execution in practice.

• Performance Impact of Instruction Complexity:

  • Early studies found that over 80% of executed instructions in typical programs were assignment statements, conditional branches, and procedure calls. This finding influenced the shift to RISC design, optimizing for these common operations.

• Machine Evolution Over Time:

  • The development of computing systems shifted from CISC designs to RISC as the need for higher clock speeds and simpler, faster execution of frequently used operations became clearer. Many modern microprocessors (like those used in smartphones, desktops, and servers) adopt the RISC model for efficiency and performance.