Architectural Design Philosophies (ISA Philosophies)

• Describe how ISA is shaped, not how the CPU internally executes instructions.

CISC (Complex Instruction Set Computer)

• CISC is a CPU design philosophy that favors powerful, complex instructions.

• Original goals (1970s)

  • reduce number of instructions per program

  • make compilers simpler

  • save memory (important at the time)

• Typical characteristics

  • variable-length instructions

  • many addressing modes

  • instructions that perform multiple operations

  • memory-to-memory operations possible

  • irregular encoding

• One instruction, many internal steps.

• Example ISA

  • x86

• Example sequence (conceptual):

  add [mem1], [mem2]
  

RISC (Reduced Instruction Set Computer)

• RISC is a philosophy favoring simple, uniform instructions.

• Original goals (1980s)

  • make pipelines efficient

  • enable higher clock speeds

  • simplify hardware

  • rely more on the compiler

• Typical characteristics

  • fixed instruction width

  • load/store architecture

  • many general-purpose registers

  • simple addressing modes

  • usually one operation per instruction

• Example ISAs

  • ARM

  • MIPS

  • RISC-V

• Example sequence:

  • Instead of one complex instruction:

  ldr r0, [mem1]
  ldr r1, [mem2]
  add r0, r0, r1
  str r0, [mem1]
  

  • More instructions, but each is simple and predictable.