CISC VS CISC
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The document discusses CPU architecture and instruction set architecture (ISA), comparing CISC (Complex Instruction Set Computing) and RISC (Reduced Instruction Set Computing) philosophies. CISC aims for fewer lines of code with varying instruction sizes, leading to slower performance, while RISC uses uniform, simple instructions executed in one cycle, enhancing speed but requiring more memory. The CPU performance equation is detailed, highlighting the balance between the number of instructions and cycles per instruction for both architectures.
CISC VS CISC
- 2. Agenda • CPU Architecture. • Instruction Set Architecture (ISA). • CISC Philosophy. >>> CISC Pros & Cons. • RISC Philosophy. >>> RISC Pros & Cons. • CPU Performance Equation. • Summary.
- 4. Instruction Set Architecture (ISA) • Group of instructions to execute a program. • Instructions are in the form of: Opcode + Operand. • An agreement between hardware and human for making interaction • Example : ADD R1, R2, R3 • Can be represented as : 00101111100001111001010101010101 10111010100011110101001011011010 • Two major schools of ISA: CISC & RISC.
- 5. CISC Philosophy (Complex Instruction Set Computing) • The primary goal is to complete a task in as few lines as possible. • Used on PCs and laptops that need to process heavy graphics and computations. • Each instruction consist of one step. (ex: MULT 2:3, 5:2, load the two values into registers, multiplies the operands, and then stores the product in appropriate register).
- 6. CISC Pros & Cons • Instruction size is different from one operation to another. • Operation size is smaller but no of cycles are more. • Needs better hardware and powerful processing. • Performance is slow due to the amount of clock time taken by different instructions.
- 7. RISC Philosophy (Reduced Instruction Set Computing) • Use only simple instructions that can be executed within one clock cycle. • Keep all instructions of same size. • Allow only load/store instruction to access the memory. (ex: MULT command divided into three separate commands: LOAD, PROD, and STORE).
- 8. RISC Pros & Cons • Allow free use of microprocessors space because of its simplicity. • Needs large memory caches on the chip itself so require very fast memory. • Give support for high level languages (like C, C++, Java). • Performance depends on the programmer or compiler.
- 9. CPU Performance Equation • The following equation is commonly used for expressing a computer's performance ability: • CISC minimize the number of instructions per program. • RISC does the opposite, reduce the cycles per instruction. 𝐶𝑃𝑈 𝑇𝑖𝑚𝑒 = 𝑆𝑒𝑐𝑜𝑛𝑑𝑠 𝑃𝑟𝑜𝑔𝑟𝑎𝑚 = 𝐼𝑛𝑠𝑡𝑟𝑢𝑐𝑡𝑖𝑜𝑛𝑠 𝑃𝑟𝑜𝑔𝑟𝑎𝑚 𝑥 𝐶𝑦𝑐𝑙𝑒𝑠 𝐼𝑛𝑠𝑡𝑟𝑢𝑐𝑡𝑖𝑜𝑛𝑠 𝑥 𝑆𝑒𝑐𝑜𝑛𝑑𝑠 𝐶𝑦𝑐𝑙𝑒
- 10. Summary