Complex Instruction Set Computer (CISC)

1. Complex Instruction Set Computer(CISC) Manish Kulkarni Department of Electrical and Computer Engineering Auburn University, Auburn, AL 36849 mmk0002@auburn.edu Computer Architecture & Design (6200) Class Presentation

2. Overview • What is CISC and Why to learn? • History • Architecture • Typical x86 design • Characteristics & Addressing modes • CISC Vs RISC • Example Programs • The Performance Equation • FAQs • Recent Developments & Future Scope • Resources • Questions Computer Architecture & Design (6200) Class Presentation

3. What is CISC? • Definition: Pronounced "sisk" and standing for Complex Instruction Set Computer, is a Microprocessor Architecture that aims at achieving complex operations with single instructions and favors the richness of the instruction set (typically as many as 200 unique instructions) over the speed with which individual instructions are executed. Why should I know about CISC? • Today’s computers still use processors which are based on CISC designs • It has been a prominent architecture since 1978 • Most Emerging Processor designs combine features of CISC and RISC to create better designs. Computer Architecture & Design (6200) Class Presentation

4. History Continued…. Computer Architecture & Design (6200) Class Presentation

5. Continued…. Computer Architecture & Design (6200) Class Presentation

6. A typical x86 Architecture Architecture Intel 8086 Architecture, the 1st member of x86 family Computer Architecture & Design (6200) Class Presentation

7. Characteristics • CISC are Mostly Von Neumann Architecture (There are few exceptions) • Same bus for program memory, data memory, I/O, registers, etc • Generally Micro-coded ,Variable length instructions • Segmentation is possible with Segment Register s like DS, ES and an offset which • can be common to all segments. • Many powerful instructions are supported, making the assembly language • programmer’s job much easier. • Physical Memory Extension Possible Addressing modes • Register Addressing Mode • Memory Addressing Modes • Displacement Only Addressing Mode • Register Indirect Addressing Modes • Indexed Addressing Modes • Based Indexed Addressing Modes • Based Indexed Plus Displacement Addressing Computer Architecture & Design (6200) Class Presentation

8. CISC Vs RISCExample Program Main Memory General Purpose Registers ALU Computer Architecture & Design (6200) Class Presentation

9. Consider following task of Multiplication 15 Operands: M[2:3] = operand 1 (15) M[5:2] = operand 2(20) Task : Multiplication Result: M[2:3] <= result 20 Computer Architecture & Design (6200) Class Presentation

10. The CISC Approach • Instruction : MULT 2:3, 5:2 • Operations: • Loads the two operands into separate registers • Multiplies the operands in the execution unit • Then stores the product in the some temporary register • Stores value back to memory location 2:3 • MULT is what is known as a "complex instruction." • Operates directly on the computer's memory banks • Does not require the programmer to explicitly call any loading or storing functions. • closely resembles a command in a higher level language. • e.g. a ‘C’ statement "a = a * b." Computer Architecture & Design (6200) Class Presentation

11. The RISC Approach • Instructions : • LW A, 2:3LW B, 5:2MULT A, BSW 2:3, A • Operations: • Load operand1 into register A • Load operand2 into register B • Multiply the operands in the execution unit and store result in A • Store value of A back to memory location 2:3 • These set of Instructions is known as a “Reduced Instructions." • Cannot Operate directly on the computer's memory banks • Requires the programmer to explicitly call any loading or storing functions. • RISC processors only use simple instructions that can be executed within one clock cycle Computer Architecture & Design (6200) Class Presentation

12. CISC RISC • Primary goal is to complete a task in as few lines of assembly as possible • Emphasis on hardware • Includes multi-clockcomplex instructions • Memory-to-memory:"LOAD" and "STORE"incorporated in instructions • Small code sizes • High cycles per second • Variable length Instructions • Primary goal is to speedup individual instruction • Emphasis on software • Single-clock,reduced instruction only • Register to register:"LOAD" and "STORE"are independent instructions • Large code sizes • Low cycles per second • Equal length instructions which make pipelining possible Computer Architecture & Design (6200) Class Presentation

13. The Performance Equation • The following equation is commonly used for expressing a computer's performance ability: • The CISC approach • minimizes the number of instructions per program (2) • sacrificing the number of cycles per instruction. (1) • RISC does the opposite • reduces the cycles per instruction (1) • sacrificing number of instructions per program (2) 1 2 Computer Architecture & Design (6200) Class Presentation

14. FAQs Which one is faster? Well, it is commonly accepted that RISC ISA's should make computers faster. The main reason why is because RISC computers figure out more words in a shorter amount of time due to pipelining. • So why isn't my computer a RISC? • CISC ISA's were implemented in the first personal computers • With more people buying computers, CISC isa's became more prominent • Software (especially OS) was developed and "translated" so that personal computers speaking x86 would be able to interact with its users • Because there was so much software written for computers "speaking" x86, people continued to buy those computers. • If we tried to switch to another ISA, we would not have all of the software choices we have now. Computer Architecture & Design (6200) Class Presentation

15. So why would someone want to develop another ISA? • x86 (and CISC) make poor use of the faster hardware we have now. • Another problem with x86 is that people have been trying to make it faster for a long time, at least 20 years, and after a while you have found most of the ways to speed the computer up significantly • Why don't we just switch to RISC? • Although it is not used on your desktop PC, RISC ISA's are implemented in many mainframe computers. • Programmers have been trying to make RISC faster for a long time, and they have found many of the areas in which it is able to be sped up significantly. Computer Architecture & Design (6200) Class Presentation

16. Where are we running into problems speeding up RISC and CISC? • We are running into problems with speeding up the computer in 2 areas • Branching Decisions and predictions consume good amount of processing time • Access to memory to fetch instruction and data So What we are going to do? Computer Architecture & Design (6200) Class Presentation

17. Recent Developments & Future Scope • The terms RISC and CISC have become less meaningful with the continued evolution of both CISC and RISC designs and implementations. • Modern x86 processors also decode and split more complex instructions into a series of smaller internal "micro-operations" which can thereby be executed in a pipelined (parallel) fashion, thus achieving high performance on a much larger subset of instructions. • Attempts have been made to combine features of both RISC and CISC to develop a new approach • Intel has teamed up with Hewlett-Packard to design a new type of ISA. They are calling it IA-64 (Intel Architecture 64) Computer Architecture & Design (6200) Class Presentation

18. IA-64 • What is IA-64? • IA-64 is a new instruction set architecture. • IA-64 seeks to address: branch delays and memory latency. • What main principles is IA-64 designed around? • IA-64 seeks to exploit instruction level parallelism to the highest degree. • Intel and HP have called their method of exploiting this parallelism in IA-64 EPIC (Explicitly Parallel Instruction Computing). • EPIC simulates parallelism by having the compiler find what instructions can be executed in parallel and "explicitly" package them for the CPU. • How does IA-64 help with branch delays? • IA-64 takes a unique approach of prediction to reduce the consequences of branch delays. • The compiler can append a predicate to any instruction it chooses. The compiler will append predicates to instructions that depend on the outcome of a branch in order to help reduce branch penalties. Computer Architecture & Design (6200) Class Presentation

19. How does IA-64 deal with memory latency issues? • Memory latency occurs because CPU processing speed is significantly faster than the speed of fetching data from memory. • IA-64 suggests a new way to eliminate some memory latency problems, speculative loading. • IA-64 Realities: • "A study in ISCA '95 by S. Malhlke, et. al. demonstrated that predication removed over 50% of the branches and 40% of the mis-predicted branches from several popular benchmark programs." ( http://www.hp.com/esy/technology/ia_64/products/isapress.html ) • IA-64 lack compatibility with Intel x86 and HP PA-RISC architectures, so this additional compatibility logic will take lot of die space. • Presently, the compilers are in experiment phase and IA-64 has no OS support. Computer Architecture & Design (6200) Class Presentation

20. Resources • http://www.pctechguide.com/glossary/WordFind.php?wordInput=CISC • http://www.cs.umd.edu/class/fall2001/cmsc411/projects/IA64/ • http://cse.stanford.edu/class/sophomore-college/projects-00/risc/risccisc/index.html • http://en.wikipedia.org/wiki/Complex_instruction_set_computer • http://en.wikipedia.org/wiki/X86 • http://arstechnica.com/cpu/4q99/risc-cisc/rvc-6.html Computer Architecture & Design (6200) Class Presentation

21. Questions ?? Computer Architecture & Design (6200) Class Presentation