RISC Architecture

Prof. Sin-Min Lee

Department of Computer Science

San Jose State University

The Basis for RISC

• Use of simple instructions

• One of their key realizations was that a sequence of simple instructions produces the same results as a sequence of complex instructions, but can be implemented with a simpler (and faster) hardware design. Reduced Instruction Set Computers---RISC machines---were the result.

Addressing modes

• Limited number of addressing modes

• The effective address is computed in a single clock cycle.

Instruction Pipeline

• Similar to a manufacturing assembly line1. Fetch an instruction2. Decode the instruction3. Execute the instruction4. Store results

• Each stage processes simultaneously (after initial latency)

• Execute one instruction per clock cycle

Pipeline Stages

• Some processors use 3, 4, or 5 stages

RISC characteristics

• Simple instruction set.

• In a RISC machine, the instruction set contains simple, basic instructions, from which more complex instructions can be composed.

• Same length instructions.

RISC characteristics

• Each instruction is the same length, so that it may be fetched in a single operation.

• 1 machine-cycle instructions. • Most instructions complete in one machine

cycle, which allows the processor to handle several instructions at the same time. This pipelining is a key technique used to speed up RISC machines.

Instructions Pipelines• It is to prepare the next instruction while

the current instruction is still executing. • A Three states RISC pipelines is :

1. Fetch instruction 2. Decode and select registers 3. Execute the instruction

Clock

Stage

1 2 3 4 5 6 7

1 i1 i2 i3 i4 i5 i6 i7

2 - i1 i2 i3 i4 i5 i6

3 - - i1 i2 i3 i4 i5

RISC vs. CISC

• RISC have fewer and simpler instructions, therefore, they are less complex and easier to design. Also, it allow higher clock speed than CISC. However, When we compiled high-level language. RISC CPU need more instructions than CISC CPU.

• CISC are complex but it doesn’t necessarily increase the cost. CISC processors are backward compactable.

Why RISC is better

The 80/20 rule: Analysis of the instruction mix generated by CISC compilers, shows that more than 80% of the instructions generated and executed used only 20% of an instruction set. It wasan obvious conclusion that if this 20% of instruction was speeded up, the performance benefits would be far greater. Further analysis shows that these instructions tend to perform the simpler operationsand use only the simpler addressing modes. For the CISC machine, all the effort invested in processor design to provide complex instructions and thereby reduce the compiler workload was being wasted. .

• Less cost: Since only the simpler instructions are needed, the processor hardware required to implement them could be reduced in complexity. Therefor it should be possible to design a more performance processor with less cost.

• Good performance: With a simpler instruction set, it should possible for a processor to execute its instruction in a single clock cycle. Higher performance can be achieved.

Pipelining: A key RISC technique

RISC designers are concerned primarily with creating the fastest chip possible, and so they use a number of techniques, including pipelining.

Pipelining is a design technique where the computer's hardware processes more than one instruction at a time, and doesn't wait for one instruction to complete before starting the next.

The advantages of RISCImplementing a processor with a simplified instruction set design provides several advantages over implementing a comparable CISC design: (1) Speed. Since a simplified instruction set allows for a pipelined, superscalar design RISC processors often achieve 2 to 4 times the performance of CISC processors using comparable semiconductor technology and the same clock rates. (2) Simpler hardware. Because the instruction set of a RISC processor is so simple, it uses up much less chip space; extra functions, such as memory management units or floating point arithmetic units, can also be placed on the same chip. Smaller chips allow a semconductor manufacturer to place more parts on a single silicon wafer, which can lower the per-chip cost dramatically. (3) Shorter design cycle. Since RISC processors are simpler than corresponding CISC processors, they can be designed more quickly, and can take advantage of other technological developments sooner than corresponding CISC designs, leading to greater leaps in performance between generations.

Early RISC Machines

IBM 801 1980 120 instructions No microcode 32 bit instructions MSI technology Berkeley RISC Coined RISC and CISC Promoted architecture and implementation innovations as RISC Single VLSI chip implementation Stanford MIPS Concentrated on compiler technology to improve system performance

IBM 801

Put in hardware what Could not be moved to compile time Could not be efficiently implemented in executable code by a compiler Could be implemented as random logic Architecture 32 32 bit registers Separate data and instruction caches Two stage pipeline, decode-operand fetch-execute, shift-set conditions-write Delayed branches, Branch with execute Compilers No intent on letting end users program in assembly

Berkeley RISC

Unlike IBM 801 No heavy reliance on compiler technology Single chip implementation Argues that RISC is the best way to use scarce silicon area Influential because Introduced RISC and CISC terms First single chip RISC processor Introduced several innovations at once Great marketing job

Current RISC

RISC -> SPARC MIPS -> MIPS R[2-4]000 IBM 801 -> IBM RT -> IBM RS/6000 HP-PA RISC ARM M88000 PowerPC i860 I960

Instruction PipelineAn instruction pipeline is very similar to a manufacturing assembly line. Imagine an assembly line partitioned into four stages:

• 1st stage receives some parts, performs its assembly task, and passes the results to the second stage;

• 2nd stage takes the partially assembled product from the first stage, performs its task, and passes its work to the third stage;

• 3rd stage does its work, passing the results to the last stage, which completes the task and outputs its results.

As the first piece moves from the first stage to the second stage, a new set of parts for a new piece enters the first stage. Ultimately, every stage processes a piece simultaneously. This is how time is saved. Each product requires the same amount of time to be processed (actually slightly more, to account for the transfers between stages), but products are manufactured more quickly because several are being created at the same time.

An instruction pipeline processes an instruction the way the assembly line processes a product.

• 1st stage: fetches the instruction from memory.

• 2nd stage: decodes the instruction and fetches any required operands.

• 3rd stage: executes the instruction, • 4th stage: stores the result.

Consider a nonpipelined machine with 6 execution stages of lengths 50 ns, 50 ns, 60 ns, 60 ns, 50 ns, and 50 ns. -  Find the instruction latency on this machine.    -  How much time does it take to execute 100 instructions?

Instruction latency = 50+50+60+60+50+50= 320 ns Time to execute 100 instructions = 100*320 = 32000 ns

Suppose we introduce pipelining on this machine. Assume that when introducing pipelining, the clock skew adds 5ns of overhead to each execution stage.

      - What is the instruction latency on the pipelined machine?       - How much time does it take to execute 100 instructions?

Solution: Remember that in the pipelined implementation, the length of the pipe stages must all be the same, i.e., the speed of the slowest stage plus overhead. With 5ns overhead it comes to:

The length of pipelined stage = MAX(lengths of unpipelined stages) + overhead = 60 + 5 = 65 ns Instruction latency =  6x65 ns =390nsTime to execute 100 instructions = 65*6*1 + 65*1*99  = 390 + 6435 = 6825 ns

Instructions Pipelines• It is to prepare the next instruction while

the current instruction is still executing. • A Three states RISC pipelines is :

1. Fetch instruction 2. Decode and select registers 3. Execute the instruction

Clock

Stage

1 2 3 4 5 6 7

1 i1 i2 i3 i4 i5 i6 i7

2 - i1 i2 i3 i4 i5 i6

3 - - i1 i2 i3 i4 i5

What is the speedup obtained from pipelining?   Solution: Speedup is the ratio of the average instruction time without pipelining to the average instruction time with pipelining. Average instruction time not pipelined = 320 ns

Average instruction time pipelined = 65 ns Speedup = 320 / 65 = 4.92

• Each instruction is the same length, so that it may be fetched in a single operation.

• 1 machine-cycle instructions. • Most instructions complete in one machine

cycle, which allows the processor to handle several instructions at the same time. This pipelining is a key technique used to speed up RISC machines.

This is one possible configuration of an RISC pipeline, the pipeline implemented in the SPARC MB86900 CPU. The IBM 801, the first RISC computer, also uses a four-stage instruction pipeline. Other processors, such as the RISC II, use only three stages; they combine the execute and store result operations in to a single stage.

The MIPS processor uses a five-stage pipeline; it decodes the instruction and selects the operand registers in separate

stages. These three configurations are shown in the following figure.

• Note that each stage has a register that latches its data at the end of the stage to synchronize data flow between stages. The flow of instructions through each pipeline is shown in the following Figure.

A Single Pipelined Control Unit Offers Several Advantage:

• The primary advantage is the reduced hardware requirements of the pipeline.

• A second advantage of instruction pipelines is the reduced complexity of the memory interface.

•Many video game systems like Sony Play Station and Nintendo use small (66MHZ in PS1) RISC processors. These machines are Single Purpose machines and always run the same types of programs, so small RISC processors give excellent performance results on machines like these.

• Pocket PC’s like the Palm Pilot and Compaq’s Ipaq series also use small RISC processors. Again, a machine like this is basically single purpose. Yes, you can do lot of things with them, but often you use a calendar, MP3 player, and maybe a word processor.

So, why don’t I have a RISC processor at home? (Continued)

• RISC based PC processors are still quite a bit more expensive than their CISC counterparts.

• When you write code for a RISC based machine, you are writing code native to that particular processor. Compatibility become an extreme issue – Another RISC processor using the same OS won’t be able to run software that you coded on the previous machine.

• The rather bright fellows at INTEL have come up with a solution for you. The current processor you own (provided that it is a x486 or higher) is a CRISC processor.

CRISC – I shouldn’t have to tell you what this stands for

• Intel realized that while the x86 CISC set is very large there are a few instructions that are quite common and only do one thing (ex. JMP, MOV, INC. etc.)

• Intel decided to take those common instructions, adjust them to be the same size and then hardwired them into the CPU’s core so they could be executed in a RISC like fashion.

• Yes, your Pentium III processor at home will behave like a RISC processor, sometimes. This helps gain more efficiency from the CPU while remaining backwards compatible

Why Use Pipelining?

• Pipelining allows you to start the process of executing one instruction before the previous one has completed

•Even if there are delays in any one stage of the process for one instruction, it is still more efficient than non-pipelined processors

•Pipelining is introduced with the 486 processor

Review of 6- Stage execution process

• FETCH – Instructions are fetched from a MICROCODE ROM (CISC)• DECODE – Instructions are decoded into simple code that the CPU understands (often called Micro-ops)• ISSUE/SCHEDULE – Once instructions have been decoded, they are placed into a pool and then issued to a unit (Integer, FPU, MMX) for execution• EXECUTE – The instruction is executed here• RETIRE – Results are analyzed and put back into their proper order• WRITE BACK – The results of the instructions are written to memory (committed to code)

Super Scalar

• Put simply, a super scalar processor has two or more integer execution units that run in parallel (they can execute instructions simultaneously)

• The Pentium Processor is the first INTEL super scalar processor

• The scheduling unit can issue instructions simultaneously to different units to be executed at the same time

Data Flow

Performance Improvement

• The speedup is the ratio of the time needed to process n instruction using a non-pipelined control unit to the time needed using a pipelined control unit

Sn = n T1 / (n + k -1) Tk

Pipeline Problems

• Memory access– Fetch an instruction in one clock cycle– Include cache memory

• Branch statements– The instruction that are in pipeline should not

be there

Register Windowing

• More than 100 registers, not always accessible

• Global registers are always accessible

• The remaining registers are windowed, accessible at specific times

SPARC Processor Register Windowing

Keeping Track

• A window point register contains the value of the window that is currently active

• A window mask register contains 1 bit per window and denotes which windows contain valid data.

Subroutine Calls

• Register windows provide greatest benefit during subroutine calls

• During the calling process, the register window is moved down one position.

• CPU can pass parameters to the subroutine via the registers that overlap

• Same register can be used to return results to the calling routine.

Example

Example (cont)

RISC Advantages

• RISC have fewer and simpler instructions.– Their control units are less complex and easier

to design– Run at higher clock frequencies– Reduced amount of space needed on the

processor chip -> more space for additional registers

– Easier to incorporate parallelism– Compilers are less complex

CISC Advantages

• New complex processors incorporate the design of the previous designs.

• Backward compatibility with other processors in their series.