The Intel Pen Ti Um Processor

8/9/2019 The Intel Pen Ti Um Processor

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The Intel Pentium

Processor

Bogdan Ilisie

&Rika Kanai

Pentium Pentium Pro Pentium II Pentium III


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The first Intel Pentium Introduced to market on March 22, 1993 with a CPU

clock cycle of 66 Mhz

With its coming, it hosted many innovations, the

most notable being:

Superscalar architecture

Dynamic Branch Prediction

Pipelined Integer Unit

These features made the newly introduced chip a very popular choice for desktop,

although it was later found that the processor had some notorious implementation

errors.

Pipelined Floating-Point Unit


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The Pentium CPU (MMX)


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The Pentium pipelined Integer Unit supports 5 stages:

1) Pre-fetch

2) Decode

3) Address generate

4) EX Execute - ALU and Cache Access

5) WB Writeback

Although different later processors like the MMX tampered with the 5 execution

steps(by adding intermediate LIFO structures to hold bulks of instructions), the steps

remain the core foundation of the pipelining.

As it can be seen from the previous diagram, the Integer unit has two pipelines(U and

V),while the Floating Point Unit (FPU) has one pipeline.


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1) In the Pre-fetch cycle, two pre-fetch buffers read instructions to be executed. Instructions can be fetched from the

U or V pipeline. The U pipeline contains more complex instructions.

2) In the Decode cycle, two decoders, decode the instructions and try to pair them together so they can run in parallel,

since the Pentium features a Superscalar architecture.

Even though the Pentium processor features a Superscalar architecture,

in order for two instructions to run concurrently, like in the diagram

below, they need to satisfy some rules. Essentially, the instructions have

to be independent otherwise they cannot be paired together.

3) In the second Decode stage, or the address generate stage, the

addresses of memory operands are calculated. After these calculations, the

EX stage of the pipeline is ready to execute.


A Floating Point instruction cannot be paired with an Integer

instruction.


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Pipelined Integer Unit(Conclusion)

4) In the Execution cycle, the ALU is reached.

5) In the Write Back cycle, information is written back to the registers.

If two instructions are executing concurrently in the pipeline (given they satisfy the

proper conditions, and are independent) and one of them stalls as a result of hazard

control, the other one will also stall.

For two instructions to be paired together in the Decode stage, they have to lack

dependencies.

The two paired instructions would also have to be basic, in the sense that they contain

no displacements or immediate addressing.

As it can be deduced, pipelines will sometimes execute an instruction at the time,

despite the Superscalar ability.


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Branch Prediction

Other than the Superscalar ability of the Pentium processor, the

branch prediction mechanism is a much-debated improvement.

Predicting the behaviors of branches can have a very strongimpact on the performance of a machine. Since a wrong

prediction would result in a flush of the pipes and wasted cycles.

The branch prediction mechanism is done through a branch target

buffer. The branch target buffer contains the information about all

branches.

The prediction of whether a jump will occur or no, is based on the branchs previous behavior.There are four possible states that depict a branchs disposition to jump:

Stage 0: Very unlikely a jump will occur

Stage 1: Unlikely a jump will occur

Stage 2: Likely a jump will occur

Stage 3: Very likely a jump will occur


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Branch Prediction

When a branch has its address in the branch target buffer, its

behavior is tracked.

This diagram portrays the four stages associated branch

prediction.

If a branch doesnt jump two times in a row, it will go down to

State 0.

Once in Stage 0, the algorithm wont predict another another

jump unless the branch will jump for two consecutive jumps (so

it will go fromS

tate 0 toS

tate 2)

Once in Stage 3, the algorithm wont predict another nojump

unless the branch is not taken for two consecutive times.


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Branch PredictionIt is actually believed that Pentiums algorithm for branch

prediction is incorrect.

As it can be seen in the diagram to the right, State 0 will jump

directly to State 3, instead of following the usual path which

would include State 1, and State 2.

This abnormality might be attributed to the way in which thebranch target buffer operates:

- If a branch is not found in the branch target buffer, then it

predicted that it wont jump.

- A branch wont get an actual entry in the branch target buffer,

until the first time it jumps, and when it does, it goes straight into

State 3.

- Because the branch wont get an entry into the branch targetbuffer until the first time it jumps, this will cause an alteration

into the actual state diagram, as it can be clearly seen.

More information about this problem can be found at http://x86.ddj.com/articles/branch/branchprediction.htm


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Branch Prediction (in later Pentium Models)

The Intel Pentium branch prediction algorithm is indeed better than a 50% guess, but it has

limitations.

In a need to increase the accuracy of branch predictions, the processors following the Pentiumadopted a different branch prediction algorithm.

Some loops have repetitive patterns and they need to be recognized. With a two bit binary counter,

it is impossible to attain any complexity.

Later generation processors, such as the Pentium MMX, Pentium Pro, Pentium II, use another

mechanism for branch prediction.

A 4 bit register is used to record the previous behavior of the branch. If the 4 bit register would be

0001, it would mean that the branch only jumped the last time out of 4.

A 4 bit register would not be of much use without any additional logic. In addition to the 4 bit

register, there are 16, 2-bit counters like the ones that were previously shown.


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Branch Prediction

(in later Pentium Models)

A 4 bit register that records the behavior of the branch

along with 16 2-bit counters, the mechanism is able to

give more accurate branching predictions.

Since the register has 4 bits, it has 16 possible values,

so the current value of the 4 bit register can always beassociated with one of the 16 bit counters, like it is

shown in the diagram to the right.

Each value in the 4 bit register, represents a trend of

that branch.

For each trend, we must be able to predict the next

value.

Since each register value will be pointing to a different 2-bit counter, the state of the 2-bitcounter will most likely return the correct prediction for that particular register pattern.

Therefore, by combining a 4 bit register that records past trends, with 16 individually updated 2-

bit counters, we end up with a much stronger mechanism for prediction, which is currently used

in Pentium MMX, Pentium II, and others.


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Newer Generation Chips

The next move up from Pentium was Pentium MMX.

The Pentium MMX, includes new instructions, registers, and data types which are aimed at

maximizing the speed of multimedia computations.

Since multimedia work requires massive data manipulation, SIMD instructions were added to

the MMX set. SIMD instructions work on multiple data values at once, in order to maximize

the amount of work done by each instruction.

The improved multimedia support of the MMX, along with lower power consumption, larger

caches, and new branch prediction mechanisms, brought about the new generations of

Pentiums (II & III)

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The Intel Pen Ti Um Processor