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Introduction to MMX, XMM, SSE and SSE2

Technology

Multimedia Extension,Streaming SIMD Extension

11/23/98, 5/6/99, 2/5/03, 5/10/04, 5/4/05

2SISD - Single Instruction, Single Data

Traditional computers

In general, one instruction processes one data item

Control Unit

ExecutionUnit

Memory

3SIMD - Single Instruction, Multiple Data

One instruction can process multiple data items

Useful when large amounts of regularly organized data is processed

Example: Matrix and vector calculations

This is the basis of MMX and XMM

Control Unit

Memory

ExecutionUnits

4

MISD

MemoryControl

UnitExecution

Units

MISD: Multiple instructions process one data item.

5

MIMD

Control Unit

Memory

ExecutionUnit

Control Unit

ExecutionUnit

Control Unit

ExecutionUnit

Control Unit

ExecutionUnit

MIMD: Multiple instructions process multiple data items.

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Your Turn

How would you classify a traditional computer under this system?

How would you classify a Shemp which has multiple processors?

How would you classify a computer having a Intel Dual Core processor?

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Potential Applications MMX and SSE

graphics MEG video/image processing music synthesis speech compression/recognition video conferencing matrix and vector calculations Advanced 3D graphics (SSE2) Speech recognition (SSE2) Scientific and engineering applications (SSE2)

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MMX

4 new data types New instructions Uses 8 existing 64 bit floating point

registers

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The floating point registers Floating point is processed by eight 80 bit registers

ST(0), ST(1), …ST(7) in the floating point unit. When doing floating point arithmetic, these registers

are organized in a stack. Programming floating point is quite different that

programming integer arithmetic. Floating point calculations are done using 80 bits even

when the program specifies storing 32 or 64 bit data values.

10Advantages of using the floating point registers in MMX. The registers already exist. Only logic had to be added to

the chip. The operating system already knows about the floating point

registers. When a computer is switches from one program to another,

the state (registers) of the current program must be saved so state can be restored when the program becomes the active program once again.

The floating point registers are automatically saved as part of the state of a program.

MMX worked under existing operating systems!

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New data types for MMX 64 bits long. One data item can store:

8 one byte integers:

4 two byte integers:

2 four byte integers

1 eight byte integer

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SSE and SSE2

SSE – Streaming SIMD Extensions SSE2 introduced eight 128 bit XMM registers These registers are disjoint from the floating

point/MMX registers SSE (Pentium III) can handle 4 single floating

point numbers SSE2 (Pentium 4) can also handle 2 double

floating point numbers

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New data types for XMM 128 bits: Can be used as:

16 one byte integers

8 two byte integers

4 doubleword integers or single precision floating

2 quadword integers or double precision floating

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Your turn Your program uses 3 arrays of 160,000

byte integers. We need to add the elements in the first two arrays to calculate the third array.

Using a standard Pentium, how many “operations” are needed? (One operation includes loading 2 values into CPU, adding, storing the result and the associate loop processing)

How many XMM operations would be needed?

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New instructions Process the new data types 16, 8,4, or 2 data items

(64 bits or 128 bits) at a time. Types of instructions:

Add / SubtractMultiply/Multiply and addShiftLogical (AND, NAND, OR, XOR)Pack and unpackMoveShuffle and unpack (SSE)

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Saturation

Handling overflow when adding 16, 8, 4, or 2 values at a time is a problem. Programmers can specify that when overflow occurs, the “sum” should be replaced by the maximum legal value.

Example: Unsigned byte addition 80h + A0h = 120h ===> overflow Instead the machine stores FFh.

Likewise when subtracting.

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Comparison operations

Consider <, >, <=, >=, =, and < > operations.

Consider comparing two 64 bits quantities each holding 8, 4, or 2 values.

Comparing multiple values at a time is a problem. So the MMX instructions store 0 for false and -1 for true for each of individual data items.

18Example 1: Calculating Dot Products

7 Consider calculating S = AiBi

i = 0using MMX

Assume Ai and Bi are stored as signed 16 bit integers.

Assume that the products and sums should be calculated using 32 bits.

Assume that all values have two “binary” places.

19Example 1: Calculating Dot Products Storing A and B (64 bit vectors)

0 2 4 6 8 10 12 14 bytes 0 1 2 3 4 5 6 7 subscriptsA

B

We store each Ai and Bi item as 16 bit integers, 4 per 64

bit data item. Assume each value has 2 binary places

20Example 1: Calculating Dot Products

Multiply and add instruction

* * * * * * * *

+ + + +

2 20

3 40

806

4 30

5 50

1520

21Example 1: MMX: Calculating Dot Products

Packed Multiply and add instruction

* * * * * * * *

+ + + +

Packed Add + +

(Normal) Add +

2 20

3 40

806

4 30

5 50

1520

2326

22Example 1: Calculating Dot Products

Approximate algorithm – Load left half of A into a FP register. – Multiply and add by left half of B.– Shift products right 2 bits. (Products should have

only two binary places.)– Repeat with right halves of A and B using a

different register.– Add the second sum to the first.– Store the result.

4 w

ords

at a

tim

e

Two doublewords

at a time

23Example 1: Calculating Dot Products

Approximate algorithm (Conclusion)– Add the two sums together in EAX to get the

final sum.

1 do

uble

wor

d at

a ti

me

24Example 1: Calculating Dot Products

Intel claims that standard Pentiums would require 40 instructions to carry this out. Using MMX technology, only 13 instructions are needed. Speed improves by even a greater ratio.

25Example 2:24-bit color video blending

Suppose we have are displaying 640 by 480 pixel video that uses 24 bit colors - 8 bits for red, 8 for green, and 8 for blue.

Suppose we are currently showing one picture which we want to fade out and replace by “fading” in a second picture.

Suppose that we want to do the fade out/in in 255 steps.

26Example 2:24-bit color video blending

For each step, for each of 3 colors and for each of the 640 by 480 pixels we must calculate:Result_pixel = NewPicture_pixel * (i/255) +OldPicture_pixel * (1-(i/255))where “i” is the step counter.

This formula must be calculated640 * 480 * 3 * 255 = 235,008,000times on 8 bit data!

27Example 2:24-bit color video blending

Intel calculates that this requires execution of 1.4 billion instructions on a standard PC even ignoring the calculation of i/255 and (1-i/255) and loop control.

With MMX, we can calculate 4 values in parallel. The number of MMX instructions would be 525 million. (Because the multiply instruction only applies to word data, the byte data must be unpacked into words and repacked after the calculation.)

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Also included in MMX Intel increased cache size when MMX was

introduced (necessary for SIMD machines) Programs run faster on MMX machines even if

the SIMD instructions are not used Excellent marketing:

– Programs run faster on MMX machine

– People want/buy MMX

– Software publishers are encouraged to rewrite programs to take advantage of the new instructions

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Information source http://www.intel.com/drg/mmx/manuals/overview/i

ndex.htm#intro

(no longer available) http://developer.intel.com/drg/mmx/manuals/

(no longer available) http://www.intel.com/design/Pentium4/manuals/24

547012.pdf (IA-32 Intel Architecture Software Developer’s Manual, vol. 1)

This slide show is MMX.PPT

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Your Turn

1. Characterize the kinds of problems where SIMD is helpful.

2. Give examples of problems where SIMD is useful.