Abstract:
In a computer system storing a first packed data and a second packed data having corresponding data elements where the data elements representing unsigned values having a method for determining the absolute difference of the corresponding data elements. The method comprising the steps of subtracting with saturation the data elements in the first packed data from the corresponding data elements in the second packed data to generate a third packed data in response to a first instruction, subtracting with saturation the data elements in the second packed data from the corresponding data elements in the first packed data to generate a fourth packed data in response to a second instruction and performing an operation to select the data elements of the third packed data and the fourth packed data of greatest value to generate a fifth packed data in response to a third instruction.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     U.S. application Ser. No. 08/576,469, titled &#34;System for Providing The Absolute Difference of Unsigned Values, filed Dec. 21, 1995, now pending. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to the field of computer systems. More specifically, the invention relates to motion estimation in multimedia applications. 
     2. Background Information 
     Multimedia applications (e.g., applications targeted at computer supported cooperation (CSC--the integration of teleconferencing with mixed media data manipulation), 2D/3D graphics, image processing, video compression/decompression, recognition algorithms and audio manipulation) require the manipulation of large amounts of data which may be represented in a small number of bits. For example, graphical data typically requires 8 bits and sound data typically requires 16 bits. Each of these multimedia application requires one or more algorithms, each requiring a number of operations. For example, an algorithm may require an add, compare and shift operations. 
     To improve efficiency of multimedia applications (as well as other applications that have the same characteristics), prior art processors provide packed data formats. A packed data format is one in which the bits typically used to represent a single value are broken into a number of fixed sized data elements, each of which represents a separate value. For example, a 64-bit register may be broken into two 32-bit elements, each of which represents a separate 32-bit value. In addition, these prior art processors provide instructions for separately manipulating each element in these packed data types in parallel. For example, a packed add instruction adds together corresponding data elements from a first packed data item and a second packed data item. Thus, if a multimedia algorithm requires a loop containing five operations that must be performed on a large number of data elements, it is desirable to pack the data and perform these operations in parallel using packed data instructions. In this manner, these processors can more efficiently process multimedia applications. 
     In performing video conferencing in multimedia applications, video data from a camera is first encoded. The encoded data is then transmitted though a channel (a CD ROM, disk) to be processed by a decoder. The decoder then outputs frame information to be displayed. The encoding typically requires the step of performing a motion estimation. Motion estimation is the process of estimating displacement of the moving objects in a video sequence. Such estimation requires searching for the best matches between data sets of a given current frame and a given reference frame, where a current frame is what is displayed at the present and a reference frame is what needs to be displayed. The sum of absolute differences is commonly used for such estimation. The displacement information is used to interpolate missing frame data or to improve the performance of compression algorithms. The searching step is typically performed by using conventional scalar mechanism to calculate the sum of absolute differences. This is performed by subtracting each pixel value in the current frame from its corresponding pixel in the reference frame, taking the absolute value of the result. This may involve a branch operation and summing the results into an accumulator. 
     The conventional scalar mechanism of performing a search for matching data sets using branching operations leads to sacrificing processing speed by paying for branch prediction penalties. More specifically, some implementations for performing branching operations try to speed up execution by attempting to reduce pipeline penalties that can result from branches by speculatively predicting where branches will go with either compile-time schemes (e.g. predict-not-taken or predict-taken) or hardware schemes (e.g. branch-prediction buffer). If a prediction is correct, the proper instruction will have been fetched and decoded in advance and there is a potential for a gain in performance. However, if a prediction is wrong, performance is lost. For example, assume the prediction scheme has predicted that the branch will be taken in the above pseudo code representation and further assume that the branch condition is not actually met. By the time the branch condition is evaluated, the pipeline already contains the decoded STORE X,C instruction. Thus, when the branch condition is determined to be false, the correct instruction (the STORE X,B instruction) must be fetched and decoded, effectively causing the pipeline to stall. This example illustrates the uncertain nature of branches based on data. The inherent problem with this technique is that branches on data are poorly predicted by branch prediction schemes. As a result, branch prediction penalties are frequently paid in situations similar to the one illustrated above. 
     Thus, a limitation of this and other prior methods of performing searches for matching data in motion estimation using branching operations, is the high cost to performance due to mispredicted branches. The processing time required to perform such branching operation increases the overall processing time. Therefore, it is desirable to incorporate in a computer system a method and an apparatus for performing the searching step required in motion estimation without the need for branching and therefore without having to sacrifice processing speed. 
     SUMMARY 
     In a computer system storing a first packed data and a second packed data having corresponding data elements where the data elements representing unsigned values having a method for determining the absolute difference of the corresponding data elements. The method comprising the steps of subtracting with saturation the data elements in the first packed data from the corresponding data elements in the second packed data to generate a third packed data in response to a first instruction, subtracting with saturation the data elements in the second packed data from the corresponding data elements in the first packed data to generate a fourth packed data in response to a second instruction and performing an operation to select the data elements of the third packed data and the fourth packed data of greatest value to generate a fifth packed data in response to a third instruction. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may best be understood by referring to the following description and accompanying drawings which illustrate the invention. In the drawings: 
     FIG. 1 shows a block diagram illustrating an exemplary computer system according to one embodiment of the invention; 
     FIG. 2 is a block diagram of a motion estimation unit as it relates to the computer system illustrated in FIG. 1; 
     FIG. 3 is a block diagram illustrating an exemplary motion estimation unit with an exemplary implementation of the present invention used in calculating the absolute differences of unsigned packed data; 
     FIG. 4 is a flow diagram illustrating the general steps followed in performing an absolute differences calculation on unsigned data according to one embodiment of the invention; 
     FIG. 5 illustrates the operation of an absolute differences calculation on unsigned data according to one embodiment of the invention; 
     FIG. 6 illustrates a packed add operation which can be used in one embodiment of the present invention in calculating the absolute difference of unsigned data. 
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are set forth to provide a thorough understanding of the invention. However, it is understood that the invention may be practiced without these specific details. In other instances, well-known circuits, structures and techniques have not been shown in detail in order not to obscure the invention. 
     According to one aspect of the invention, a method and apparatus for calculating absolute differences of unsigned numbers for unsigned packed data is described. In one embodiment of the invention, packed subtractions are performed with unsigned saturating arithmetic. Unlike prior art methods, no branching operations are necessary. 
     FIG. 1 shows a block diagram illustrating an exemplary computer system 100 according to one embodiment of the invention. The exemplary computer system 100 includes a processor 105, a storage device 110, and a bus 115. The processor 105 is coupled to the storage device 110 by the bus 115. In addition, a number of user input/output devices, such as a keyboard 120, a camera 123 and a display 125, are also coupled to the bus 115. A network 130 may also be coupled to bus 115. The processor 105 represents a central processing unit of any type of architecture, such as a CISC, RISC, VLIW, or hybrid architecture. In addition, the processor 105 could be implemented on one or more chips. The storage device 110 represents one or more mechanisms for storing data. For example, the storage device 110 may include read only memory (ROM), random access memory (RAM), magnetic disk storage mediums, optical storage mediums, flash memory devices, and/or other machine-readable mediums. The bus 115 represents one or more busses (e.g., PCI, ISA, X-Bus, EISA, VESA, etc.) and bridges (also termed as bus controllers). While this embodiment is described in relation to a single processor computer system, the invention could be implemented in a multiprocessor computer system. In addition, while this embodiment is described in relation to a 64-bit computer system, the invention is not limited to a 64-bit computer system. 
     FIG. 1 also illustrates that the storage device 110 has stored therein complex data 135 and software 136. Complex data 135 represents data stored in one or more of the formats described herein. Software 136 represents the necessary code for performing any and/or all of the techniques described with reference to FIGS. 3, 4 and 5. Of course, the storage device 110 preferably contains additional software (not shown), which is not necessary to understanding the invention. 
     FIG. 1 additionally illustrates that the processor 105 includes an execution unit 140 for executing instruction. The execution unit 140 is shown including packed data instruction set 145 for performing operations on packed data. In one embodiment, the packed data instruction set 145 includes the following instructions: a packed or instruction 146, a packed subtract unsigned with saturation instruction 147, a packed multiply-add instruction 150, a pack with unsigned saturation instruction 152, a pack instruction 155, an unpack/interleave instruction 160, a packed shift instruction 165, an XOR instruction 170, and a packed add instruction 175. Alternative embodiments of the invention may contain more or less, as well as different, packed data instructions and still utilize the teachings of the invention. Of course, the processor 105 contains additional circuitry, which is not necessary to understanding the invention. The term data processing system is used herein to refer to any machine for processing data, including the computer system(s) described with reference to FIG. 1. 
     FIG. 2 is a block diagram of a motion estimation unit with an exemplary implementation of the present invention as it relates to the computer system illustrated in FIG. 1. Camera 123 of computer system 100 captures external images for display at remote computer system 214&#39;s display device 212. The captured image is transmitted to encoder 202 of computer system 100 for processing. Encoder 202 is part of software 136 illustrated in FIG. 1. In one embodiment of the present invention, an H.261 video conferencing standard may be used to process video data. With this standard, a given reference frame which is the image captured by camera 123 is altered using the information on the differences between the reference frame and a current frame. The current frame as used herein refers to the image currently being displayed by remote display device 212. The altered reference frame results in a new current frame to be displayed on remote display device 212. Motion estimation unit 204 of encoder 202 estimates the displacement of moving objects in a video sequence of the transmitted data (the reference frame) and the current frame on remote display device 212. The encoded information is then transmitted to remote computer system 214&#39;s decoder 208 through some type of transmission channel (e.g. CD ROM, disk, and telephone line). Motion compensation unit 210 of decoder 208 then outputs a new current frame for display on display device 212. 
     FIG. 3 is a block diagram illustrating an exemplary implementation of a motion estimation unit for calculating absolute differences of unsigned data. Motion estimation unit 202 performs motion estimation in video algorithms for encoding video streams. Motion compensation unit 210 decodes the motion vector produced by the motion estimation process of motion estimation unit 202 and reconstructs the original video streams. 
     In FIG. 3, encoder 202 receives video stream input which is processed by motion estimation unit 204. Motion estimation unit 204 estimates the displacement of moving objects in a video sequence, by for example, examining blocks of pixels (e.g. 16×16 or 8×8) from the current frame and comparing the data block form the current frame to a data block of a reference frame for matching data sets. Energy differences is used as a metric for finding the best matches between the data sets and the sum of absolute differences is used for the energy difference calculations by motion estimation unit 204. More specifically, motion estimation unit 204 with the implementation of the present invention performs an absolute difference calculation on unsigned packed data. The details of such a method is illustrated in FIGS. 4 and 5 and the accompanying text. 
     Motion estimation unit 204 produces residual information 302 and motion vector 312. Transform unit 304 receives the residual information 312. Residual information 312 are pixel differences between a block of the current frame and a matching block of a reference frame. When the search for matching data sets is performed by motion estimation unit 204, an absolute difference calculation is performed on unsigned packed data. The residual information produced is signed. 
     Transform unit 304 compresses the energy information in residual information 204. Transform unit 304 may be implemented with discreet cosign transform (DCT) for MPEG standard or H.261 video conferencing standard. In the alternative, transform unit 304 may be implemented with wavelet transformation, for example, the Intel Internal Indio algorithms. Quantizer unit 306 performs the process of converting the continuous valued signal of the transformed residual information into a discrete valued signal. Code unit 308 takes the quantized residual information from quantizer unit 306 and encodes the residual information through for example the Huffman code. Motion estimation unit 204 also outputs motion vector 312 having X and Y axis values corresponding to the location of the block of reference frame selected as matching the given block of the current frame. 
     More specifically, given a current frame and a reference frame of blocks 16×16 or 8×8 pixels, the correct motion vector for a given block is searched in the given reference flames. The number of matches searched is dependent on the algorithm used. Motion estimation unit 204 performs these searches and looks for the minimal sum of the absolute differences. Whatever block or match it finds through its pattern of searches, i.e. blocks in the reference frame which is the most similar to the current block being searched, then one is subtracted from the other. The residue then is the pixel values which are different between the block of the current frame and the block of the reference frame. If everything in the respective blocks are the same, then the residual information is zero. The subtraction yielding all residual values is then processed by transform unit 304, quantizer unit 306 and code unit 308 respectively. Motion compensation unit 210 of decoder 208 receives this residual information 302 and motion vector 312. 
     On the motion compensation side (decoder 208), the encoded stream is unencoded by decoder unit 314 and then untransformed by inverse transform unit 316. Motion compensation unit 210 then processes residual information 302 and motion vector 312 to produce a new current frame. Residual information 302 and motion vector 312 are used to select the block to be decoded. 8 bit values of the reference frame are unpacked into 16 bit values. The block of reference frame is altered according to the residual information which is added. Once the residual information is added to the block of reference frame the 16 bit values of the block is packed back into 8 bit values. A pack operation with unsigned saturation may be used to perform this packing operation. This creates a new current block. 
     FIG. 4 illustrates a technique for performing an absolute difference calculation on two complex numbers according to one embodiment of the invention. In one exemplary application of the invention, the technique described herein is implemented in the motion estimation unit described in FIGS. 2 and 3. More specifically, the technique is utilized when the motion estimation unit is performing a search of matching data sets between a block of a given current frame and a block of a given reference frame. In this application, data is represented by dotted rectangles, while instructions are represented by rectangles. 
     At step 401, a complex number A and a complex number B are stored in a first packed data item 410 and a second packed data item 420. The first packed data item 410 stores data elements representing the complex number A in a first format (such that the data elements are X 1  and X 2 ), while the second packed data item 420 stores data elements representing the complex number B in a second format (such that the data elements are Y 1  and Y 2 ). 
     At step 402, subtraction is performed using saturation arithmetic by subtracting Y i  from X i  and by subtracting X i  from Y i . At step 403, data element X 1  is replaced by Y 1  subtracted from X 1  if X 1  is greater than Y 1 . Otherwise, X 1  is replaced by zero. X 2  is replaced by Y 2  subtracted from X 2  if X 2  is greater than Y 2 . Otherwise, X 2  is replaced by zeros. Y 1  is replaced by X 1  subtracted from Y1 if Y 1  is greater than X 1 . Otherwise Y 1  is set to zeros. Finally, Y 2  is set to Y 2  less X 2  if Y 2  is greater than X 2 . Otherwise, Y 2  is set to all zeros. At step 404, the greater data element from each pair of corresponding data elements are selected. At step 405, depending on the results from step 404, the most significant bits are equal to X 1  -Y 1  or Y 1  -X 1 . The least significant bits are equal to X 2  -Y 2  or Y 2  -X 2 . In this way, the absolute differences of the unsigned values are calculated. 
     FIG. 5 illustrates one embodiment of how an absolute difference calculation is performed for unsigned packed data. The term operand is interchangeably used herein to refer to the data on which an instruction operates or the storage area (e.g., register, memory location, etc.) in which that data can be found. The first operand 510 is packed data item containing X i+3 , X i+2 , X i+1 , and X i  as its data elements, while the second operand 520 is packed data item containing Y i+3 , Y i+2 , Y i+1 , and Y i  as it data elements. 
     Differences are taken, for instance, between blocks of pixels and the sum of these differences&#39; absolute values is computed. Since color components are often eight bit unsigned quantities, their absolute differences can also be expressed in eight bits. In FIG. 5, four absolute differences between sixteen bit unsigned values are computed in parallel. The same method could also be used to simultaneously calculate eight absolute differences between eight bit unsigned values (pixel components which are usually represented by eight bits for example). 
     When packed subtractions are performed with unsigned saturating arithmetic 502 and 506, any negative results are saturated to zero (0×0000). By subtracting the values X i  and Y i  in both directions with this type of saturation, only positive results are generated. The results always fit into the same amount of bits as the original operands. If saturating subtract was not used, then if X i  is greater than Y i  and X i  is subtracted from Y i , a negative result is produced that needs one more bit beyond the original bits. Saturation ensures that the result is a zero and not a negative result that needs more bits than are available. Thus the usage of saturating subtract enables full parallelism without having to have to unpack the results to a higher precision data type in order to accommodate the possibility of the result being negative. The results can be combined using the logical operation performed by packed or (POR) operation 507 to produce the desired vector of absolute differences 510. While the packed data instructions can be implemented to perform any number of different operations, in one embodiment these packed data instructions are those described in &#34;A Set of Instructions for Operating on Packed Data,&#34; filed on Aug. 31, 1995, U.S. Ser. No. 08/521,360. 
     FIG. 6 illustrates a packed add operation according to one embodiment of the invention. The illustrated packed add operation may be used as an alternative to the packed or (POR) operation illustrated in FIG. 5. The data elements of a first operand 610 are added to the respective packed data elements of a second operand 620 to generate a result 630. For example, data element 0 of the first operand 610 is added to data element 0 the second operand 620 and the result is stored as data element 0 of the result 630. The packed add operation replacing packed or operation (POR) 507 illustrated in FIG. 5 produces the same results. 
     Example code for an absolute difference of unsigned values routine that is software pipelined for the embodiment described in FIGS. 1 through 6 is included at the end of this description. The registers used for performing the packed data operations are referred to herein as mm0-mm7. The code also includes references to registers ebx, edi, and esi and instructions MOV, JNZ, and SUB (further description of these items may be found in Pentium Processor&#39;s Users Manual-Volume 3: Architecture and Programming Manual, 1994, available from Intel Corporation of Santa Clara, Calif.). While this code is written for one embodiment of the processor, alternative embodiments that have different instruction names or different instructions are still within the scope of the invention. 
     In one embodiment of the code, a sufficient number of multiply-accumulate iterations may be unrolled to minimize the overhead of loop code if necessary. Then, for the duration of each PMADDwd operation stage, instructions related to the previous and next PMADDwd operation stage are issued that do not depend of the current PMADDwd operation stage result. As a result of this software pipelining technique, a two clock throughput per complex multiply-accumulate operation can be achieved within the inner loop. 
     As previously described, one embodiment of processor 105 includes two pipelines (the U- and V-pipelines). Both the U- and the V-pipelines are capable executing certain of the packed data instructions. However, in one embodiment, only the U-pipeline is capable of executing packed data instructions that require accesses to memory or register files other than those used to perform the packed data operations (e.g., MOVQ). To maximize throughput of the computation flow, the instruction sequence may be scheduled properly to minimize data dependencies. To establish a framework for how the instructions may be scheduled, it is useful to first breakdown the number and type of operations that are required per iteration. Thus, similar software pipelining techniques may be used as described with reference to the motion estimation. 
     Example code for a complex absolute difference of unsigned values routine that is software pipelined for the described embodiment is included at the end of this description. The pairing in the pipes of the P55C is designated by the pairs of instructions in the code. Expected cycle count is in the comments. The illustrated code takes 16 values from source 1 and 16 values from source 2, each value 8 bit unsigned and generates the absolute difference of each pair of respective values. In the pairs illustrated, the first instruction goes in the first pipe and the second in the second pipe. 
     While the invention has been described in terms of several embodiments, those skilled in the art will recognize that the invention is not limited to the embodiments described. The method and apparatus of the invention can be practiced with modification and alteration within the spirit and scope of the appended claims. For example, the absolute difference of unsigned values can also be used in different recognition processes such as speech and handwriting recognition processes. The description is thus to be regarded as illustrative instead of limiting on the invention. 
     
         ______________________________________EXAMPLE CODE______________________________________movq      mm0, (% ebp) /     4- get 8 pixelsmovq      mm1, (% edx) /     1- get 8 pixelsmovq      mm0, mm2     /     copy one inputmovq      mm3, 8 (% ebp)                  /     4- get next 8 pixelspsubusb   mm1, mm0           get diff one waypsubusb   mm2, mm1     /     1- get diff other waymovq      mm4, 8 (% edx)     get next 8 pixelspor       mm1, mm0     /     1- make absolute diffmovq      mm3, mm5           copy one inputmovq      mm0, mm1     /     1- copypsubusb   mm4, mm3           get diff one waypunpckbw  mm6, mm0     /     1- unpack with zero regpsubusb   mm5, mm4           get diff other waypsrlq     $32, mm1     /     1- shift for upper pixelspor       mm4, mm3     /     make absolute diffpunpckbw  mm6, mm1     /     1- unpack upper pixelsmovq      mm3, mm4           copypunpckbw  mm6, mm3     /     1- unpack with zero regpaddw     mm0, mm7           add 4 lowerpsrlq     $32, mm4     /     1- shift for upper pixelspaddw     mm1, mm7           add 4 upperpunpckbw  mm6, mm4     /     1- unpack upper pixelspaddw     mm3, mm7           add 4 lowerpaddw     mm4, mm7     /     1- add 4 upper______________________________________