Abstract:
Aspects of the present invention provide a “SuperTag” cache that manages cache at three granularities: (i) coarse grain, multi-block “super blocks,” (ii) single cache blocks and (iii) fine grain, fractional block “data segments.” Since contiguous blocks have the same tag address, by tracking multi-block super blocks, the SuperTag cache inherently increases per-block tag space, allowing higher compressibility without incurring high area overheads. To improve compression ratio, the SuperTag cache uses variable-packing compression allowing variable-size compressed blocks without requiring costly compactions. The SuperTag cache also stores data segments dynamically. In addition, the SuperTag cache is able to further improve the compression ratio by co-compressing contiguous blocks. As a result, the Super Tag cache improves energy and performance for memory intensive applications over conventional compressed caches.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with government support under 1218323, 1117280, 1017650, and 0916725 awarded by the National Science Foundation. The government has certain rights in the invention. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to the field of computer systems, and in particular, to an energy optimized cache memory architecture exploiting spatial locality. 
     Improvements in technology scaling continue to bring new power and energy challenges in computer systems as the amount of power consumed per transistor does not scale down as quickly as the total density of transistors. In such systems, a significant amount of energy is consumed by the memory hierarchy which has long focused on improving memory latency and bandwidth by minimizing the gap between processor speeds and memory speeds. 
     Caches memories, or caches, play a critical role in reducing system energy. A typical cache memory is a fast access memory that stores data reflecting selected locations in a corresponding main memory of the computer system. Caches are usually comprised of Static Random Access Memory (“SRAM”) cells. Typically, the data stored in caches is organized into data sets which are commonly referred to as cache lines or cache blocks. Caches usually include storage areas for a set of tags that correspond to each block. Such tags typically include address tags that identify an area of the main memory that maps to the corresponding block. In addition, such cache tags usually provide status information for the corresponding block. 
     Although caches consume significant power, they can also save system power by filtering, and thereby reducing, costly off-chip accesses to main memory. Consequently, effectively utilizing caches is not only important for system performance, but also for system energy. 
     Cache compression is a known technique for increasing the effective cache capacity by compressing and compacting data, which reduces cache misses. Cache compression can also improve cache power by reading and writing less data for each cache access. Cache compression techniques may include targeting limited data patterns, such as dynamic zero compression and significance compression, to alternatives targeting more complex patterns. The “C-PACK” (Cache Packer) algorithm, for example, as described in “C-pack: a high-performance microprocessor cache compression algorithm,” IEEE Transactions on VLSI Systems, 2010 by X. Chen, L. Yang, R. Dick, L. Shang and H. Lekatsas, the contents of which is hereby expressly incorporated by reference, applies a pattern-based partial dictionary match compression technique with fixed packing, and uses a pair matching technique to locate cache blocks with sufficient unused space for newly allocate blocks, thereby offering a compression technique with lower hardware overhead. In general, cache compression can improve system energy if its energy overheads due to compressing and packing cache blocks are lower than the energy it saves by reducing accesses to the next level of memory in the memory hierarchy, such as to main memory. 
     However, existing cache compression techniques limit the effectiveness in optimizing system energy by lowering compressibility and incurring high energy overheads. Conventional compressed caches typically have three main drawbacks. First, to fit more cache blocks, conventional compressed caches typically double the tag array size, and as such, can only typically double the effective cache capacity. Second, packing more cache blocks often results in higher energy overheads. Variable packing techniques, which compress cache blocks into variable, sizes, improve compressibility, but incur higher energy overheads. These techniques need to frequently compact invalid cache blocks to make contiguous free space, called compaction or repacking, and as such, they significantly increase the number of accessed cache blocks. Thus, they remove the potential energy benefits of the compression. Third, conventional compressed caches limit the compression ratio. Several proposals, including those targeting energy-efficiency, use fixed-packing techniques that at most fit two compressed cache blocks in the space of one uncompressed block. In addition, all of the existing cache compression proposals compress small blocks, for example, 64 Bytes, not allowing higher compression ratios made possible by compressing larger blocks of data. 
     SUMMARY OF THE INVENTION 
     The present inventors have recognized that several contiguous blocks often co-exist in memory, such as in the last level cache (“LLC”); that contiguous blocks often have a similar compression ratio; and that large block sizes typically offer higher compression ratios. As such, by exploiting spatial locality, compression effectiveness may be maximized, thus optimizing the cache system. 
     The present inventors propose a compressed cache called “SuperTag” cache that improves compression effectiveness and reduces system energy by exploiting spatial locality. SuperTag cache manages cache, such as the last level cache, at three granularities: (i) coarse grain, multi-block “super blocks,” (ii) single cache blocks, and (iii) fine grain, fractional block “data segments.” Since contiguous blocks have the same tag address, by tracking multi-block super blocks, the SuperTag cache inherently increases per-block tag space, allowing higher compressibility without incurring high area overheads. A super block may comprise, for example, a group of four aligned contiguous blocks of 64 bytes in size each, for a total 256 Byte super block. 
     To improve the compression ratio, the SuperTag cache uses a variable-packing compression scheme allowing variable-size compressed blocks without requiring costly compactions. The SuperTag cache then stores compressed data segments, such as data segments of 16 Bytes in size each, dynamically. 
     In addition, the SuperTag cache is able to further improve the compression ratio by co-compressing contiguous blocks. As a result, the SuperTag cache improves energy and performance for memory intensive applications over conventional compressed caches. 
     As described herein, aspects of the present invention provide a cache memory system comprising: a cache memory having a plurality of index addresses, wherein the cache memory stores a plurality of data segments at each index address; a tag memory array coupled to the cache memory and the plurality of index addresses, wherein the tag memory array stores a plurality of tag addresses at each index address with each tag address corresponding to a data block originating from a higher level of memory; and a back pointer array coupled to the cache memory, the tag memory array and the plurality of index addresses, wherein the back pointer array stores a plurality of back pointer entries at each index address with each back pointer entry corresponding to a data segment at an index address in the cache memory and each back pointer entry identifying a data block associated with a tag address in the tag memory array. The data blocks are compressed into one or more data segments. 
     In addition, each tag address may correspond to a plurality of data blocks originating from a higher level of memory. 
     A first data block may also be compressed with a second data block into one or more data segments, the first and second data blocks may be from the same plurality of data blocks corresponding to a tag address, and each back pointer entry may identify the tag address in the tag memory array. 
     Data segments compressed from a data block may be stored non-contiguously in the cache memory, a data block may be compressed using the C-PACK algorithm. 
     The cache memory may comprise the last level cache, or another level of cache. 
     The tag memory array may store the cache coherency state and/or the compression status for each data block. The tag memory array and the back pointer array may be accessed in parallel during a cache lookup. Each tag address may correspond, for example, to four contiguous data blocks. Each data block may be, for example, 64 Bytes in size, and each data segment may be, for example, 16 Bytes in size. 
     An alternative embodiment may provide a method for caching, data in a computer system comprising: (a) compressing a plurality of contiguous data blocks originating from a higher level of memory into a plurality of data segments; (b) storing the plurality of data segments at an index address in a cache memory; (c) storing a tag address in a tag memory array at the index address, the tag address corresponding to the plurality of contiguous data blocks originating from the higher level of memory; and (d) storing a plurality of back pointer entries in a back pointer array at the index address, each of the plurality of back pointer entries corresponding to a data segment at an index address in the cache memory and identifying a data block associated with a tag address in the tag memory array. 
     The method may further comprise compressing a first data block with a second data block into a plurality of data segments. Also, data segments compressed from a data block may be stored contiguously or non-contiguously in the cache memory, data blocks may be compressed using the C-PACK algorithm, for example, and the tag memory array may store the cache coherency state and/or compression status for each data block. 
     Another alternative embodiment may provide a computer system with a cache memory comprising: a data array having a plurality of data segments at a cache address; a back pointer array having a plurality of back pointer entries at the cache address, each back pointer entry corresponding to a data segment; a tag array having a plurality of group identification entries at the cache address, each group identification entry having a group identification number; and a cache controller in communication with the data array, the back pointer array, the tag array and a higher level of memory. The cache controller may operate to: (a) obtain from the higher level of memory a plurality of contiguous data blocks at a memory address, each of the plurality of contiguous data blocks receiving a sub-group identification number; (b) compress the plurality of data blocks into a plurality of data segments; (c) store the plurality of data segments in the data array at the cache address (d) store the memory address and the sub-group identification numbers in a group identification entry having a group identification number in the tag array; and (e) in each back pointer entry corresponding to a stored data segment, store the group and sub-group identification numbers corresponding to the data block from which the stored data segment was compressed. 
     The cache controller, may further operate to compress a first data block with a second data block into a plurality of data segments. Also, data segments may be stored contiguously or non-contiguously in the data array. 
     These and other objects, advantages and aspects of the invention will become apparent from the following description. The particular objects and advantages described herein may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention and reference is made, therefore, to the claims herein for interpreting the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a logical diagram of a computer system in accordance with an embodiment of the present invention, including a plurality of processors and caches, a memory controller, a main memory and a mass storage device; 
         FIG. 2  is a SuperTag cache system in accordance with an embodiment of the present invention, including a super tag array, a segmented back pointer array and a segmented data array; 
         FIG. 3  is a depiction of the fields for mapping and indexing the cache system of  FIG. 2 ; 
         FIG. 4  is a depiction of an exemplar super tag set from the super tag array of the cache system of  FIG. 2 ; 
         FIG. 5  is a depiction of an exemplar segmented back-pointer set from the segmented back pointer array of the cache system of  FIG. 2 ; 
         FIGS. 6A-D  depict a multi-block super block that is variable-packed, co-compressed and dynamically stored in cache in accordance with an embodiment of the present invention; and 
         FIG. 7  is a flow chart illustrating the operation of a SuperTag cache system in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     One or more specific embodiments of the present invention will be described below. It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. Nothing in this application is considered critical or essential to the present invention unless explicitly indicated as being “critical” or “essential.” 
     Referring now to the drawings wherein like reference numbers correspond to similar components throughout the several views and, specifically, referring to  FIG. 1 , the present invention shall be described in the context of a computer system  10  in accordance with an embodiment of the present invention. The computer system  10  includes one or more processors, such as processors  12 ,  14  and  16 , coupled together on a common bus, switched interconnect or other interconnect  18 . Additional processors may also be coupled together via the same bus, switched interconnect or other interconnect  18 , or via additional buses or interconnects comprising additional nodes (not shown), as understood in the art. 
     Each processor, such as processor  12 , further includes one or more processor cores  20  and a plurality of caches comprising a cache memory hierarchy. In alternative embodiments, one or more caches may be external to the processor/processor module, and/or one or more caches may be integrated with the one or more processor cores. 
     The plurality of, caches may include, at a first level, a Level 1 Instruction (“IL1”) cache  22  and a Level 1 Data (“DL1”) cache  24 , each coupled in parallel to the processor cores  20 . The IL1 cache  22  and DL1 cache  24  may each be, for example, private, 32 Kilobyte, 8-way associative caches with a 3-cycle hit latency. The plurality of caches may next include, at a second level, a larger Level 2 (“L2”) cache  26  coupled to each of the IL1 cache  22  and DL1 cache  24 , respectively, which, may be, for example, a private, 256 Kilobytes, single bank, 8-way associative cache with a 10-cycle hit latency. The plurality of caches may next include, at a third level, and perhaps last level, an even larger Level 3 (“L3”) last level cache (“LLC”)  28  coupled to the L2 cache  26 . The last level cache  28  may be, for example, a shared, 8 Megabytes, divided into 8 banks, 16-way associative cache with a 17-cycle hit latency. The plurality of caches may implement, for example, the “MESI” protocol or any other protocol for maintaining cache coherency as understood in the art. 
     Each processor, in turn, couples via the bus, switched interconnect or other interconnect  18  to a memory controller  50 . The memory controller  50  may communicate directly with the last level cache  28  in the processor  12 , or in an alternative embodiment, indirectly with the last level cache  28  via the processor cores  20  in the processor  12 . The memory controller  50  may then communicate with main memory  52 , such as Dynamic Random Access Memory (“DRAM”) modules  54 , which may be, for example, 4 Gigabytes, divided into 16 banks, of Double Data Rate Type 3 (“DDR3”) Synchronous DRAM (“SDRAM”) operating at 800 MHz. The memory controller  50  may also communicate via one or more expansion buses  54  with more distant data containing devices, such as a mass storage device  58  (e.g., a hard disk drive, magnetic tape drive, optical disc drive, flash memory, etc.). 
     Referring now to  FIG. 2 , a SuperTag cache  80  in accordance with an embodiment of the present invention is shown. The SuperTag cache  80  may be implemented, for example, at the last level cache  28  as shown in  FIG. 1 . As will be described below, the SuperTag cache  80  provides a decoupled, segmented cache which may be managed at three granularities: coarse grain, multi-block “super blocks,” such as every four blocks of 64 Bytes each, via a super tag memory array  110 , (ii) single cache blocks, such as individual 64 Byte blocks, and (iii) fine grain, fractional block “data segments,” such as at 16 Byte data segments, via a segmented back, pointer array  112 . SuperTag cache  80  explicitly tracks super blocks and data segments, while it implicitly tracks single cache blocks by storing them as a plurality of data segments. 
     In alternative embodiments, the sizes of super blocks, cache blocks and data segments may vary. For example, another embodiment may provide larger size super blocks, such as every eight blocks of 128 Bytes each, and/or smaller data segments, such as 8 Byte data segments. This might improve compression ratio, for example, but at the cost of additional area and power overheads. In yet another embodiment, the super block may comprise a single block which may incur more area and power, but provide increased performance. 
     Referring briefly to  FIG. 3 , a depiction of the fields for mapping and indexing the cache system in accordance with an embodiment of the present invention is shown. The SuperTag cache  80  maps super blocks to locations in the higher level of memory via a tag address field  132 . The SuperTag cache  80  also indexes cached data via an index field  134 , a block number field  136  and an offset field  138 . The sizes of each bit field may vary according to the cache architecture and addressing schemes. For example, in an embodiment comprising super blocks consisting of four contiguous blocks, the block number field  136  may comprise only 2 bits for uniquely identifying each of the four contiguous blocks. 
     Referring back to  FIG. 2 , the SuperTag cache  80  explicitly tracks super blocks in the super tag array  110 , and also breaks each cache block into smaller data segments  104  that are dynamically allocated in a cache memory or segmented data array  100 . In this way, it can exploit the spatial similarities among multiple blocks while it does not incur the internal fragmentation and false sharing overheads of large blocks. 
     Unlike conventional caches, the SuperTag cache  80  does not require data segments  104  of a cache block to be stored adjacently. The SuperTag cache  80  stores data segments  104  in-order, but not necessarily contiguously. For example, data segments  104  and  106  may originate from the same cache block while being stored non-contiguously. As such, the SuperTag cache  80  does not require repacking cache sets to make contiguous space, and as a result, eliminates compaction overheads while keeping the benefits of variable-size compressed cache blocks. 
     In addition to separately compressing cache blocks into variable sizes, to further improve compression ratio, the SuperTag cache  80  may further exploit spatial locality by co-compressing cache blocks, including within a super block. In other words, a first data block may be compressed with a second data block, or with a second and a third data block, etc., including within the same super block, to produce one or more data segments. 
     The SuperTag cache  80  organizes data space by data segments in a cache memory comprised of a segmented data array  100 . For example, for the 16-way last level cache  28  described above, there may be 64 data segments in each set, such as exemplar data set  102  having individual data segments numbered from 0 to 63. With cache blocks of 64 Bytes in size, multiple data segments may be divided into 16 Bytes in size each, such as exemplar data segments  104  and  106 , and stored in order, but not necessarily contiguously, within the data set. In this way, each data set can store, for example, up to 16 uncompressed blocks, or up to 64 compressed blocks. 
     To track cache blocks at both coarse and fine granularities, a super tag array (“STA”)  110 , which tracks coarse grain, multi-block super blocks, and a segmented back-pointer array (“SBPA”), which tracks fine grain, data segments, are both used. The super tag array  110  and the segmented back-pointer array  112  may be accessed in parallel on a cache lookup, and in serial with the segmented data array  100 . 
     The main source of area overheads in the SuperTag cache  80  may be the back pointer array which tracks each data segment assignment. However, an alternative embodiment may provide, for example, limiting how segments are assigned to blocks by using a hybrid packing technique, such as fixing the assignment at super block boundaries. 
     Referring briefly to  FIG. 4 , a depiction of an exemplar super tag set  114  of the super tag array  110  is shown. The exemplar super tag set  114  may include a least recently used (“LRU”) field  140  for implementing a cache replacement policy. Each super tag entry within the super tag set, such as exemplar super tag entry  142 , shares one tag address  144  for each of the related blocks within the super block, such as exemplar block  146  (“Block  3 ”). Each of the related blocks within the super block stores per-block information separately, such as the cache coherency state  150  and optionally the compression status  152  for the block. For example, as shown in  FIG. 4 , the super tag array  110  is tracking for “SuperTag  14 ,” “Blk  3 ” the tag address, the cache coherency state and the compression status for that block. 
     Referring back to  FIG. 2 , since the SuperTag cache  80  does not store segments of a cache block in contiguous space, it uses the segmented back-pointer array  112  to resolve which block each data segment in the segmented data array  100  refers. Referring briefly to  FIG. 5 , a depiction of an exemplar segmented back-pointer entry set  160  of the segmented back pointer array  112  is shown. The exemplar segmented back-pointer entry set  160  includes sixty-four back-pointer entries in, the set, individually numbered from 0 to 63, and corresponding to the same number data segment in the corresponding data set in the segmented data array  100 . Each back pointer entry within the back-pointer set, such as exemplar back pointer entry  162 , stores the super tag number and the block number being tracked. For example, referring to  FIGS. 2-5 , for at a particular tag address and index, back-pointer entries “ 58 ” and “ 62 ” correspond to segmented data entries “ 58 ” and “ 62 ” in the segmented data array  100 , and are tracking data for “SuperTag  14 ,” “Blk  3 .” 
     Referring back to  FIG. 2 , during a cache lookup, both the super tag array  110  and the segmented back-pointer array  112  may be accessed in parallel. In the case of a cache hit, both the block and its corresponding super block are found available, meaning, for example, the SuperTag cache  80  has matched  170  a super tag entry  142 , and the block&#39;s  146  coherency state  150  shows that it is valid. In this case, using the corresponding exemplar back pointer entries  162  and  163  from the back pointer entry set  160 , corresponding exemplar data segments  104  and  106  from the data set  102  in the segmented data array  100  may be accessed. 
     Referring now to  FIG. 6A-D , a multi-block super block that is variable-packed, co-compressed and dynamically stored in cache in accordance with an embodiment of the present invention is shown. Referring to  FIG. 6A , a multi-block super block  180  stored in a main memory  182 , beginning at a particular address  184 , may include contiguous blocks “A,” “B,” “C” and “D,” each block 64 Bytes in size and divisible into 16 Byte segments. Referring to  FIG. 6B , each block within the super block  180  may be individually compressed into fewer 16 Byte data segments  186 . For example, the 64 Byte block “A,” comprised of four 16 Byte segments “A 1 ,” “A 2 ,” “A 3 ” and “A 4 ,” may be compressed into two 16 Byte data segments, A′ and “A″,” Similarly, the 64 Byte block “B,” comprised of four 16 Byte segments “B 1 ,” “B 2 ,” “B 3 ” and “B 4 ,” may be compressed into two 16 Byte data segments, “B′” and “B″,” and so forth. A C-PACK pattern-based partial dictionary compression algorithm, for example, which has low hardware overheads, may be used in a preferred embodiment. 
     Alternatively, referring to  FIG. 6C , blocks of the super block  180  may be co-compressed together, including within the super block, into fewer 16 Byte co-compressed data segments  188 . For example, blocks “A,” “B,” “C” and “D,” a 256 Byte super block, may be co-compressed as a whole into four 16 Byte data segments, “X 1 ,” “X 2 ,” “X 3 ” and “X 4 .” Alternatively, block “A” may be co-compressed with block “B” and block “C” may be co-compressed with block “D,” or any other similar arrangement may be made. 
     Co-compression on larger scales may advantageously improve the compression ratio. Co-compression includes providing one or more compressors and de-compressors. A single compressor/de-compressor may be used to compress and decompress blocks serially, however, this may reduce compression benefits by increasing cache hit latency. In a preferred embodiment, a plurality of compressors/de-compressors may be used in parallel, such as four compressors and, de-compressors for super blocks comprising four cache blocks. In this manner, co-compression would not incur additional latency overhead. This is particularly the case given the typically low area and energy overheads of compressor/de-compressor units, thereby incurring low overhead. 
     In an alternative embodiment, the SuperTag cache may consistently use co-compression for every block within a super block as a whole, and thereby avoid tracking individual block numbers in the segmented back pointer array. 
     Referring to  FIG. 6D , the co-compressed 16 Byte data segments  188  may, in turn, be dynamically stored in order in a data set  190  in a segmented data array  192 . Alternatively, however, the individually compressed 16 Byte data segments  186  may, in turn, be dynamically stored in order in the data set  190  in the segmented data array  192  (not shown). The 16 Byte data segments  186  or  188  need not be stored contiguously, however, due to the utilization of corresponding back pointer entries by the SuperTag cache. 
     Referring now to  FIG. 7 , a flow chart illustrating the operation of a SuperTag cache system in accordance with another embodiment of the present invention is shown. In step  200 , during a cache lookup for a particular block, both a super tag array and a segmented back-pointer array may be accessed in parallel using a cache index. In decision step  202 , a matching super block, or cache hit, using the index address, tag address and block number is determined. If no matching super block is found in decision step  202 , a victim super block may be selected for replacement in step  206 , for example, based on an LRU replacement policy, and data may be retrieved from higher in the memory hierarchy, such as from main memory in an embodiment implemented in the last level cache. As such, a victim block may then be replaced with the data being sought in step  210 . Then, in decision step  211 , it is determined if the replacement block will fit in the data array. If the replacement block does not fit, in step  213  an additional block may be replaced, then the system may return to decision step  211  to repeat as necessary. If, however, the replacement block does fit, the system may then update the LRU field in step  212  accordingly. 
     However, if a matching super block is found in decision step  202 , the validity, or cache coherency state, for the block within the super block is then determined in decision step  208  to ensure that the block is valid. If the block is found to be invalid, then the victim block within the super block may be replaced with the data being sought in step  210 , then it may be determined if the replacement block will fit in decision step  211 , and if the replacement block does not fit, an additional block may be replaced in step  213 , repeating as necessary. Then, the system may update the LRU field in step  212  accordingly. Alternatively, if the block is found to be valid in step  208 , then the LRU field may then be directly updated in step  212  without any replacement activity occurring. 
     Next, in step  214 , the corresponding super tags and back pointer entries may be accessed and/or updated accordingly. Then, if decision step  216  indicates a read operation, the corresponding data segments are read in step  218  and then decompressed in step  220  before the cycle ends at step  230 . Alternatively, if decision step  216  indicates a write operation, the data segments are compressed in step  222 , and in decision step  224 , it is determined if the data segments will fit in the data array. If the data segments will not fit, in step  226  an additional block may be replaced, then the system may return to decision step  224  to repeat as necessary. If, however, the data segments will fit, the data segments are written in step  228  before the cycle ends at step  230 . The cycle may repeat, or cycles may perform in parallel, for each cache lookup. 
     Certain terminology is used herein for purposes of reference only, and thus is not intended to be limiting. For example, terms such as “upper,” “lower,” “above,” and “below” refer to directions in the drawings to which reference is made. Terms such as “front,” “back,” “rear,” “bottom,” “side,” “left” and “right” describe the orientation of portions of the component within a consistent but arbitrary frame of reference which is made clear by reference to the text and the associated drawings describing the component under discussion. Such terminology may include the words specifically mentioned above, derivatives thereof, and words of similar import. Similarly, the terms “first,” “second” and other such numerical terms referring to structures do not imply a sequence or order unless clearly indicated by the context. 
     When introducing elements or features of the present disclosure and the exemplary embodiments, the articles “a,” “an,” “the” and “said” are intended to mean that there are one or more of such elements or features. The terms “comprising,” “including” and “having” are intended to be inclusive and mean that there may be additional elements or features other than those specifically noted. It is further to be understood that the method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed. 
     References to “a microprocessor” and “a processor” or “the microprocessor” and “the processor” can be understood to include one or more microprocessors that can communicate in a stand-alone and/or a distributed environment(s), and can thus be configured to communicate via wired or wireless communications with other processors, where such one or more processor can be configured to operate on one or more processor-controlled devices that can be similar or different devices. Furthermore, references to memory, unless otherwise specified, can include one or more processor-readable and accessible memory elements and/or components that can be internal to the processor-controlled device, external to the processor-controlled device, and can be accessed via a wired or wireless network. 
     It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein and the claims should be understood to include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as coming within the scope of the following claims. All of the publications described herein including patents and non-patent publications are hereby incorporated herein by reference in their entireties.