WO1998034172A1 - A cache system - Google Patents
A cache system Download PDFInfo
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- WO1998034172A1 WO1998034172A1 PCT/GB1998/000268 GB9800268W WO9834172A1 WO 1998034172 A1 WO1998034172 A1 WO 1998034172A1 GB 9800268 W GB9800268 W GB 9800268W WO 9834172 A1 WO9834172 A1 WO 9834172A1
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- cache
- partition
- item
- main memory
- memory
- Prior art date
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- 238000005192 partition Methods 0.000 claims abstract description 147
- 230000015654 memory Effects 0.000 claims abstract description 124
- 230000007246 mechanism Effects 0.000 claims abstract description 13
- 238000000034 method Methods 0.000 claims description 53
- 230000008569 process Effects 0.000 claims description 47
- 238000000638 solvent extraction Methods 0.000 description 19
- 230000006399 behavior Effects 0.000 description 7
- 238000010586 diagram Methods 0.000 description 7
- 230000008859 change Effects 0.000 description 4
- 230000006870 function Effects 0.000 description 4
- 230000000694 effects Effects 0.000 description 3
- 229940002865 4-way Drugs 0.000 description 1
- 230000004075 alteration Effects 0.000 description 1
- 230000001934 delay Effects 0.000 description 1
- 230000008014 freezing Effects 0.000 description 1
- 238000007710 freezing Methods 0.000 description 1
- 230000006872 improvement Effects 0.000 description 1
- 238000013507 mapping Methods 0.000 description 1
Classifications
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- G—PHYSICS
- G06—COMPUTING; CALCULATING OR COUNTING
- G06F—ELECTRIC DIGITAL DATA PROCESSING
- G06F12/00—Accessing, addressing or allocating within memory systems or architectures
- G06F12/02—Addressing or allocation; Relocation
- G06F12/08—Addressing or allocation; Relocation in hierarchically structured memory systems, e.g. virtual memory systems
- G06F12/0802—Addressing of a memory level in which the access to the desired data or data block requires associative addressing means, e.g. caches
- G06F12/0804—Addressing of a memory level in which the access to the desired data or data block requires associative addressing means, e.g. caches with main memory updating
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- G—PHYSICS
- G06—COMPUTING; CALCULATING OR COUNTING
- G06F—ELECTRIC DIGITAL DATA PROCESSING
- G06F12/00—Accessing, addressing or allocating within memory systems or architectures
- G06F12/02—Addressing or allocation; Relocation
- G06F12/08—Addressing or allocation; Relocation in hierarchically structured memory systems, e.g. virtual memory systems
- G06F12/0802—Addressing of a memory level in which the access to the desired data or data block requires associative addressing means, e.g. caches
- G06F12/0806—Multiuser, multiprocessor or multiprocessing cache systems
- G06F12/0842—Multiuser, multiprocessor or multiprocessing cache systems for multiprocessing or multitasking
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- G—PHYSICS
- G06—COMPUTING; CALCULATING OR COUNTING
- G06F—ELECTRIC DIGITAL DATA PROCESSING
- G06F12/00—Accessing, addressing or allocating within memory systems or architectures
- G06F12/02—Addressing or allocation; Relocation
- G06F12/08—Addressing or allocation; Relocation in hierarchically structured memory systems, e.g. virtual memory systems
- G06F12/0802—Addressing of a memory level in which the access to the desired data or data block requires associative addressing means, e.g. caches
- G06F12/0844—Multiple simultaneous or quasi-simultaneous cache accessing
- G06F12/0846—Cache with multiple tag or data arrays being simultaneously accessible
- G06F12/0848—Partitioned cache, e.g. separate instruction and operand caches
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- G—PHYSICS
- G06—COMPUTING; CALCULATING OR COUNTING
- G06F—ELECTRIC DIGITAL DATA PROCESSING
- G06F12/00—Accessing, addressing or allocating within memory systems or architectures
- G06F12/02—Addressing or allocation; Relocation
- G06F12/08—Addressing or allocation; Relocation in hierarchically structured memory systems, e.g. virtual memory systems
- G06F12/0802—Addressing of a memory level in which the access to the desired data or data block requires associative addressing means, e.g. caches
- G06F12/0864—Addressing of a memory level in which the access to the desired data or data block requires associative addressing means, e.g. caches using pseudo-associative means, e.g. set-associative or hashing
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- G—PHYSICS
- G06—COMPUTING; CALCULATING OR COUNTING
- G06F—ELECTRIC DIGITAL DATA PROCESSING
- G06F12/00—Accessing, addressing or allocating within memory systems or architectures
- G06F12/02—Addressing or allocation; Relocation
- G06F12/08—Addressing or allocation; Relocation in hierarchically structured memory systems, e.g. virtual memory systems
- G06F12/10—Address translation
- G06F12/1027—Address translation using associative or pseudo-associative address translation means, e.g. translation look-aside buffer [TLB]
- G06F12/1045—Address translation using associative or pseudo-associative address translation means, e.g. translation look-aside buffer [TLB] associated with a data cache
Definitions
- the present invention relates to a cache system for operating between a processor and a main memory of a computer.
- cache memories are used in computer systems to decrease the access latency to certain data and code and to decrease the memory bandwidth used for that data and code .
- a cache memory can delay, aggregate and reorder memory accesses.
- a cache memory operates between a processor and a main memory of a computer. Data and/or instructions which are required by the process running on the processor can be held in the cache while that process runs. An access to the cache is normally much quicker than an access to main memory. If the processor does not locate a required data item or instruction in the cache memory, it directly accesses main memory to retrieve it, and the requested data item or instruction is loaded into the cache. There are various known systems for using and refilling cache memories .
- the present invention provides a cache system for operating between a processor and a main memory of a computer, the cache system comprising: a cache memory having a set of cache partitions, each cache partition comprising a plurality of addressable storage locations for holding items fetched from said main memory for use by the processor, a cache refill mechanism arranged to fetch an item from the main memory and to load said item into the cache memory at one of said addressable storage locations, wherein the cache refill mechanism is operable to allocate to each said item fetched from the main memory one or more of said cache partitions in dependence on the address of said item in the main memory.
- a cache partition access table holds in association with addresses of items to be cached respective multi-bit partition indicators identifying into which cache partition the item may be loaded.
- each address in main memory comprises a page number and a line-in-page number, the page numbers being held in a look-up table in association with their respective partition indicators .
- the processor issues addresses comprising a virtual page number and line- in-page number.
- the system can comprise a translation look-aside buffer for translating the virtual page number to a real page number for accessing the main memory, the translation look-aside buffer also holding respective partition indicators in association with the real page numbers for identifying the cache partition into which the addressed item is to be loaded.
- a set of partition selector bits which are held as part of the virtual address itself. This set of partition selector bits is used to generate a partition mask which is used to control access to the cache.
- the line-in-page number of the items addressed can be used to identify the addressable storage location within the cache partition into which the item is to be located. That is, each cache partition could be direct-mapped. It will be apparent that it is not necessary to use all of the end bits of the item's address as the line-in-page number, but merely a set of appropriate bits. These will normally be near the least significant end of the address. Moreover, it will be appreciated that within a cache partition the addressing mechanism need not be direct mapped but could be fully associative .
- One or more cache partitions may be allocated to a page in main memory .
- the system can include a cache access circuit which accesses items from the cache memory according to the address in main memory of said items and regardless of the cache partition in which the item is held in the cache memory. That is, the partition indicator is only used on refill and not on look-up. Thus, a cached item can be retrieved from its partition even if subsequent to its caching that partition is now allocated to a different set of addresses.
- a method of operating a cache memory arranged between a processor and a main memory of a computer wherein, when the processor requests an item from main memory using an address in main memory for said item and that item is not held in the cache memory, said item is fetched from the main memory and loaded into one of a plurality of addressable storage locations in the cache memory, the addressable storage locations being arranged as a set of cache partitions and wherein one or more of said cache partitions is allocated to said item in dependence on the address of said item in main memory.
- each address is associated with a multi-bit partition indicator identifying into which cache partition the item may be loaded.
- the partition indicator is held in a cache partition access table in association with the addresses of items to be cached.
- a set of partition selector bits is held as part of the virtual address itself. This set of partition selector bits is used to generate a partition mask for controlling access to the cache.
- the main memory can hold a plurality of processes, each process including one or more sequence of instructions held at addresses in the main memory within a common page number.
- Cache partitions can be allocated by associating each cache partition with page numbers of a particular process in the main memory.
- the number of addressable storage locations in each cache partition can be alterable. Also, the association of cache partitions to page numbers can be alterable while a process using these page numbers is being run by the processor.
- Inter-thread partitioning can be used to prevent trashing of the entire cache when accessing a large array. This is particularly effective when the routine is reading or writing each element only once .
- Inter-thread partitioning prevents the caching behaviour of a critical process being perturbed by the cache activities of other processes. For instance, a real time task would not wish critical data to be removed from the cache.
- the following described embodiments illustrate a cache system which gives protection of the contents of the cache against unexpected eviction by reading from or writing to cache lines from other pages of data which are placed in other partitions. It also provides a system in which the contents of the cache may be predicted.
- Figure 1 is a block diagram of a computer incorporating a cache system
- Figure 2 is a sketch illustrating a four way set associative cache
- Figure 3 is an example of an entry in a translation lookaside buffer
- Figure 4 is a block diagram of the refill engine
- Figure 5 is a diagram illustrating the operation of a multitasking processor
- Figure 6 is a diagram illustrating the alteration in caching behaviour for the system of Figure 5 ;
- Figure 7 is an example of a virtual address holding partition selector bits;
- Figure 8 is a table showing example partition selectors
- Figure 9 is a schematic diagram illustrating a cache partitioning mechanism.
- FIG. 1 is a block diagram of a computer incorporating a cache system.
- the computer comprises a CPU 2 which is connected to an address bus 4 for accessing items from a main memory 6 and to a data bus 8 for returning items to the CPU 2.
- the data bus 8 is referred to herein as a data bus, it will be appreciated that this is for the return of items from the main memory 6, whether or not they constitute actual data or instructions for execution by the CPU.
- the system described herein is suitable for use on both instruction and data caches. As is known, there may be separate data and instruction caches, or the data and instruction cache may be combined.
- the addressing scheme is a so-called virtual addressing scheme although it will be understood that the cache partitioning schemes defined herein may be used with real addresses.
- the address is split into a line-in-page address 4a and a virtual page address 4b.
- the virtual page address 4b is supplied to a translation look-aside buffer (TLB) 10.
- the line-in-page address 4a is supplied to a look-up circuit 12.
- the translation lookaside buffer 10 supplies a real page address 14 converted from the virtual page address 4b to the look-up circuit 12.
- the lookup circuit 12 is connected via address and data buses 16,18 to a cache access circuit 20. Again, the data bus 18 can be for data items or instructions from the main memory 6.
- the cache access circuit 20 is connected to a cache memory 22 via an address bus 24, a data bus 26 and a control bus 28 which transfers replacement information for the cache memory 22.
- a refill engine 30 is connected to the cache access circuit 20 via a refill bus 32 which transfers replacement information, data items (or instructions) and addresses between the refill engine and the cache access circuit.
- the refill engine 30 is itself connected to the main memory 6.
- the refill engine 30 receives from the translation look-aside buffer 10 a full real address 34, comprising the real page address and line- in-page address of an item in the main memory 6.
- the refill engine 30 also receives a partition indicator from the translation look-aside buffer 10 on a four bit bus 36. The function of the partition indicator will be described hereinafter.
- the refill engine 30 receives a miss signal on line 38 which is generated in the look-up circuit 12 in a manner which will be described more clearly hereinafter.
- the cache memory 22 described herein is a direct mapped cache. That is, it has a plurality of addressable storage locations, each location constituting one row of the cache. Each row contains an item from main memory and the address in main memory of that item. Each row is addressable by a row address which is constituted by a number of bits representing the least significant bits of the address in main memory of the data items stored at that row. For example, for a cache memory having eight rows, each row address would be three bits long to uniquely identify those rows. For example, the second row in the cache has a row address 001 and thus could hold any data items from main memory having an address in the main memory which ends in the bits 001. Clearly, in the main memory, there would be many such addresses and thus potentially many data items to be held at that row in the cache memory. Of course, the cache memory can hold only one data item at that row at any one time.
- the CPU 2 requests an item from main memory 6 using the address in main memory and transmits that address on address bus 4.
- the virtual page number is supplied to the translation look- aside buffer 10 which translates it into a real page number 14 according to a predetermined virtual to real page translation algorithm.
- the real page number 14 is supplied to the look-up circuit 12 together with the line-in-page number 4a of the original address transmitted by the CPU 2.
- the line-in-page address is used by the cache access circuit 20 to address the cache memory 22.
- the line-in-page address includes a set of least significant bits (not necessarily including the end bits) of the main address in memory which are equivalent to the row address in the cache memory 22.
- a miss signal is generated on line 38 to the refill engine 30. It is the task of the refill engine 30 to retrieve the correct item from the main memory 6, using the real address which is supplied from the translation look-aside buffer 10 on bus 34.
- the data item, once fetched from main memory 6 is supplied to the cache access circuit 20 via the refill bus 32 and is loaded into the cache memory 22 together with the address in main memory.
- the data item itself is also returned to the CPU along data bus 8 so that the CPU can continue to execute .
- FIG. 2 An example of a 4- way set associative cache is illustrated in Figure 2.
- the cache memory is divided into four banks B1,B2,B3,B4.
- the banks can be commonly addressed row-wise by a common row address, as illustrated schematically for one row in Figure 2.
- that row contains four cache entries, one for each bank.
- the cache entry for bank Bl is output on bus 26a
- the cache entry for bank B2 is output on bus 26b
- this allows four cache entries for one row address (or line-in- page address) .
- the refill engine 30 retrieves the requested item from the main memory 6 and loads it into the correct row in one of the banks, in accordance with a refill algorithm which is based on, for example, how long a particular item has been held in the cache, or other program parameters of the system. Such replacement algorithms are known and are not described further herein.
- n-way set associative cache (where n is the number of banks and is equal to four in Figure 2) , while being an improvement on a single direct mapped system is still inflexible and, more importantly, does not allow the behaviour of the cache to be properly predictable .
- the systems described herein provides a cache partitioning mechanism which allows the optimisation of the computer's use of the cache memory by a more flexible cache refill system.
- a first cache partitioning scheme will be described with reference to Figure 3.
- each TLB entry has associated with the virtual page number, a real page number and an information sequence.
- An example entry is shown in Figure 3, where VP represents the virtual page number, RP represents the real page number and INFO represents the information sequence.
- the information sequence contains various information about the address in memory in a manner which is known and which will not be described further herein. However, according to the presently described system the information sequence additionally contains a partition indicator PI, which in the described embodiment is four bits long. Thus, bits 0 to 3 of the information sequence INFO constitute the partition indicator.
- the partition indicator gives information regarding the partition into which the data item may be placed when it is first loaded into the cache memory 22.
- each partition can constitute one bank of the cache.
- each bit refers to one of the banks.
- the value of 1 in bit j of the partition indicator means that the data in that page may not be placed in partition j .
- the value of 0 in bit j means that the data in that page may be placed in partition j .
- Data may be placed in more than one partition by having a 0 in more than one bit of the partition indicator.
- a partition indicator which is all zeros allows the data to be placed in any partition of the cache.
- a partition indicator which is all ones does not allow any data items to be loaded into the cache memory. This could be used for example for "freezing" the contents of the cache, for example for diagnostic purposes.
- the partition indicator indicates that replacement of data items which have that real page number in main memory may not use banks Bl or B3 but may use banks B2 or B4 .
- the partitions would behave as a k-way set associative cache, where k partitions are allocated to a page.
- the real page number of Figure 3 can use banks B2 and B4. However, it may not use banks Bl and B3.
- the partition information is not used on cache look-up, but only upon cache replacement or refill.
- the cache access can locate data items held anywhere in the cache memory, whereas a replacement will only replace data into the allowed partitions for that page address.
- FIG 4 illustrates in more detail the content of the refill engine 30.
- the refill bus 32 is shown in Figure 4 as three separate buses, a data bus 32a, an address bus 32b and a bus 32c carrying replacement information.
- the address and data buses 32a and 32c are supplied to a memory access circuit 50 which accesses the main memory via the memory bus 54.
- the replacement information is fed to a decision circuit 52 which also receives the real address 34, the partition indicator PI on bus 36 and the miss signal 38.
- the decision circuit 52 determines the proper partition of the cache into which data accessed the main memory is to be located.
- the partition indicator PI can be set in the TLB like any other TLB entry.
- the partition indicators are set by kernel mode software running on the CPU 2 and it is the responsibility of that kernel mode software to ensure that pages which should not be placed in a particular cache partition do not have their partition indicator bits set for that partition.
- a user may alter partitions by requesting that the cache partitions be altered. In that event, the CPU 2 would change to kernel mode to implement the request, change the TLB entries accordingly and then return to the user mode to allow the user to continue.
- a user can alter the partitioning behaviour of the cache, thus providing much greater flexibility than has hitherto been possible.
- a multi-tasking processor is capable of running more than one process "simultaneously".
- the processor executes part of a process and, when that process is halted for some reason, perhaps in need of data or a stimulus to proceed, the processor immediately begins executing another process.
- Figure 5 illustrates diagrammatically such a situation.
- Figure 5 is illustrated the sequence which a processor may undertake to run different processes P1,P2,P3,P4.
- P1,P2,P3,P4 On the right hand side of Figure 5 is an illustration of where these processes may expect their data to be held in memory.
- the data for the process PI are held on page 0.
- the data for process P2 are held on pages 1 and 2.
- Data for processes P3 and P4 share page
- the processor executes a first sequence of process PI, a first sequence of process P2 , a second sequence of process PI, a second sequence of process P2 and then a first sequence of process P3.
- the second sequence of the process executes a first sequence of process PI, a first sequence of process P2 , a second sequence of process PI, a second sequence of process P2 and then a first sequence of process P3.
- the process PI has been fully run by the processor. It will readily be apparent that in a conventional cache system, once the processor has started executing the first sequence of the process P2 , and is thus requesting accesses from page 1, the data items and instructions in these lines will replace in the cache the previously stored data items and instructions from page 0. However, these may soon again be required when the second sequence of the process PI is executed.
- Figure 6 shows the partitioning of the cache while the processor is running process PI, and the change in the partitioning when the processor switches to running P3 etc.
- Figure 6 also shows the TLB cache partition indicators for each case.
- Figure 5 shows the cache partitioned while the processor is running processes PI and P2.
- the process PI may use banks Bl and B2 of the cache, but may not use banks B3 and B4.
- the process P2 may use banks B3 and B4 , but not banks Bl and B2. This can be seen in the TLB entries below.
- page 0 has a cache partition indicator allowing it to access banks Bl and B2 , but not B3 and B4.
- Pages 1 and 2 have cache partition indicators allowing them to access banks B3 and B4 but not Bl and B2.
- Page 3 has a cache partition indicator which prevents it from accessing the cache.
- the processor is not intending to execute any part of the process P3 until it has finished executing process PI. If it did for some reason have to execute P3 , the only downside would be that it would have to make its accesses from direct memory and would not be allowed use of the cache.
- the processor can request kernel mode to allow it to alter the cache partition indicators in the TLB.
- kernel processes do not have access to the cache. Instead they modify the TLB entries for the partition indicators to modify the behaviour of the cache. The change is illustrated on the right hand side of Figure 6.
- the cache partition indicators prevent the process PI from using the cache at all, but allocate banks Bl and B2 to the processes P3 and P4 , by altering the cache partition indicator for page 3 so that it can access these banks of the cache.
- the processor is expecting to execute the process P3 , it now has a cache facility.
- FIG. 7 illustrates the arrangement in which bits 0 to 57 hold the virtual page number and page offset (referred to earlier as line-in-page) and bits 58 to 63 are the partition selector bits.
- the partition selector bits include a partition index (which is a bit sequence comprising any number of bits between zero and five) and a guard bit GB set to one. If there are any remaining bits to the right hand side of the guard bit (depending on the length of the partition index sequence), these bits are set to zero.
- the partition selector bits are used to generate a partition mask which serves the same function as the partition indicator described earlier in that it controls access of data items to the cache.
- the guard bit GB marks the start of the partition index sequence and controls into how many portions the cache is divisible.
- the scheme described herein supports sixteen cache partitions. Note that in Figure 8 the partitions are numbered from right to left from partition zero. The size of the partition depends on how many partitions the cache is divided into .
- Figure 8 illustrates some examples.
- the shaded part indicates the available partition for that address.
- the guard bit is not set at all and therefore all of the cache is available.
- the guard bit is set in the left-most position, again indicating that the entire cache is available.
- the guard bit is set in the second position which indicates that the cache is to be divided into two partitions.
- the partition index is set to 1 indicating that PARTITION 1 is available. Thus, half of the cache is shown shaded.
- the guard bit is set in the third bit position, indicating that there are four cache partitions.
- the partition index sequence is 01, indicating that PARTITION 1 is available for use.
- the guard bit is set in the fourth bit position, indicating that there are eight cache partitions.
- the partition index sequence 010 denotes that PARTITION 2 is available.
- the guard bit is set in the fifth bit position indication that there are sixteen partitions and the partition index denotes that PARTITION 15 is available.
- guard bit denotes the number (and then the size) of cache partitions.
- partition index denotes the position of the cache partition available for use.
- FIG 9 is a schematic diagram illustrating implementation of the second cache partitioning scheme.
- the circles marked TLB (reference number 10) and Mask (reference numeral 33) denote hardware, while the rectangles denote bit sequences.
- the TLB 10 performs the same function as in Figure 1.
- the mask circuit 33 receives the partition selector bits from the virtual address and generates the partition mask as described above with reference to Figure 8.
- the partition mask is used together with the offset to generate set select bits and line index bits for addressing the cache.
- the second cache partitioning scheme is used in the same context as described earlier for the first cache partitioning scheme.
- the address issued by the CPU on address bus 4 is split into a virtual page number 4b and a line-in-page 4a.
- the invention can also be used in a situation where the entire virtual address is sent from the CPU to the look-up circuit for the cache. Conversely, the invention is also applicable in a situation where the CPU issues real addresses.
- no TLB would be required. What is important is that the cache partition indicator is provided in association with the address in main memory .
- a single cache access circuit 20 is shown for accessing the cache both on look-up and refill.
- the refill engine 30 and cache access circuit 20 are shown in individual blocks. However, it would be quite possible to combine their functions into a single cache access circuit which performs both look-up and refill.
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Abstract
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Priority Applications (4)
Application Number | Priority Date | Filing Date | Title |
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EP98902083A EP0890149A1 (en) | 1997-01-30 | 1998-01-29 | A cache system |
JP53262498A JP3370683B2 (en) | 1997-01-30 | 1998-01-29 | Cash system |
US09/155,607 US6594729B1 (en) | 1997-01-30 | 1998-01-29 | Cache system |
US11/046,580 US20050132141A1 (en) | 1997-01-30 | 2005-01-28 | Cache system |
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GBGB9701960.8A GB9701960D0 (en) | 1997-01-30 | 1997-01-30 | A cache system |
GB9701960.8 | 1997-01-30 |
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US09155607 A-371-Of-International | 1998-01-29 | ||
US10/446,280 Continuation US6871266B2 (en) | 1997-01-30 | 2003-05-23 | Cache system |
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WO1998034172A1 true WO1998034172A1 (en) | 1998-08-06 |
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Also Published As
Publication number | Publication date |
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US20050132141A1 (en) | 2005-06-16 |
GB9701960D0 (en) | 1997-03-19 |
JPH11509356A (en) | 1999-08-17 |
JP3370683B2 (en) | 2003-01-27 |
US20080034162A1 (en) | 2008-02-07 |
US6594729B1 (en) | 2003-07-15 |
US7437514B2 (en) | 2008-10-14 |
EP0890149A1 (en) | 1999-01-13 |
US6871266B2 (en) | 2005-03-22 |
US20030196041A1 (en) | 2003-10-16 |
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