Abstract:
A method, system and program for controlling access to computer memory are provided. The present invention comprises receiving a work request from a user, wherein the work request comprises an index portion and a protection portion. The index portion of the work request is used to locate an element in an address translation and protection table. The protection portion of the work request is then compared with a protection key in the table element, and access to memory is granted only if the protection portion and protection key match.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Technical Field  
           [0002]    The present invention relates to memory access in computer systems, and more specifically, how to control user access to particular areas of memory.  
           [0003]    2. Description of Related Art  
           [0004]    In a System Area Network (SAN), the hardware provides a message passing mechanism which can be used for Input/Output devices (I/O) and interprocess communications between general computing nodes (IPC). Consumers access SAN message passing hardware by posting send/receive messages to send/receive work queues on a SAN channel adapter (CA). The send/receive work queues (WQ) are assigned to a consumer as a queue pair (QP). The messages can be sent over five different transport types: Reliable Connected (RC), Reliable datagram (RD), Unreliable Connected (UC), Unreliable Datagram (UD), and Raw Datagram (RawD). Consumers retrieve the results of these messages from a completion queue (CQ) through SAN send and receive work completions (WC). The source channel adapter takes care of segmenting outbound messages and sending them to the destination. The destination channel adapter takes care of reassembling inbound messages and placing them in the memory space designated by the destination&#39;s consumer. Two channel adapter types are present, a host channel adapter (HCA) and a target channel adapter (TCA). The host channel adapter is used by general purpose computing nodes to access the SAN fabric. Consumers use SAN verbs to access host channel adapter functions. The channel interface (CI) interprets verbs and directly accesses the channel adapter.  
           [0005]    The HCA transfers data received on Infiniband (IB) links directly to its host system main memory and also fetches data from system main memory to be transmitted on these IB links. Currently, there are no mechanisms to provide protection against unauthorized access of this memory or to point to specific areas (regions and windows) of memory, each of which uses its own translation tables to translate the virtual addresses that reference this memory into the real addresses that the CI hardware needs to directly access it.  
           [0006]    Therefore, it would be desirable to have mechanisms to provide protection against unauthorized access of host system main memory, as well as mechanisms that point to specific areas of this memory, which translate virtual addresses for the memory into real addresses for the CI hardware.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention provides a method, system and program for controlling access to computer memory. The present invention comprises receiving a work request from a user, wherein the work request comprises an index portion and a protection portion. The index portion of the work request is used to locate an element in an address translation and protection table. The protection portion of the work request is then compared with a protection key in the table element, and access to memory is granted only if the protection portion and protection key match.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objectives and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein:  
         [0009]    [0009]FIG. 1 is a diagram of a network computing system is illustrated in accordance with a preferred embodiment of the present invention;  
         [0010]    [0010]FIG. 2 is a functional block diagram of a host processor node in accordance with a preferred embodiment of the present invention;  
         [0011]    [0011]FIG. 3 is a diagram of a host channel adapter in accordance with a preferred embodiment of the present invention;  
         [0012]    [0012]FIG. 4 is a diagram illustrating processing of work requests in accordance with a preferred embodiment of the present invention;  
         [0013]    [0013]FIG. 5 is an illustration of a data packet in accordance with a preferred embodiment of the present invention;  
         [0014]    [0014]FIG. 6 depicts a schematic diagram illustrating memory access through a Protection/Translation Table in accordance with the present invention;  
         [0015]    [0015]FIG. 7 depicts a flowchart illustrating memory access through a Protection/Translation Table in accordance with the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0016]    The present invention provides a distributed computing system having end nodes, switches, routers, and links interconnecting these components. Each end node uses send and receive queue pairs to transmit and receives messages. The end nodes segment the message into packets and transmit the packets over the links. The switches and routers interconnect the end nodes and route the packets to the appropriate end node. The end nodes reassemble the packets into a message at the destination.  
         [0017]    With reference now to the figures and in particular with reference to FIG. 1, a diagram of a network global change computing system is illustrated in accordance with a preferred embodiment of the present invention. The distributed computer system represented in FIG. 1 takes the form of a system area network (SAN)  100  and is provided merely for illustrative purposes, and the embodiments of the present invention described below can be implemented on computer systems of numerous other types and configurations. For example, computer systems implementing the present invention can range from a small server with one processor and a few input/output (I/O) adapters to massively parallel supercomputer systems with hundreds or thousands of processors and thousands of I/O adapters. Furthermore, the present invention can be implemented in an infrastructure of remote computer systems connected by an internet or intranet.  
         [0018]    SAN  100  is a high-bandwidth, low-latency network interconnecting nodes within the distributed computer system. A node is any component attached to one or more links of a network and forming the origin and/or destination of messages within the network. In the depicted example, SAN  100  includes nodes in the form of host processor node  102 , host processor node  104 , redundant array independent disk (RAID) subsystem node  106 , and I/O chassis node  108 . The nodes illustrated in FIG. 1 are for illustrative purposes only, as SAN  100  can connect any number and any type of independent processor nodes, I/O adapter nodes, and I/O device nodes. Any one of the nodes can function as an endnode, which is herein defined to be a device that originates or finally consumes messages or frames in SAN  100 .  
         [0019]    In one embodiment of the present invention, an error handling mechanism in distributed computer systems is present in which the error handling mechanism allows for reliable connection or reliable datagram communication between end nodes in a distributed computing system, such as SAN  100 .  
         [0020]    A message, as used herein, is an application-defined unit of data exchange, which is a primitive unit of communication between cooperating processes. A packet is one unit of data encapsulated by a networking protocol headers and/or trailer. The headers generally provide control and routing information for directing the frame through SAN. The trailer generally contains control and cyclic redundancy check (CRC) data for ensuring packets are not delivered with corrupted contents.  
         [0021]    SAN  100  contains the communications and management infrastructure supporting both I/O and interprocessor communications (IPC) within a distributed computer system. The SAN  100  shown in FIG. 1 includes a switched communications fabric  116 , which allows many devices to concurrently transfer data with high-bandwidth and low latency in a secure environment. Endnodes can communicate over multiple ports and utilize multiple paths through the SAN fabric. The multiple ports and paths through the SAN shown in FIG. 1 can be employed for fault tolerance and increased bandwidth data transfers.  
         [0022]    The SAN  100  in FIG. 1 includes switch  112 , switch  114 , switch  146 , and router  117 . A switch is a device that connects multiple links together and allows routing of packets from one link to another link within a subnet using a small header Destination Local Identifier (DLID) field. A router is a device that connects multiple subnets together and is capable of routing frames from one link in a first subnet to another link in a second subnet using a large header Destination Globally Unique Identifier (DGUID).  
         [0023]    In one embodiment, a link is a full duplex channel between any two network fabric elements, such as endnodes, switches, or routers. Example of suitable links include, but are not limited to, copper cables, optical cables, and printed circuit copper traces on backplanes and printed circuit boards.  
         [0024]    For reliable service types, endnodes, such as host processor endnodes and I/O adapter endnodes, generate request packets and return acknowledgment packets. Switches and routers pass packets along, from the source to the destination. Except for the variant CRC trailer field which is updated at each stage in the network, switches pass the packets along unmodified. Routers update the variant CRC trailer field and modify other fields in the header as the packet is routed.  
         [0025]    In SAN  100  as illustrated in FIG. 1, host processor node  102 , host processor node  104 , and I/O chassis  108  include at least one channel adapter (CA) to interface to SAN  100 . In one embodiment, each channel adapter is an endpoint that implements the channel adapter interface in sufficient detail to source or sink packets transmitted on SAN fabric  100 . Host processor node  102  contains channel adapters in the form of host channel adapter  118  and host channel adapter  120 . Host processor node  104  contains host channel adapter  122  and host channel adapter  124 . Host processor node  102  also includes central processing units  126 - 130  and a memory  132  interconnected by bus system  134 . Host processor node  104  similarly includes central processing units  136 - 140  and a memory  142  interconnected by a bus system  144 .  
         [0026]    Host channel adapters  118  and  120  provide a connection to switch  112  while host channel adapters  122  and  124  provide a connection to switches  112  and  114 .  
         [0027]    In one embodiment, a host channel adapter is implemented in hardware. In this implementation, the host channel adapter hardware offloads much of central processing unit and I/O adapter communication overhead. This hardware implementation of the host channel adapter also permits multiple concurrent communications over a switched network without the traditional overhead associated with communicating protocols. In one embodiment, the host channel adapters and SAN  100  in FIG. 1 provide the I/O and interprocessor communications (IPC) consumers of the distributed computer system with zero processor-copy data transfers without involving the operating system kernel process, and employs hardware to provide reliable, fault tolerant communications.  
         [0028]    As indicated in FIG. 1, router  116  is coupled to wide area network (WAN) and/or local area network (LAN) connections to other hosts or other routers.  
         [0029]    The I/O chassis  108  in FIG. 1 include an I/O switch  146  and multiple I/O modules  148 - 156 . In these examples, the I/O modules take the form of adapter cards. Example adapter cards illustrated in FIG. 1 include a SCSI adapter card for I/O module  148 ; an adapter card to fiber channel hub and fiber channel-arbitrated loop (FC-AL) devices for I/O module  152 ; an ethernet adapter card for I/O module  150 ; a graphics adapter card for I/O module  154 ; and a video adapter card for I/O module  156 . Any known type of adapter card can be implemented. I/O adapters also include a switch in the I/O adapter backplane to couple the adapter cards to the SAN fabric. These modules contain target channel adapters  158 - 166 .  
         [0030]    In this example, RAID subsystem node  106  in FIG. 1 includes a processor  168 , a memory  170 , a target channel adapter (TCA)  172 , and multiple redundant and/or striped storage disk unit  174 . Target channel adapter  172  can be a fully functional host channel adapter.  
         [0031]    SAN  100  handles data communications for I/O and interprocessor communications. SAN  100  supports high-bandwidth and scalability required for I/O and also supports the extremely low latency and low CPU overhead required for interprocessor communications. User clients can bypass the operating system kernel process and directly access network communication hardware, such as host channel adapters, which enable efficient message passing protocols. SAN  100  is suited to current computing models and is a building block for new forms of I/O and computer cluster communication. Further, SAN  100  in FIG. 1 allows I/O adapter nodes to communicate among themselves or communicate with any or all of the processor nodes in distributed computer system. With an I/O adapter attached to the SAN  100 , the resulting I/O adapter node has substantially the same communication capability as any host processor node in SAN  100 .  
         [0032]    Turning next to FIG. 2, a functional block diagram of a host processor node is depicted in accordance with a preferred embodiment of the present invention. Host processor node  200  is an example of a host processor node, such as host processor node  102  in FIG. 1. In this example, host processor node  200  shown in FIG. 2 includes a set of consumers  202 - 208 , which are processes executing on host processor node  200 . Host processor node  200  also includes channel adapter  210  and channel adapter  212 . Channel adapter  210  contains ports  214  and  216  while channel adapter  212  contains ports  218  and  220 . Each port connects to a link. The ports can connect to one SAN subnet or multiple SAN subnets, such as SAN  100  in FIG. 1. In these examples, the channel adapters take the form of host channel adapters.  
         [0033]    Consumers  202 - 208  transfer messages to the SAN via the verbs interface  222  and message and data service  224 . A verbs interface is essentially an abstract description of the functionality of a host channel adapter. An operating system may expose some or all of the verb functionality through its programming interface. Basically, this interface defines the behavior of the host. Additionally, host processor node  200  includes a message and data service  224 , which is a higher level interface than the verb layer and is used to process messages and data received through channel adapter  210  and channel adapter  212 . Message and data service  224  provides an interface to consumers  202 - 208  to process messages and other data.  
         [0034]    With reference now to FIG. 3, a diagram of a host channel adapter is depicted in accordance with a preferred embodiment of the present invention. Host channel adapter  300  shown in FIG. 3 includes a set of queue pairs (QPs)  302 - 310 , which are used to transfer messages to the host channel adapter ports  312 - 316 . Buffering of data to host channel adapter ports  312 - 316  is channeled through virtual lanes (VL)  318 - 334  where each VL has its own flow control. Subnet manager configures channel adapters with the local addresses for each physical port, i.e., the port&#39;s LID. Subnet manager agent (SMA)  336  is the entity that communicates with the subnet manager for the purpose of configuring the channel adapter. Memory translation and protection (MTP)  338  is a mechanism that translates virtual addresses to physical addresses and to validate access rights. Direct memory access (DMA)  340  provides for direct memory access operations using memory  340  with respect to queue pairs  302 - 310 .  
         [0035]    A single channel adapter, such as the host channel adapter  300  shown in FIG. 3, can support thousands of queue pairs. By contrast, a target channel adapter in an I/O adapter typically supports a much smaller number of queue pairs.  
         [0036]    Each queue pair consists of a send work queue (SWQ) and a receive work queue. The send work queue is used to send channel and memory semantic messages. The receive work queue receives channel semantic messages. A consumer calls an operating-system specific programming interface, which is herein referred to as verbs, to place work requests (WRs) onto a work queue.  
         [0037]    With reference now to FIG. 4, a diagram illustrating processing of work requests is depicted in accordance with a preferred embodiment of the present invention. In FIG. 4, a receive work queue  400 , send work queue  402 , and completion queue  404  are present for processing requests from and for consumer  406 . These requests from consumer  406  are eventually sent to hardware  408 . In this example, consumer  406  generates work requests  410  and  412  and receives work completion  414 . As shown in FIG. 4, work requests placed onto a work queue are referred to as work queue elements (WQEs).  
         [0038]    Send work queue  402  contains work queue elements (WQEs)  422 - 428 , describing data to be transmitted on the SAN fabric. Receive work queue  400  contains work queue elements (WQEs)  416 - 420 , describing where to place incoming channel semantic data from the SAN fabric. A work queue element is processed by hardware  408  in the host channel adapter.  
         [0039]    The verbs also provide a mechanism for retrieving completed work from completion queue  404 . As shown in FIG. 4, completion queue  404  contains completion queue elements (CQEs)  430 - 436 . Completion queue elements contain information about previously completed work queue elements. Completion queue  404  is used to create a single point of completion notification for multiple queue pairs. A completion queue element is a data structure on a completion queue. This element describes a completed work queue element. The completion queue element contains sufficient information to determine the queue pair and specific work queue element that completed. A completion queue context is a block of information that contains pointers to, length, and other information needed to manage the individual completion queues.  
         [0040]    Example work requests supported for the send work queue  402  shown in FIG. 4 are as follows. A send work request is a channel semantic operation to push a set of local data segments to the data segments referenced by a remote node&#39;s receive work queue element. For example, work queue element  428  contains references to data segment  4   438 , data segment  5   440 , and data segment  6   442 . Each of the send work request&#39;s data segments contains a virtually contiguous memory region. The virtual addresses used to reference the local data segments are in the address context of the process that created the local queue pair.  
         [0041]    A remote direct memory access (RDMA) read work request provides a memory semantic operation to read a virtually contiguous memory space on a remote node. A memory space can either be a portion of a memory region or portion of a memory window. A memory region references a previously registered set of virtually contiguous memory addresses defined by a virtual address and length. A memory window references a set of virtually contiguous memory addresses which have been bound to a previously registered region.  
         [0042]    The RDMA Read work request reads a virtually contiguous memory space on a remote endnode and writes the data to a virtually contiguous local memory space. Similar to the send work request, virtual addresses used by the RDMA Read work queue element to reference the local data segments are in the address context of the process that created the local queue pair. For example, work queue element  416  in receive work queue  400  references data segment  1   444 , data segment  2   446 , and data segment  448 . The remote virtual addresses are in the address context of the process owning the remote queue pair targeted by the RDMA Read work queue element.  
         [0043]    A RDMA Write work queue element provides a memory semantic operation to write a virtually contiguous memory space on a remote node. The RDMA Write work queue element contains a scatter list of local virtually contiguous memory spaces and the virtual address of the remote memory space into which the local memory spaces are written.  
         [0044]    An Atomic Operation work queue element provides a memory semantic operation to perform an atomic operation on a remote word. The Atomic Operation work queue element is a combined RDMA Read, Modify, and RDMA Write operation. The Atomic Operation work queue element can support several read-modify-write operations, such as Compare and Swap if equal.  
         [0045]    A bind (unbind) remote access key (R_Key) work queue element provides a command to the host channel adapter hardware to modify (destroy) a memory window by associating (disassociating) the memory window to a memory region. The R_Key is part of each RDMA access and is used to validate that the remote process has permitted access to the buffer.  
         [0046]    In one embodiment, receive work queue  400  shown in FIG. 4 only supports one type of work queue element, which is referred to as a receive work queue element. The receive work queue element provides a channel semantic operation describing a local memory space into which incoming send messages are written. The receive work queue element includes a scatter list describing several virtually contiguous memory spaces. An incoming send message is written to these memory spaces. The virtual addresses are in the address context of the process that created the local queue pair.  
         [0047]    For interprocessor communications, a user-mode software process transfers data through queue pairs directly from where the buffer resides in memory. In one embodiment, the transfer through the queue pairs bypasses the operating system and consumes few host instruction cycles. Queue pairs permit zero processor-copy data transfer with no operating system kernel involvement. The zero processor-copy data transfer provides for efficient support of high-bandwidth and low-latency communication.  
         [0048]    When a queue pair is created, the queue pair is set to provide a selected type of transport service. In one embodiment, a distributed computer system implementing the present invention supports four types of transport services.  
         [0049]    Reliable and Unreliable connected services associate a local queue pair with one and only one remote queue pair. Connected services require a process to create a queue pair for each process which is to communicate over the SAN fabric. Thus, if each of N host processor nodes contain P processes, and all P processes on each node wish to communicate with all the processes on all the other nodes, each host processor node requires P 2 ×(N−1) queue pairs. Moreover, a process can connect a queue pair to another queue pair on the same host channel adapter.  
         [0050]    Reliable datagram service associates a local end-end (EE) context with one and only one remote end-end context. The reliable datagram service permits a client process of one queue pair to communicate with any other queue pair on any other remote node. At a receive work queue, the reliable datagram service permits incoming messages from any send work queue on any other remote node. The reliable datagram service greatly improves scalability because the reliable datagram service is connectionless. Therefore, an endnode with a fixed number of queue pairs can communicate with far more processes and endnodes with a reliable datagram service than with a reliable connection transport service. For example, if each of N host processor nodes contain P processes, and all P processes on each node wish to communicate with all the processes on all the other nodes, the reliable connection service requires P 2 ×(N−1) queue pairs on each node. By comparison, the connectionless reliable datagram service only requires P queue pairs+(N−1) EE contexts on each node for exactly the same communications.  
         [0051]    The unreliable datagram service is connectionless. The unreliable datagram service is employed by management applications to discover and integrate new switches, routers, and endnodes into a given distributed computer system. The unreliable datagram service does not provide the reliability guarantees of the reliable connection service and the reliable datagram service. The unreliable datagram service accordingly operates with less state information maintained at each endnode.  
         [0052]    Turning next to FIG. 5, an illustration of a data packet is depicted in accordance with a preferred embodiment of the present invention. Message data  500  contains data segment  1   502 , data segment  2   504 , and data segment  3   506 , which are similar to the data segments illustrated in FIG. 4. In this example, these data segments form a packet  508 , which is placed into packet payload  510  within data packet  512 . Additionally, data packet  512  contains CRC  514 , which is used for error checking. Additionally, routing header  516  and transport header  518  are present in data packet  512 . Routing header  516  is used to identify source and destination ports for data packet  512 . Transport header  518  in this example specifies the destination queue pair for data packet  512 . Additionally, transport header  518  also provides information such as the operation code, packet sequence number, and partition for data packet  512 . The operating code identifies whether the packet is the first, last, intermediate, or only packet of a message. The operation code also specifies whether the operation is a send RDMA write, read, or atomic. The packet sequence number is initialized when communications is established and increments each time a queue pair creates a new packet. Ports of an endnode may be configured to be members of one or more possibly overlapping sets called partitions.  
         [0053]    Each memory region has an associated Address Translation Table (ATT). The entries in the ATT are real addresses of the pages that make up part of the memory region. The entries are arranged in ascending order corresponding to the incrementing virtual address associated with the memory region. When the HCA hardware translates from a virtual address to a real address, it indexes into the ATT based on the virtual address offset into the memory region.  
         [0054]    Both memory regions and memory windows are accessed through a Protection/Translation Table (PTT). Each memory window belongs to a memory region and defines a portion (or subset) of the region. For memory regions, each PTT Element contains a real address pointer to the beginning of each ATT in main memory. For memory windows, each PTT Element contains a pointer to the associated memory region. Also, each access to main memory includes either a Local Key (L_Key) or a Remote Key (R_Key) that is supplied by the user. The L_Keys and R_Keys are divided into two portions. The first portion is called the index and is used to index into the PTT, and the second portion is a protection key. The user provides the L_Key and R_Key and the HCA hardware uses the keys to find the PTT Element. Within each PTT Element is a protection key, and the HCA hardware compares this protection key to the second portion of the L_Key or R_Key. If the protection keys match, memory access may be given to the user depending on the particular access rights requested (i.e. Read, Write, or Atomic operation).  
         [0055]    Referring to FIG. 6, a schematic diagram illustrating memory access through a Protection/Translation Table is depicted in accordance with the present invention. The hardware structure in FIG. 6 is part of a HCA. The HCA accesses main memory on behalf of its users in two cases. First, local users from the host system (the one to which the HCA is attached) supply work requests that are comprised of virtual addresses and byte counts. Collectively, these addresses and lengths are called scatter/gather lists. All access to main memory for a particular work request must be from the same memory region, and therefore use the same Address Translation Table (ATT). The user supplies a L_Key with each work request.  
         [0056]    A second source for main memory access is from external users who are performing Remote Direct Memory Access (RDMA) and Atomic operations. These accesses may be for either a memory region or a portion of a region (memory window). In either case, the HCA translates these addresses using the same translation mechanisms that are used for local accesses. However, in this case, the external user supplies a R_Key for the incoming packets.  
         [0057]    Referring to FIG. 7, a flowchart illustrating memory access through a Protection/Translation Table is depicted in accordance with the present invention. FIG. 6 shows that the user (internal or external)  604  supplies a Key (L_Key or R_Key)  602  to the HCA hardware when the user wants access to main memory (step  701 ). The Key  602  is divided into two portions: the index portion  602   a  and the protection portion  602   b.  The HCA hardware has a Base Real Address Register (BRAR)  606  that points to the beginning of the Protection/Translation Table (PTT)  610  which is in either main memory or HCA local memory (depending on the specific implementation). At the beginning of each access to main memory, the HCA shifts the Key index portion  602   a  to the left by the number of bits required for each PTT Element  612  (step  702 ). This number is the power of 2 representing the size of the PTT in bytes. For example, if each PTT is 64 bytes, the shift operation would be 6 bits (2 to the 6th power). The HCA then adds this number to the BRAR  606  using adder  608  (step  703 ). The resulting composite address is the base address (the first byte) of the PTT Element  612  in main or local memory for the memory region or window.  
         [0058]    The HCA then uses this address from adder  608  to fetch the PTT Element  612  from memory (step  704 ). Within each PTT Element  612  is a protection key  614 , along with other validity and access rights information.  
         [0059]    The HCA hardware first checks that the index  602   a  is valid (step  705 ), meaning the corresponding PTT  610  actually exists. This check is performed because empty, invalid or unused PTT&#39;s might be interspersed among the valid ones. If the index  602   a  is valid, the HCA then uses the comparator hardware  616  to compare the L_Key protection portion  602   b  to the protection key  614  in the PTT Element  612  (step  706 ). If the comparator  616  determines that the protection keys  602   b  and  614  match (step  707 ), the access rights are granted to the user (step  708 ). If protection keys  602   b  and  614  do not match, access is denied (step  709 ). Finally, the PTT Element  612  also contains specific access rights information describing the types of main memory operation that the user is allowed (i.e. Read, Write, Atomic operations).  
         [0060]    It is important to note that while the present invention has been described in the context of a fully functioning data processing system, those of ordinary skill in the art will appreciate that the processes of the present invention are capable of being distributed in the form of a computer readable medium of instructions and a variety of forms and that the present invention applies equally regardless of the particular type of signal bearing media actually used to carry out the distribution. Examples of computer readable media include recordable-type media, such as a floppy disk, a hard disk drive, a RAM, CD-ROMs, DVD-ROMs, and transmission-type media, such as digital and analog communications links, wired or wireless communications links using transmission forms, such as, for example, radio frequency and light wave transmissions. The computer readable media may take the form of coded formats that are decoded for actual use in a particular data processing system.  
         [0061]    The description of the present invention has been presented for purposes of illustration and description, and is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. The embodiment was chosen and described in order to best explain the principles of the invention, the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.