Abstract:
A circuit and method is provided for selectively stalling interrupt requests originating devices coupled to a multiprocessor system. The multiprocessor system includes a plurality of circuit nodes each one of which is coupled to an individual memory. An I/O bridge coupled to a first circuit node is configured to generate non-coherent memory access command packets and non-coherent interrupt command packets. The first circuit node also generates a coherent interrupt command packet in response to receiving the non-coherent interrupt command packet. The first circuit node transmits the coherent interrupt command packet to another circuit node, possibly the second circuit node. However, the transmission of the coherent interrupt command packet may be delayed. Any delay in transmission is based on a comparison of the pipe identifications of the non-coherent command packets.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of computer systems, and more particularly, to a multiprocessor computer system coupled to devices external to the multiprocessor computer system via an input/output bridge. 
     2. Description of the Relevant Art 
     Personal computers (PCs) and other types of computer systems have been designed around a shared bus system for accessing memory. One or more processors and one or more input/output (I/O) devices may be coupled to the memory through the shared bus. The I/O devices may be coupled to the shared bus through an I/O bridge which manages the transfer of information between the shared bus and the I/O devices, while processors are typically coupled directly to the shared bus or coupled through a cache hierarchy to the shared bus. 
     Unfortunately, shared bus systems suffer from several drawbacks. For example, since there are multiple devices attached to the shared bus, the bus is typically operated at a relatively low frequency. The multiple attachments present a high capacitive load to a device driving a signal on the bus, and the multiple attach points present a relatively complicated transmission line model for high frequencies. Accordingly, the frequency remains low, and the bandwidth available of the shared bus is similarly relatively low. A low bandwidth presents a barrier to attaching additional devices to the shared bus, as performance may be limited by available bandwidth. 
     Another disadvantage of the shared bus system is a lack of scalability to larger numbers of devices. As mentioned above, the amount of bandwidth is fixed and may decrease if additional devices are added. Once the bandwidth requirements of the devices attached to the bus, either directly or indirectly, exceeds the available bandwidth of the bus, devices will frequently be stalled when attempting to access the bus. As a result, overall performance may be decreased. 
     One or more of the above problems may be addressed using a distributed memory system. A computer system employing a distributed memory system includes multiple nodes. Two or more of these nodes are connected to individual memories, respectively, and the nodes are interconnected using any suitable interconnect. For example, each node may be connected to each other node using dedicated lines. Alternatively, each node may connect to a fixed number of other nodes, and transactions between nodes may be routed from a first node to a second node to which the first node is not directly connected via one or more intervening nodes. A memory address space is assigned across the memories in each node. 
     Nodes may additionally include a processor. The processor typically includes a cache which stores cache blocks of data read from the memories. Furthermore, a node may include one or more caches external to the processors. Since the processors and/or nodes may be storing cache blocks accessed by other nodes, it is desirable to maintain coherency within the nodes. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method and circuit for selectively stalling interrupt requests initiating from one or more devices coupled to a multiprocessor system via an I/O bridge. In one embodiment, the multiprocessor system includes a plurality of circuit nodes and a plurality of memories. Each circuit node may include at last one microprocessor coupled to a memory controller which, in turn, is coupled to one of the memories. The plurality of circuit nodes are in data communication with each other so that one circuit node may communicate with another circuit node directly or indirectly through one or more intermediate circuit nodes. 
     A first circuit node is coupled to an I/O bridge to affect data communication there between. Other circuit nodes may likewise be coupled to separate I/O bridges. The first I/O bridge, in turn, may be coupled to one or more I/O buses or devices such as graphic devices. The I/O buses, in turn, may be coupled to one or more I/O devices or a secondary bus bridge. 
     In one embodiment of the present invention, the I/O bridge generates a non-coherent memory access command packet. This non-coherent memory access command packet comprises a first pipe identification. Pipe identifications are used to identify, for example, one or more I/O devices coupled to the I/O bridge via an I/O bus. The I/O bridge generates the non-coherent memory access command packet in response to a memory access request originating with one of the I/O devices coupled directly or indirectly to the I/O bridge. The I/O bridge transmits the non-coherent memory access command packet to the first circuit node via a non-coherent link connected between the I/O bridge and the first circuit node. The first circuit node receives the non-coherent memory access command packet and generates a coherent memory access command packet using information in the received non-coherent memory access command packet, such as, for example, a memory address where data is to be written or read. The first circuit node then transmits the non-coherent memory access command packet to a second circuit node within the multiprocessor system. 
     Subsequent to generating the non-coherent memory access command packet, the I/O bridge generates a non-coherent interrupt command packet. The I/O bridge generates the non-coherent interrupt command packet in response to, for example, an interrupt request generated by an I/O device coupled directly or indirectly to the I/O bridge. This non-coherent interrupt command packet includes a second pipe identification which may or may not equate to the first pipe identification of the non-coherent memory access command packet. The first circuit node receives the non-coherent interrupt command packet and, in response, generates a coherent interrupt command packet using information contained within the received non-coherent interrupt command packet. While the coherent interrupt command packet will be transmitted to another circuit node, the transmission may be delayed. More particularly, after the first circuit node receives the non-coherent interrupt command packet, the first circuit node compares the first and second pipe identifications contained within the non-coherent memory access command packet and non-coherent interrupt command packet, respectively. If the first and second pipe identifications compare unequally, then the first circuit transmits the coherent interrupt command packet to another circuit node without the above-mentioned delay. If, however, the first and second pipe identifications compare equally, the first circuit node may delay transmission of a coherent interrupt command packet. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the accompanying drawings in which: 
     FIG. 1 is a block diagram of one embodiment of a multiprocessor computer system; 
     FIG. 2A is a block diagram of a pair of processing nodes shown in FIG. 1; 
     FIG. 2B is a block diagram showing a processing node coupled to an I/O bridge; 
     FIG. 3 is a block diagram of one embodiment of a coherent information packet; 
     FIG. 4 is a block diagram of one embodiment of a coherent command or address packet; 
     FIG. 5 is a block diagram of one embodiment of a coherent response packet; 
     FIG. 6 is a block diagram of one embodiment of a coherent data packet; 
     FIG. 7 is a block diagram of one embodiment of a non-coherent information packet; 
     FIG. 8 is a block diagram of one embodiment of a non-coherent address or command packet; 
     FIG. 9 is a block diagram of one embodiment of a non-coherent response packet; 
     FIG. 10 is a block diagram of one embodiment of a non-coherent data packet; 
     FIG. 11 is a table illustrating exemplary packet types; 
     FIG. 12 is a block diagram of one embodiment of the circuit node of FIG. 1 coupled to the I/O bridge; 
     FIG. 13 is a flow chart illustrating operational aspects of the circuit node shown in FIG.  12 . 
    
    
     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawing and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Turning now to FIG. 1, one embodiment of a computer system  10  is shown. Other embodiments are possible and contemplated. In the embodiment of FIG. 1, computer system  10  includes several processing nodes  12 A,  12 B,  12 C, and  12 D. Processing nodes  12 A- 12 D may include a memory controller  16 A- 16 D coupled to a respective memory  14 A- 14 D, respectively. Additionally, processing nodes  12 A- 12 D may include interface logic used to communicate between the processing nodes  12 A- 12 D. For example, processing node  12 A may include interface logic  18 A for communicating with processing node  12 B, interface logic  18 B for communicating with processing node  12 C, and interface logic  18 C for communicating with yet another processing node (not shown). Similarly, processing node  12 B may include interface logic  18 D,  18 E, and  18 F; processing node  12 C may include interface logic  18 G,  18 H, and  18 I; and processing node  12 D may include interface logic  18 J,  18 K, and  18 L. 
     Processing node  12 D may be coupled to communicate with an I/O bridge circuit  20  (I/O bridge  20 ) via interface logic  18 L and  18 M, with interface logic  18 M included in I/O bridge  20 . Other processing nodes of FIG. 1 may communicate with other I/O bridges in a similar fashion. FIG. 1 shows I/O bridge  20  in communication only with processing node  12 D. Although not shown in FIG. 1, it is to be noted that I/O bridge  20  may be coupled to an additional processing node. Moreover, FIG.  1 . shows just one I/O bridge coupled to processing node  12 D. However, several I/O bridges could be coupled to processing node  12 D in series or parallel. 
     I/O bridge  20  may be coupled to an I/O bus  22  and a graphics device  26  as shown in FIG.  1 . Although not shown in FIG. 1, I/O bridge  20  may also be connected to another I/O bus. I/O bus  22  is shown coupled to several I/O devices  28  one of which may be a secondary I/O bridge coupled to a secondary I/O bus. 
     Processing nodes  12 - 12 D may implement a packet-based inter-node communication, i.e., communication between nodes. Inter-node communication may occur through coherent transactions of one or more packets. Transaction packets transmitted between processing nodes will often be referred to as coherent packets. Processing nodes  12 - 12 D, if coupled to an I/O bridge, may implement a packet-based intra-node communication, i.e., communication between a node and an I/O bridge. Intra-node communication occurs through non-coherent transactions of one or more packets. Transaction packets transmitted between a processing node and an I/O bridge will often be referred to as non-coherent packets. 
     In the present embodiment, the links between nodes or between a node and an I/O bridge may be implemented as sets of unidirectional lines (e.g. lines  24 A are used to transmit coherent packets from processing node  12 A to processing node  12 B and lines  24 B are used to transmit coherent packets from processing node  12 B to processing node  12 A). Other sets of lines  24 C- 24 J may be used to transmit packets between processing nodes or between a processing node and an I/O bridge as illustrated in FIG.  1 . The links may be operated either in a cache coherent fashion for communication between processing nodes or in a non-coherent fashion for communication between a processing node and an I/O bridge. The link between nodes will often be referred to as a coherent link whereas the link between a node and an I/O bridge will often be referred to as a non-coherent link. 
     It is noted that a coherent packet to be transmitted from one processing node to another may pass through one or more intermediate processing nodes. For example, a coherent packet transmitted by processing node  12 A to processing node  12 D may pass through either processing node  12 B or processing node  12 C as shown in FIG.  1 . Any suitable routing algorithm may be used. Other embodiments of computer system  10  may include more or fewer processing nodes and I/O bridges then the embodiment shown in FIG.  1 . 
     Processing nodes  12 - 12 D, in addition to a memory controller and interface logic, may include one or more processors. Broadly speaking, a processing node comprises at least one processor and may optionally include a memory controller for communicating with a memory and other logic as desired. Additionally, a processing node may include a host bridge circuit (host bridge) coupled to at least one I/O bridge via a non-coherent link for transforming non-coherent transactions into coherent transactions, and vice versa. 
     Memories  14 - 14 D may comprise any suitable memory device. Memories  14 A- 14 D may comprise one or more RAMBUS DRAMs (RDRAMs), synchronous DRAMs (SDRAMs), static RAM, etc. The address space of computer system  10  may be divided among memories  14 - 14 D. Each processing node  12 - 12 D may include a memory map for determining which addresses are mapped to which memories  14 - 14 D, and hence to which processing node  12 - 12 D a memory access transaction for a particular address should be routed. In one embodiment, the coherency point for an address within computer system  10  may be the memory controller  16 - 16 D coupled to the memory storing bytes corresponding to the address. In other words, the memory controllers  16 A- 16 D may be responsible for ensuring that each memory access to corresponding memories  14 - 14 D occurs in a cache coherent fashion. Memory controllers  16 - 16 D may comprise control circuitry for interfacing to memories  14 - 14 D. Additionally, memory controllers  16 - 16 D may include request queues for queuing memory access transactions. 
     Generally, interface logic  18 A- 18 M may comprise a variety of buffers for receiving packets and for buffering packets to be transmitted. Computer system  10  may employ any suitable flow control mechanism for transmitting packets. For example, in one embodiment, each interface logic  18  may store a count of the number of each type of buffer within the receiver at the other end of the link to which that interface logic is connected. The interface logic does not transmit a packet unless the receiving interface logic has a free buffer to store the packet. As a receiving buffer is freed by routing a packet onward, the receiving interface logic transmits a message to the sending interface logic to indicate that the buffer has been freed. Such a mechanism may be referred to as a “coupon-based” system. 
     Turning next to FIG. 2A, a block diagram with processing nodes  12 A and  12 B is shown to illustrate one embodiment of the coherent link between nodes in more detail. Other embodiments are possible and contemplated. In the embodiment of FIG. 2A, lines  24 A may include a clock line  24 AA, a control line  24 AB, and a control/address/data bus  24 AC. Similarly, lines  24 B may include a clock line  24 BA, a control line  24 BB, and a control/address/data bus  24 BC. 
     The clock line of the coherent link may transmit a clock signal that indicates a sample point for the control line and the control/address/data bus. In one particular embodiment, data/control bits may be transmitted on each edge (i.e. rising edge and falling edge) of the clock signal. Accordingly, two data bits per line may be transmitted per clock cycle. The amount of time employed to transmit one bit per line is referred to herein as a “bit time”. The above-mentioned embodiment includes two bit times per clock cycle. A coherent packet may be transmitted across two or more bit times. Multiple clock lines may be used depending upon the width of the control/address/data bus. For example, two clock lines may be used for a 32 bit control/address/data bus (with one half of the control/address/data bus referenced to one of the clock lines and the other half of the control/address/data bus and the control line referenced to the other one of the clock lines. 
     The control line may indicate whether or not the data transmitted upon the control/address/data bus is either a bit time of a coherent control packet or a bit time of a coherent data packet. The control line, in one embodiment, may be asserted to indicate a coherent control packet, and deasserted to indicate a coherent data packet. Certain coherent control packets may indicate that a coherent data packet follows. The coherent data packet may immediately follow the corresponding coherent control packet. In one embodiment, other coherent control packets may interrupt the transmission of a coherent data packet. Such an interruption may be performed by asserting the control line for a number of bit times during transmission of the coherent data packet and transmitting the bit times of the coherent control packet while the control line is asserted. Coherent control packets which interrupt a coherent data packet may not indicate that a coherent data packet will be following. 
     The control/address/data bus comprises a set of lines for transmitting data/control bits. In one embodiment, the control/address/data bus may comprise 8, 16, or 32 lines. Each coherent link may employ any one of the supported numbers of lines according to design choice. Other embodiments may support other sizes of control/address/data bus as desired. 
     According to one embodiment, the command/address/data bus lines and the clock line may carry inverted data (i.e. a logical one is represented as a low voltage on the line, and a logical zero is represented as a high voltage). Alternatively, lines may carry non-inverted data (in which a logical one is represented as a high voltage on the line, and logical zero is represented as a low voltage). 
     Turning next to FIG. 2B, a block diagram with processing node  12 D and I/O bridge  20  is shown to illustrate one embodiment of the non-coherent link between a node and I/O bridge in more detail. Other embodiments are possible and contemplated. In the embodiment of FIG. 2B, lines  24 I may include a clock line  24 IA, a control line  24 IB, and a control/address/data bus  24 IC. Similarly, lines  24 J may include a clock line  24 JA, a control line  24 JB, and a control/address/data bus  24 JC. 
     Similar to the coherent link, the clock line in the non-coherent link between a node and an I/O bridge may transmit a clock signal that indicates a sample point for the control line and the control/address/data bus. In one particular embodiment, data/control bits may be transmitted on each edge (i.e. rising edge and falling edge) of the clock signal. Accordingly, two data bits per line may be transmitted per clock cycle in the non-coherent coherent link. The amount of time employed to transmit one bit per line is referred to herein as a “bit time”. The above-mentioned embodiment may include two bit times per clock cycle. A non-coherent packet may be transmitted across two or more bit times. Multiple clock lines may be used depending upon the width of the control/address/data bus. For example, two clock lines may be used for a 32 bit control/address/data bus (with one half of the control/address/data bus referenced to one of the clock lines and the other half of the control/address/data bus and the control line referenced to the other one of the clock lines. 
     The control line may indicate whether or not the data transmitted upon the control/address/data bus is either a bit time of a non-coherent control packet or a bit time of a non-coherent data packet. The control line, in one embodiment, may be asserted to indicate a non-coherent control packet, and deasserted to indicate a non-coherent data packet. Certain non-coherent control packets may indicate that a non-coherent data packet follows. The non-coherent data packet may immediately follow the corresponding non-coherent control packet. In one embodiment, other non-coherent control packets may interrupt the transmission of a non-coherent data packet. Such an interruption may be performed by asserting the control line for a number of bit times during transmission of the non-coherent data packet and transmitting the bit times of the non-coherent control packet while the control line is asserted. Non-coherent control packets which interrupt a non-coherent data packet may not indicate that a non-coherent data packet will be following. 
     The control/address/data bus comprises a set of lines for transmitting the data/control bits. In one embodiment, the control/address/data bus may comprise 8, 16, or 32 lines. Each non-coherent link may employ any one of the supported numbers of lines according to design choice. Other embodiments may support other sizes of control/address/data bus as desired. 
     According to one embodiment, the command/address/data bus lines and the clock line may carry inverted data (i.e. a logical one is represented as a low voltage on the line, and a logical zero is represented as a high voltage). Alternatively, lines may carry non-inverted data (in which a logical one is represented as a high voltage on the line, and logical zero is represented as a low voltage). 
     Turning now to FIGS. 3-6, exemplary coherent packets employed in coherent transactions according to one embodiment of computer system  10  are shown. FIGS. 3-5 illustrate coherent control packets and FIG. 6 illustrates a coherent data packet. Other embodiments may employ different coherent packet definitions, as desired. Each of the coherent packets are illustrated as a series of bit times enumerated under the “bit time” heading. The bit times of the coherent packet are transmitted according to the bit time order listed. FIGS. 3-6 illustrate coherent packets for an eight bit coherent control/address/data bus implementation. Accordingly, each bit time comprises eight bits numbered seven through zero. Bits for which no value is provided in the figures may either be reserved for a given coherent packet, or may be used to transmit packet-specific information. 
     FIG. 3 illustrates a coherent information packet (info packet)  30 . Coherent info packet  30  comprises two bit times on an eight bit coherent link. A command encoding (CMD[ 5 : 0 ]) may be transmitted during bit time one, and comprises six bits in the present embodiment. Each of the other coherent control packets shown in FIGS. 4 and 5 may include the command encoding in the same bit positions during bit time  1 . Coherent info packet  30  may be used to transmit messages between processing nodes when the messages do not include a memory address. 
     FIG. 4 illustrates a coherent command packet  32 . Coherent command packets may also be referred to as coherent address packets. Coherent command packet  32  comprises eight bit times on an eight coherent bit link. The command encoding may be transmitted during bit time  1 , along with a portion of a destination node number (DestNode[ 1 : 0 ]). The remainder of the destination node number (DestNode[ 3 : 2 ]) and a source node number (SrcNode[ 3 : 0 ]) may be transmitted during bit time two. A node number (i.e., a destination node number) unambiguously identifies one of the processing nodes  12 A- 12 D within computer system  10 , and may be used in routing a coherent packet through computer system  10 . Additionally, the source of the coherent packet may be assigned a coherent source tag (SrcTag[ 6 : 0 ]) transmitted during bit times  2  and  3 . The coherent source tag may identify coherent packets corresponding to a particular coherent transaction initiated by the source node (i.e. each coherent packet corresponding to a particular coherent transaction includes the same coherent source tag). Bit times four through eight may be used to transmit the memory address affected or targeted by the coherent transaction. Coherent command packet  32  may be used to initiate a coherent memory access transaction (e.g. a coherent read or write transaction). 
     FIG. 5 illustrates a coherent response packet  34 . Coherent response packet  34  may include the command encoding, destination node number, source node number, and coherent source tag similar to the coherent command packet  32 . Various types of coherent response packets may include additional information. For example, a coherent read response packet may indicate the amount of read data provided in a following coherent data packet. Coherent probe response packets may indicate whether a hit was detected for the requested cache block. Generally, coherent response packet  34  may be used when addresses are not required to be transmitted. Furthermore, coherent response packet  34  may be used to a transmit positive acknowledgement to terminate a coherent transaction. 
     FIG. 6 illustrates coherent data packet  36 . Coherent data packet  36  may include eight bit times on an eight bit coherent link in the embodiment of FIG.  6 . Coherent data packet  36  may comprise different numbers of bit times dependent upon the amount of data being transferred. For example, in one embodiment coherent data packet  36  may comprise a cache block of 64 bytes and hence 64 bit times. Other embodiments may define a cache block to be of a different size, as desired. Additionally, data may be transmitted in less than cache block sizes for non-cacheable reads and writes. Coherent data packets for transmitting data less than cache block size employ fewer bit times. 
     FIGS. 3-6 illustrate coherent packets for an eight bit coherent link. Packets for 16 and 32 bit coherent links may be formed by concatenating consecutive bit times illustrated in FIGS. 3-6. For example, bit time one of a coherent packet on a 16 bit coherent link may comprise the information transmitted during bit times one and two on the eight coherent bit link. Similarly, bit time one of the coherent packet on a 32 bit coherent link may comprise the information transmitted during bit times one through four on the eight bit coherent link. Formulas 1 and 2 below illustrate the formation of bit time one of a 16 bit coherent link and bit time one of a 32 bit coherent link according to bit times from an eight bit coherent link. 
     
       
           BT   1   16 [ 15 : 0 ]= BT   2   8 [ 7 : 0 ]∥ BT   1   8 [ 7 : 0 ]  (1) 
       
     
     
       
           BT   1   32 [ 31 : 0 ]= BT   4   8 [ 7 : 0 ]∥ BT   3   8 [ 7 : 0 ]∥ BT   2   8 [ 7 : 0 ]∥ BT   1   8 [ 7 : 0 ]  (2) 
       
     
     FIGS. 7-10 illustrate exemplary packets employed in non-coherent transactions (i.e., communication between a processing node and an I/O bridge) according to one embodiment of the of computer system  10  shown in FIG.  1 . FIGS. 7-9 illustrate non-coherent control packets and FIG. 10 illustrates a non-coherent data packet. Other embodiments may employ different packet definitions, as desired. Each of the non-coherent packets is illustrated as a series of bit times enumerated under the “bit time” heading. The bit times of the non-coherent packet may be transmitted according to the bit time order listed. FIGS. 7-10 illustrate non-coherent packets for an eight-bit control/address/data bus implementation. Accordingly, each bit time comprises eight bits numbered seven through zero. Bits for which no value is provided in the figures may either be reserved for a given non-coherent packet, or may be used to transmit packet-specific information. 
     FIG. 7 illustrates a non-coherent information packet (info packet)  40 . Non-coherent info packet  40  comprises two bit times on an eight bit non-coherent link. A command encoding (CMD[ 5 : 0 ]) may be transmitted during bit time one, and comprises six bits in the present embodiment. Each of the other non-coherent control packets shown in FIGS. 8 and 9 may include the command encoding in the same bit positions during bit time  1 . Non-coherent info packet  40  may be used to transmit messages between a processing node and an I/O bridge when the messages do not include a memory address. 
     FIG. 8 illustrates a non-coherent command packet  42 . Non-coherent command packets may also be referred to as non-coherent address packets. Non-coherent command packet  42  comprises eight bit times on an eight bit non-coherent link. The command encoding of non-coherent packet  42  may be transmitted during bit time  1 . A pipe identification (PipeID[ 4 : 0 ]) may be transmitted during bit time two. Non-coherent command packet  42  may include non-coherent source tag (SrcTag[ 4 : 0 ]) transmitted during bit time  3 . Bit times four through eight may be used transmit the memory address affected by the non-coherent command packet. 
     As noted above, I/O bridge (see e.g., FIG. 1) may connect to I/O buses or devices such as a graphic device. TheI/O buses in turn may connect to one or more I/O devices including a secondary I/O bridge. The secondary I/O bridge, the graphic device, and I/O devices may be examples of “functions.” A function is logical entity that may act as a source or destination of, for example, a memory access request or an interrupt request. Each function may be distinguished by a unique pipe identification. Non-coherent source tags may be used to distinguish memory access requests corresponding to the same pipe identification. Pipe identifications may allow streams of memory access and interrupt requests from different functions to be combined onto a non-coherent link between a processing node and an I/O bridge without losing information about the origins (i.e., sources) of the streams. 
     Non-coherent command packet  42  such as a non-coherent memory access command packet, may be generated by an I/O bridge and transmitted to a host bridge of a processing node in response to a memory access request generated by, for example, an I/O device directly or indirectly coupled to the I/O bridge. A non-coherent memory access command packet initiates a non-coherent memory access transaction. However, a non-coherent memory access transaction doesn&#39;t directly result in access of one of the memories coupled to a processing node. Rather, a non-coherent memory access transaction must be converted to a coherent memory access transaction before one of the memories is accessed. Thus, a non-coherent memory access command packet must be transformed into a coherent memory access command packet which, in turn, initiates a corresponding coherent memory access transaction. 
     A non-coherent command packet  42  in the form of a non-coherent interrupt command packet, may also be generated by an I/O bridge and transmitted to a host bridge of a processing node in response to an interrupt request generated by, for example, an I/O device directly or indirectly coupled to the I/O bridge. A non-coherent interrupt command packet received at the processing node initiates a coherent interrupt transaction. In general, the coherent interrupt transaction begins with the processing node generating an Interrupt Broadcast packet. The remaining nodes in the system receive the Interrupt Broadcast packet and determine whether they are one of the targeted destinations for the interrupt. A targeted node may be a node that is capable of servicing the interrupt request. All nodes respond with a packet identifying themselves as a targeted node or not. Nodes that are not targeted indicate this in an LpaInfo field of the response packet. Nodes that are targeted supply this information in the LpaInfo, and place their interrupt priority in a priority field of the response packet. A node that is the focus processor for the interrupt can also indicate this information in the LpaInfo field. 
     When the processing node has received the responses from the remaining nodes, the processing node selects the processor of one of the remaining nodes to be interrupted. The processing node will select the focus processor if there is one (selecting randomly if there is more than one focus processor). Otherwise, the processing node will select the targeted processor with the lowest priority. In the event of a tie, the processing node picks one of the lowest priority processors at random. In one embodiment, selection of the target or focus processor is compatible with the advanced programmable interrupt controller (APIC) provided by Intel Corporation. Ultimately, the coherent interrupt command packet (or Directed Interrupt packet) is sent by the processing node to the selected node with the focus processor or the targeted processor. 
     The processing node generates the coherent interrupt command packet by transforming the non-coherent interrupt command packet received from the I/O  20 . Thus, a non-coherent interrupt command packet must be transformed into a coherent interrupt command packet. 
     Non-coherent command packet  42 , as shown in FIG. 8, is similar to the coherent command packet  32  shown in FIG.  4 . However, some differences are important to note. More particularly, the coherent command packet  32  of FIG. 4 does not use the pipe identification or non-coherent source tag of coherent command packet  42  shown in FIG.  8 . Rather, the destination node number replaces the pipe identification, and the coherent source tag replaces the non-coherent source tag. When a host bridge receives a non-coherent memory access command packet or a non-coherent interrupt command packet from an I/O bridge, the host bridge transforms the received non-coherent memory access command packet or non-coherent interrupt command packet into a coherent memory access command packet or coherent interrupt transaction packet, respectively, for transmission on a coherent link. Coherent memory access transactions initiated with non-coherent memory access command packets having the same pipe identification, are often required to complete in order. Likewise, a coherent interrupt transaction initiated with non-coherent interrupt command packet having the same pipe identification of previously initiated non-coherent memory access transactions, are often required to complete after completion of coherent memory access transactions corresponding to the previously initiated non-coherent memory access transactions. The transformed non-coherent memory access command packet and transformed non-coherent interrupt command packets, however, no longer contain pipe identifications. Without a pipe identification in the transformed non-coherent memory access and interrupt command packets, transformed non-coherent interrupt command packets might be completed prior to completion of earlier transformed non-coherent memory access transactions having the same pipe identification. 
     FIG. 9 illustrates one embodiment of non-coherent response packet  44 . Non-coherent response packet  44  may include the command encoding, pipe identification, and non-coherent source tag similar to the non-coherent command packet  42 . Various types of non-coherent response packets may include additional information. For example, a non-coherent read response packet may include a Count to indicate the amount of data provided in a following non-coherent data packet. Unlike coherent transactions, non-coherent transactions do not employ probes nor probe responses, commands which will be further explained below. Generally, non-coherent response packet  44  may be used when addresses are not required to be transmitted. Furthermore, non-coherent response packet  44  may be used to transmit a positive acknowledgement to terminate a non-coherent transaction. 
     FIG. 10 illustrates the non-coherent data packet  46 . Non-coherent data packet  36  may include eight bit times on an eight bit non-coherent link in the embodiment of FIG.  10 . Non-coherent data packet  46  may comprise different numbers of bit times dependent upon the amount of data being transferred. 
     FIGS. 7-10 illustrate non-coherent packets for an eight bit non-coherent link. Packets for 16 and 32 bit non-coherent links may be formed by concatenating consecutive bit times illustrated in FIGS. 7-10. For example, bit time one of a non-coherent packet on a 16 bit non-coherent link may comprise the information transmitted during bit times one and two on the eight non-coherent bit link. Similarly, bit time one of the non-coherent packet on a 32 hit coherent link may comprise the information transmitted during bit times one through four on the eight bit non-coherent link. Formulas 3 and 4 below illustrate the formation of bit time one of a 16 bit non-coherent link and bit time one of a 32 bit non-coherent link according to bit times from an eight bit non-coherent link. 
     
       
           BT   1   16 [ 15 : 0 ]= BT   2   8 [ 7 : 0 ]∥ BT   1   8 [ 7 : 0 ]  (3) 
       
     
     
       
           BT   1   32 [ 31 : 0 ]= BT   4   8 [ 7 : 0 ]∥ BT   3   8 [ 7 : 0 ]∥ BT   2   8 [ 7 : 0 ]∥ BT   1   8 [ 7 : 0 ]  (4) 
       
     
     Turning now to FIG. 11, a table  48  is shown illustrating commands employed in transmitted coherent or non-coherent packets according to one exemplary embodiment of the computer system  10 . Other embodiments are possible and contemplated. Table  48  includes a command code column illustrating the command encoding assigned to each command, a command column naming the command, and a packet type column indicating which packet is employed for that command. 
     A coherent memory read transaction might be initiated by transmitting a coherent command packet containing one of the ReadSized, RdBlk, RdBlkS or RdBlkMod commands. The ReadSized command may be used for non-cacheable reads or reads of data other than a cache block in size. The amount of data to be read may be encoded into the coherent ReadSized command packet. For reads of a cache block, the RdBlk command may be used unless: (i) a writeable copy of the cache block is desired, in which case the RdBlkMod command may be used; or (ii) a copy of the cache block is desired but no intention to modify the block is known. The RdBlkS command may be used to make certain types of coherency schemes (e.g. directory based coherency schemes) more efficient. 
     In general, the appropriate read command may be transmitted in a coherent command packet from a source node initiating the memory access transaction to a destination node coupled to the memory which owns the requested cache block. The memory controller in the destination node may broadcast a Probe/Src command in coherent command packets to the other nodes in the system to maintain coherency by changing the state of the cache block in those nodes and by causing a node including an updated copy of the cache block to send the cache block to the source node. Each node receiving a Probe/Src may transmit a coherent response packet containing a ProbeResp command to the source node. 
     If a probed node has an updated copy of the read data (i.e. dirty data), that node may transmit a RdResponse command in a coherent command packet and the dirty data in a subsequent coherent data packet. A node transmitting dirty data may also transmit a MemCancel command in a coherent response packet to the destination node in an attempt to cancel transmission by the destination node of the requested read data. Additionally, the memory controller in the destination node may transmit the requested read data using a RdResponse command in a coherent command packet followed by the data in a coherent data packet. If the source node receives a RdResponse command from a probed node, the associated read data is used. Otherwise, the data from the destination node is used. 
     Once each of the coherent response packets and the read data is received in the source node, the source node transmits a coherent SrcDone response packet to the destination node as a positive acknowledgement of the termination of the coherent memory read transaction. 
     A coherent memory write transaction may be initiated by transmitting a coherent WrSized or VicBlk command packet followed by corresponding coherent data packet. The coherent WrSized command packet may be used for non-cacheable writes or writes of data other than a cache block in size. To maintain coherency for WrSized commands, the memory controller in the destination node may broadcast a coherent Probe/Tgt command packet to each of the other nodes in the system. In response to the Probe/Tgt command, each probed node may transmit a ProbeResp packet to the destination node. If a probed node is storing dirty data, the probed node may respond with a RdResponse packet, and the dirty data is subsequently transmitted in a coherent data packet. In this manner, a cache block updated by the WrSized command is returned to the memory controller for merging with the data provided with the WrSized command. The memory controller, upon receiving coherent probe response packets from each of the probed nodes, may transmit a TgtDone command in a coherent response packet to the source node to provide a positive acknowledgement of the termination of the coherent memory write transaction. The source node may reply with a SrcDone command in a coherent response packet. 
     A victim cache block that has been modified by a node and is being replaced in a cache within the node may be transmitted back to memory using the coherent VicBlk command packet. Probes are not needed for the coherent VicBlk command packet. Accordingly, when the destination memory controller is prepared to commit victim block data, the destination memory controller may transmit a TgtDone packet in a coherent response packet to the source node of the victim block. The source node may reply with either a SrcDone in a coherent response packet to indicate that the data should be committed or a MemCancel in a coherent response packet to indicate that the data has been invalidated between transmission of the coherent VicBlk command packet and receipt of the TgtDone response packet (e.g. in response to an intervening probe). 
     The coherent ChangetoDirty command packet may be transmitted by a source node in order to obtain write permission for a cache block stored by the source node in a non-writeable state. A transaction initiated with a coherent ChangetoDirty command packet may operate similar to a read except that the destination node does not return data. The coherent ValidateBlk command packet may be used to obtain write permission to a cache block not stored by a source node if the source node intends to update the entire cache block. No data is transferred to the source node for such a transaction, but otherwise operates similar to a coherent memory read transaction. 
     The coherent Interrupt Broadcast (Intr/Brd), Interrupt Target (Int/Tgt), and IntrResponse packets may be used to broadcast interrupts, to send an interrupt to a particular destination node, and to respond to interrupts, respectively. The coherent CleanVicBlk command packet may be used to inform a memory controller that a clean victim block has been discarded from a node (e.g. for directory based coherency schemes). The coherent TgtStart command packet may be used by a destination node to indicate that a first coherent transaction has been started and is complete to the extent necessary for a subsequent coherent transaction to begin without violating transaction ordering. The TgtStart command packet is useful in situations when initiation of the subsequent is dependent upon completion of the first transaction. The TgtStart command packet enables early initiation of the subsequent coherent transaction which otherwise would have been delayed until completion of the first coherent transaction. For example, the TgtStart command may enable early initiation of a coherent memory access transaction corresponding to a non-coherent memory access transaction or the early initiation of a coherent interrupt transaction corresponding to a non-coherent interrupt transaction as will be more fully described below. The Error command is used to transmit an error indication. 
     Non-coherent transactions use many of the same commands of FIG. 11 as the coherent transactions. With reference to FIG. 1, non-coherent memory access transactions may begin with a memory access request generated by one of the I/O devices  28  or graphic device  26  connected directly or indirectly to an I/O bridge  20 . The received I/O device requests may be transformed into non-coherent memory access transactions by the I/O bridge  20 , and the subsequent non-coherent memory access transactions are transmitted via non-coherent link to a connecting host bridge (not shown in FIG.  1 ). For example, the I/O bridge  20  may generate a non-coherent command packet containing a ReadSized command in response to receiving a memory read request from a coupled I/O device  28 . The non-coherent ReadSized command packet, including the memory address of the memory read request, is then transmitted over the non-coherent link to the host bridge connected thereto. The host bridge in turn may start a coherent memory read transaction by transforming the non-coherent ReadSized command packet into a coherent ReadSized command packet and transmitting the resulting coherent ReadSized command packet to the destination node that owns the memory address where the requested data is stored. Thereafter, the coherent memory read transaction proceeds as described above, and a coherent RdResponse packet and a subsequent coherent data packet may be returned to the source node containing the host bridge. The host bridge in turn may transform the received coherent RdResponse and data packets into non-coherent RdResponse and data packets, and transmit the non-coherent RdResponse and data packets over the non-coherent link to the I/O bridge connected thereto. The I/O bridge may then route the read data to the source I/O device  28  requesting the read data. 
     A non-coherent memory write transaction may be initiated by the I/O bridge in response to receiving a write memory request from an I/O device coupled directly or indirectly thereto. For example, the I/O bridge may generate a non-coherent command packet containing a WrSized command in response to receiving memory write request from a coupled I/O device. The non-coherent WrSized command packet, including the memory address of the memory write request, may then be transmitted over the non-coherent link to the host bridge connected thereto. The host bridge in turn may start a coherent memory write transaction by transforming the non-coherent WrSized command packet into a coherent WrSized command packet and transmitting the coherent WrSized command packet to the destination node that owns the memory address where the requested data, which follows in a coherent data packet, is to be written. The destination node may proceed as described above and send a coherent TgtDone response packet back to the source node containing the host bridge. Optionally, the host bridge may in turn generate a non-coherent TgtDone response packet for transmission to the I/O bridge. 
     When a non-coherent memory access transaction or non-coherent interrupt transaction is initiated by the I/O bridge, the host bridge may respond by starting a new coherent memory access transaction or a new coherent interrupt transaction, respectively. As noted earlier, a coherent memory access transaction converted from a non-coherent memory access transaction, may be required to complete or start at a target node in order and prior to a coherent interrupt transaction converted from a non-coherent interrupt transaction if the non-coherent memory access transactions and the subsequent non-coherent interrupt transaction have the same pipe identification. Further, a coherent interrupt transaction converted from a non-coherent interrupt transaction may be required to complete or start at the target node before initiation of a subsequent coherent memory access transaction converted from a non-coherent memory access transaction having the same pipe identification of the non-coherent interrupt transaction. However, the coherent memory access and interrupt transactions representing the transformed non-coherent memory access and interrupt transactions, respectively, lack pipe identifications. Pipe identifications are used to insure non-coherent ordering of certain transactions. Since the non-coherent transactions are converted into coherent transactions, the resulting coherent transactions lose the information necessary to maintain transaction order. As a result an error may arise when a coherent memory access transaction representing a transformed non-coherent memory access transaction completes or starts out of order with a coherent interrupt transaction representing a transformed non-coherent interrupt transaction. Therefore, before a new coherent interrupt transaction converted from a non-coherent interrupt transaction, is started, all pending coherent memory access transactions converted from non-coherent memory access transactions, must complete or start if the pending coherent memory access transactions are transformed non-coherent memory access transactions having the same pipe identification as the non-coherent interrupt transaction corresponding to the coherent interrupt transaction. Likewise, all non-coherent memory access transactions transmitted to the host node after the non-coherent interrupt transaction, must be delayed until the completion or the start of the prior non-coherent interrupt transaction if the non-coherent transactions have the same pipe identifications. 
     With reference to FIGS. 1 and 12, one embodiment of host bridge circuit (host bridge)  50  mentioned above will be described in greater detail. Other embodiments of the host bridge  50  are contemplated. Host bridge  50  may include non-coherent transaction buffers  52  for receiving non-coherent packets from I/O bridge  20 , a pending coherent memory access transaction circuit  56  which stores pipe identifications corresponding to pending coherent memory access transactions wherein the pending coherent memory access transactions represent transformed non-coherent memory access transactions, host bridge control circuit  62 , and coherent transaction packet generator circuit  64 . 
     Transaction packet buffers  52  optionally may be included in interface node  18 L and may receive non-coherent packets from I/O bridge circuit  20  if the buffers have sufficient space. Non-coherent info packets may be used to control flow of non-coherent packets to buffers  52  when one or more buffers lack space to receive non-coherent packets. Coherent transaction packet generator  64 , and pending coherent memory access transaction circuit  56  are coupled to one or more buffers of the non-coherent transaction packet buffers  52 . Host bridge control circuit  62  may control pending coherent memory access transaction circuit  56  and coherent packet generator circuit  64 . Pending coherent memory access transaction circuit  56  is updated by host bridge control circuit  62  each time a coherent memory access transaction converted from a non-coherent memory access transaction, is completed so that only pipe identifications corresponding to pending coherent memory access transactions are stored therein. Optionally, pending coherent memory access transaction circuit  56  may be updated by host bridge control circuit  62  each time a coherent memory access transaction converted from a non-coherent memory access transaction, is completed or started so that only pipe identifications corresponding to pending coherent memory access transactions, which have not completed or started, are stored therein. Coherent memory access transaction circuit  56  may be updated upon receipt by host bridge control circuit  62  of the TrgStart command mentioned above. 
     One of the buffers of non-coherent transaction buffers  52  may receive a non-coherent interrupt command packet. Thereafter the pipe identification contained in the received non-coherent interrupt command packet may be extracted. The extracted pipe identification may be provided to pending coherent memory access transaction circuit  56  for comparison with pipe identifications stored therein. Pending coherent memory access transaction circuit  56  generates a match or mismatch signal depending on the outcome of the comparison. The signal is provided to host bridge circuit  62 . 
     If a match does not exist between the extracted pipe identification and the stored pipe identifications, then the coherent transaction generation circuit is instructed by the host bridge control circuit  62  to generate a coherent interrupt transaction including transmission of a corresponding coherent interrupt command packet using information in the received non-coherent interrupt command packet. The generated coherent interrupt command packet is subsequently transmitted to one of the processing nodes via one or more coherent links. If a match occurs between the extracted pipe identification and a pipe identification in a pending coherent memory access transaction circuit  56 , the coherent transaction generation circuit  64  is directed by the host bridge control circuit  62  to stall generation of a coherent interrupt transaction corresponding to the received non-coherent interrupt transaction. The coherent transaction generation circuit may continue to stall until the match signal is deasserted. Normally, the match signal is deasserted once one or more of coherent memory access transactions causing the stall, have completed or started. 
     Turning now to FIG. 13, a more detailed explanation regarding operation of the host bridge  50  shown in FIG. 12 is described. More particularly, FIG. 13 shows a flow chart illustrating operational aspects of the host bridge  50  shown in FIG.  12 . In step  100 , the host bridge  40  may receive a non-coherent interrupt command packet from I/O bridge  20  (not shown in FIG.  10 ). The received non-coherent packet is stored in one of the buffers of buffers  52  if space is available therein. 
     In step  102 , a pipe identification extracted from the received non-coherent command packet may be read from the received non-coherent interrupt command packet. In step  104  pending memory access coherent transaction circuit  64  may be accessed with the extracted pipe identification to determine if the extracted pipe identification matches a pipe identification stored therein. If a match does not exist, the process may proceed to step  106  wherein a coherent interrupt command packet corresponding to the received non-coherent interrupt command packet, is generated and transmitted to an appropriate destination node via one or more coherent links. However, if a match does exist within pending transaction circuit  64 , the process may be stalled by repeatedly reinitiating step  106  until a the extracted pipe identification fails to match a pipe identification stored in coherent pending memory access transaction circuit  56 . 
     While the present invention has been described with reference to particular embodiments, it will be understood that the embodiments are illustrated and that the invention scope is not so limited. Any variations, modifications, additions and improvements to the embodiments described are possible. These variations, modifications, additions and improvements may fall within the scope of the invention as detailed within the following claims.