METHOD AND APPARATUS FOR INITIALIZING A COMPUTER INTERFACE

Abstract: According to one embodiment, a computer system includes a first hub agent and a hub interface coupled to the first hub agent. The first hub agent is adaptable to sample the hub interface in order to detect the presence of a second hub agent upon initiation of the computer system. In a further embodiment, the first hub agent comprises a presence detect module and control logic coupled to the presence detect module. The control logic responds to a central processing unit (CPU) poll request if the second hub agent is detected and does not respond to the CPU if the first device is not detected.

Description: METHOD AND APPARATUS FOR INITIALIZING A COMPUTER INTERFACE The present invention pertains to the field of computer systems. More particularly, the invention pertains to the field of detecting devices coupled to a hub interface in a computer system. BACKGROUND OF THE INVENTION Prior computer systems typically rely on standardized busses, such as the Peripheral Component Interconnect (PCI) bus adhering to a Specification Revision 2.1 bus developed by the PCI Special Interest Group of Portland Oregon, to allow computer system chipset components to communicate one with another. For example, a transaction originating at a processor and intended for a disk drive might first be delivered to a first chipset component that serves as an intermediary between the processor bus and a PCI bus. The first chipset component would then deliver the transaction over the PCI bus to a second system chipset component which would then deliver the transaction to the disk drive. Busses such as the PCI bus also provide for communication with other computer system devices such as graphics controllers and network adapters. Because busses such as the PCI bus must interface with a variety of component types, each with varying requirements, the busses are not necessarily optimized for allowing communication between chipset components. Further, chipset manufacturers who rely on standardized busses such as the PCI bus must adhere to bus standards in order to ensure compatibility with other components, and are not at liberty to make substantial changes in how the chipset components communicate with each other. Another issue that faces chipset component manufacturers in designing and manufacturing chipset components is the need to conform to standardized supply and signaling voltages when relying on busses such as PCI for communication between chipset components, thereby locking the manufacturers into certain design practices and manufacturing technologies. Therefore, it would be desirable to provide a flexible point to point interface that provides optimal communication between chipset components. In addition, it would be desirable to have a mechanism for initializing such an interface wherein the presence of a device coupled to the chipset via the interface is ascertained. Further, it would be advantageous to assign device identification numbers if a device is detected so that a processor may poll and read the devices. SUMMARY OF THE INVENTION According to one embodiment, a system is disclosed that includes a central processing unit (CPU) and a memory controller hub (MCH) coupled to the CPU. The MCH includes a first interface controller that is operable to detect the presence of a hub agent coupled to the MCH. BRIEF DESCRIPTION OF THE DRAWINGS The invention is illustrated by way of example and not limitation in the figures of the accompanying drawings, in which like references indicate similar elements, and in which: Figure 1 is a block diagram of one embodiment of a computer system; Figure 2 is a block diagram of one embodiment of agents connected by a hub interface; Figure 3 is a block diagram of one embodiment of a presence detection mechanism; Figure 4 is a flow diagram for one embodiment of the initialization of a hub interface; Figure 5 is a timing diagram illustrating a split transaction implemented by one embodiment of an interface; Figure 6 is a timing diagram illustrating arbitration and transmission of data packets, according to one embodiment; Figure 7 is a timing diagram illustrating flow control of data packets, according to one embodiment; Figure 8 illustrates a flow diagram describing the steps of responding to flow control operations according to one embodiment; Figure 9 illustrates the physical signal interface according to one embodiment; and Figure 10 is a timing diagram illustrating source synchronous clocking according to one embodiment. DETAILED DESCRIPTION A method and apparatus for initializing a hub interface is described. In the following detailed description of the present invention numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form, rather than in detail, in order to avoid obscuring the present invention. Some portions of the detailed descriptions that follow are presented in terms of algorithms and symbolic representations of operations on data bits within a computer memory. These algorithmic descriptions and representations are the means used by those skilled in the data processing arts to most effectively convey the substance of their work to others skilled in the art. An algorithm is here, and generally, conceived to be a self- consistent sequence of steps leading to a desired result. The steps are those requiring physical manipulations of physical quantities. Usually, though not necessarily, these quantities take the form of electrical or magnetic signals capable of being stored, transferred, combined, compared, and otherwise manipulated. It has proven convenient at times, principally for reasons of common usage, to refer to these signals as bits, values, elements, symbols, characters, terms, numbers, or the like. It should be borne in mind, however, that all of these and similar terms are to be associated with the appropriate physical quantities and are merely convenient labels applied to these quantities. Unless specifically stated otherwise as apparent from the following discussions, it is appreciated that throughout the present invention, discussions utilizing terms such as "processing" or "computing" or "calculating" or "determining" or "displaying" or the like, refer to the action and processes of a computer system, or similar electronic computing device, that manipulates and transforms data represented as physical (electronic) quantities within the computer system's registers and memories into other data similarly represented as physical quantities within the computer system memories or registers or other such information storage, transmission or display devices. The present invention also relates to apparatus for performing the operations herein. This apparatus may be specially constructed for the required purposes, or it may comprise a general-purpose computer selectively activated or reconfigured by a computer program stored in the computer. Such a computer program may be stored in a computer readable storage medium, such as, but is not limited to, any type of disk including floppy disks, optical disks, CD-ROMs, and magneto-optical disks, read-only memories (ROMs), random access memories (RAMs), EPROMs, EEPROMs, magnetic or optical cards, or any type of media suitable for storing electronic instructions, and each coupled to a computer system bus. The algorithms and displays presented herein are not inherently related to any particular computer or other apparatus. Various general-purpose machines may be used with programs in accordance with the teachings herein, or it may prove convenient to construct more specialized apparatus to perform the required method steps. The required structure for a variety of these machines will appear from the description below. In addition, the present invention is not described with reference to any particular programming language. It will be appreciated that a variety of programming languages may be used to implement the teachings of the invention as described herein. The programs including executable instructions may be executed by one or more programming devices (e.g., a central processing unit (CPU), processor, controller, etc.) in one or more personal computer systems, servers, workstations, etc. Figure 1 is a block diagram of one embodiment of a computer system 100. Computer system 100 includes a central processing unit (CPU) 102 coupled to bus 105. In one embodiment, CPU 102 is a processor in the Pentium® family of processors including the Pentium® π processor family and Pentium® EH processors available from Intel Corporation of Santa Clara, California. Alternatively, other CPUs may be used. A memory control hub (MCH) 110 is also coupled to bus 105. MCH 110 may include a memory controller 112 that is coupled to a main system memory 115. Main system memory 115 stores data sequences of instructions that are executed by CPU 102 or any other device included in system 100. In one embodiment, main system memory 115 includes dynamic random access memory (DRAM); however, main system memory 115 may be implemented using other memory types. Additional devices may also be coupled to bus 105, such as multiple CPUs and/or multiple system memories. MCH 110 may also include a graphics interface 113 coupled to a graphics accelerator 130. In one embodiment, graphics interface 113 is coupled to graphics accelerator 130 via an accelerated graphics port (AGP) that operates according to an AGP Specification Revision 2.0 interface developed by Intel Corporation of Santa Clara, California. In other embodiments, graphics interface 1 13 may be implemented using a hub interface controller 120 coupled to graphics accelerator 130. MCH 1 10 may also include a hub interface 120 that is coupled to a bridge 125. Bridge 125 may provide an interface between MCH 110 and a system bus. Bridge 125 is coupled to MCH 1 10 via hub interface A. Further, MCH 1 10 is coupled to an input/output control hub (ICH) 140 via hub interface B. ICH 140 provides an interface to input/output (I/O) devices within computer system 100. ICH 140 may include one or more interface controllers 120. For example, one interface controller 120 may be coupled to a network interface 160 via hub interface C. In addition, another interface controller 120 may be coupled to a fibre channel 165. Devices coupled together via a hub interface may be referred to as hub agents. A hub agent that is positioned closer to CPU 102 in computer system 100 in terms of travel distance may be referred to as an upstream agent, while an agent that is further away from CPU 102 is referred to as a downstream agent. For example, bridge 125 is downstream of MCH 110, ICH 140 is downstream of MCH 110, and network interface 160 and fibre channel 165 are downstream agents of both MCH 110 and ICH 140. However, one of ordinary skill in the art will appreciate that hub interfaces 120 may be coupled to other devices. ICH 140 may also include a bridge 146 that provides a conventional interface to a PCI bus. Bridge 146 provides a data path between CPU 102 and peripheral devices. Devices that may be coupled to PCI bus 142 include an audio device 150 and a disk drive 155. However, one of ordinary skill in the art will appreciate that other devices may be coupled to PCI bus 142. In addition, one of ordinary skill in the art will recognize that CPU 102 and MCH 110 could be combined to form a single chip. Further graphics accelerator 130 may be included within MCH 110 in other embodiments. Figure 2 is a block diagram of one embodiment of a hub interface coupling an upstream agent 220 and downstream agent 230. A hub interface is a mechanism for connecting main building blocks of the core logic of a computer system, such as the system 100, via a relatively narrow and relatively high bandwidth data path. Between individual components in computer system 100, such as between MCH 1 10 and bridge 125, the connection is implemented in a point-to-point fashion. According to one embodiment, transfer of information across the hub interface bus is accomplished using a packet-based split-transaction protocol. Hub interfaces will be discussed in more detail below. The hub interface includes a bi-directional data path 251, a stop signal 253, a request A (RQA) signal 254 and a request B (RQB) signal 258 and a system reset signal 255. According to one embodiment, the data path is 8 bits wide. However, the data path width may be any width that is a power of 2. Stop signal 243 is a bi-directional signal used for flow control. RQA signal 254 and RQB signal 258 are request signals that, during normal system operation, are asserted in order to request for control of the hub interface. In addition, RQB signal 258 may be used during a system reset to detect whether a downstream agent 230 is actually coupled to upstream agent 220. The hub interface may also include other signal paths, such as a clock signal, one or more data strobes that operate at four times the frequency of the clock signal and a voltage reference signal (not shown). Alternatively, the data strobes may operate at a multiple of the clock signal other than four. For example, the data strobes may run at a rate of eight times that of the system clock signal. The hub interface is coupled to interface controllers 120 and 232 within upstream agent 220 and downstream agent 230, respectively. Interface controllers 120 and 232 control the interaction between the hub interface agents. Interface controller 120 includes a presence detect module 225 that detects whether downstream agent 230 is present. Interface controller 232 includes a presence acknowledge unit 235 that transmits a signal to presence detect module 225 acknowledging the presence of downstream agent 230. According to one embodiment, the presence detect process is executed during system initialization. System initialization is detected at a hub interface upon reset signal 255 being de-asserted. Upstream agent 220 also includes control logic 228. Control logic 228 receives signals from presence detect module 225 identifying whether a device is coupled to upstream agent 220. Control logic 228 also stores a device identification (ID) number assignment for upstream agent 220. Control logic 228 maintains this information and responds to CPU 102 whenever CPU 102 addresses the device ID for the agent including control logic 228. Figure 3 is a block diagram of one embodiment of the device presence detect mechanism between presence detect module 225 and presence acknowledge unit 235. Presence detect module 225 and presence acknowledge unit 235 are coupled via RQB signal 258. According to one embodiment, presence detect module 225 and presence acknowledge unit 235 include resistors Rl and R2, respectively. Rl is a pull-up resistor coupled to a line voltage Vcc, while R2 is a pull-down resistor coupled to ground. Further, resistor Rl is designed to have a small resistance in relation to resistor R2. During an interface initialization (i.e., the de-assertion of reset signal 255), control logic 228 monitors RQB signal 258 to determine whether a downstream agent 230 is coupled to upstream agent 220. According to one embodiment, control logic 228 samples RQB signal 258 on the inactive going edge of a power-on reset in order to determine whether a downstream agent is present. If no downstream agent 230 is present, RQB signal 258 is pulled high, resulting in a high logic level being transmitted to control logic 228. If a downstream agent 230 is detected, RQB signal 258 is pulled to ground since resistor Rl has a high resistance compared to resistor R2. Consequently, a low logic level is transmitted to control logic 228 representing that upstream agent 230 is present. One of ordinary skill in the art will recognize that resistors Rl and R2 may be replaced with other devices, such as transistors. Further, presence detect module 225 and presence acknowledge unit 235 may be configured such that a high logic level is transmitted whenever a device is detected and a low logic level is transmitted if no device is present. Upon system initialization, CPU 102 polls control logic 228 within each upstream agent 220 in order to ascertain whether a downstream agent 230 is coupled to each particular upstream agent 220. If no agent is detected, control logic 228 does not respond to the CPU 102 poll request. Moreover, if a device is not detected at upstream agent 220, CPU 102 manages the upstream agent 220 as if it does not exist. If a downstream agent 230 is detected, CPU 102 receives a response from the upstream agent and subsequently polls the downstream agent. According to one embodiment, device ED numbers are assigned to all components within computer system 100 upon configuration. In addition, component device ID numbers are split between MCH 1 10 and ICH 140. For example, components within MCH 1 10 may be assigned device ID numbers between 0 and 7, and components within ICH 140 may be assigned device ED numbers between 8 and 31. Further, if an upstream agent 220 maps logical internal devices onto a downstream agent, the upstream agent 220 uses device ED numbers 16-31 for any internal devices mapped onto the downstream agent. Figure 4 is a flow diagram for one embodiment of the initiation of a hub interface 120 (e.g., upstream agent 220). At process block 410, a hub interface is reset following a computer system 100 power-on reset. At process block 420, upstream agent 220 samples RQB signal 258 in order to determine whether a downstream agent is coupled to the hub interface. At process block 430, a signal is transmitted to control logic 228 indicating whether a downstream agent has been detected. At process block 434, it is determined whether a downstream agent has been detected by upstream agent 220. If a downstream device has not been detected, the upstream agent disables the hub interface, process block 436. At process block 440, CPU 102 polls upstream agent 220. At process block 450, it is determined whether CPU 102 receives a poll response from upstream agent 220. If a CPU 102 receives a response from upstream agent 220, CPU 102 polls downstream agent 230, process block 460. If it has been determined that no downstream agent is coupled to upstream agent 220, control logic 228 does not respond to the CPU 102 poll request since the hub interface has been disabled, process block 470. Consequently, upstream agent 220 is rendered invisible to CPU 102 and computer system 100. Referring back to Figure 2, the hub agents provide a central connection between two or more separate buses and/or other types of communication lines. By using the hub interface to interconnect the MCH 110 and the ICH 140, improved access is provided between I/O components and the CPU/memory subsystem (e.g., increased bandwidth, protocol independence, and lower latency.) In addition, the hub interface may also improve the scalability of a computer system (e.g., upgrading from a base desktop platform to high-end desktop platforms or workstation platform) by providing a backbone for I O building blocks. To provide the improved interface, the hub interface includes one or more unique features. In one embodiment, transactions are transferred across the hub interface using a packet based split-transaction protocol. For example, a Request Packet is used to start a transaction and a separate Completion Packet may subsequently be used to terminate a transaction, if necessary. Figure 5 illustrates an example of a split transaction across the hub interface. As illustrated in Figure 5, a hub agent initially obtains ownership of the hub interface via arbitration 502. Following the arbitration, there is a request phase 504. If necessary (e.g., in the case of returning data for a read transaction), a completion phase 508 will follow the request phase. Prior to the completion phase, however, the responding hub agent, will first arbitrate 506 for ownership of the hub interface. In between the time of transmitting a request packet and a corresponding completion packet across the hub interface, separate unrelated packets may be transmitted across the hub interface in accordance with predetermined order rules, as discussed below in more detail. For example in the case of a read request from a peripheral to memory, providing the requested data may take multiple clock cycles to have the data ready to be returned in a completion packet. During the time it takes to obtain the requested data, separate unrelated completion and/or request packets waiting in a queue/pipe of the MCH 110, may be transmitted to the ICH 140. Furthermore, as shown in Figure 5, each request or completion is transmitted as a packet across the interface. For write type transactions, data is associated with the request. For read type transactions, there will be data associated with the completion. In some cases, there will be more than one completion for a request for the case where the completion packet is disconnected, effectively splitting it into multiple completion packets. In addition, in one embodiment, the hub interface uses transaction descriptors for routing of hub interface traffic as well as identifying the attributes of a transaction. For instance, the descriptors may be used to define a transaction as isochronous or asynchronous, which, as a result, may then be handled in accordance with a predefined protocol. Furthermore, in one embodiment, the bandwidth of the interface is increased in part by transmitting the data packets via a source synchronous clock mode. Moreover, in one embodiment, the hub interface provides the increased bandwidth despite using a narrow connection (e.g., less pins/pads). In alternative embodiments, however, a hub interface may be implemented with less than all of the unique features as discussed above, without departing from the scope of the invention. Moreover, the hub interface could also be used to interconnect bridges and and/or other components within or external to a chipset, without departing from the scope of the present invention. TRANSACTION. PROTOCOL AND PHYSICAL LAYERS For greater clarity, the hub interface is described in three parts: a transaction layer; a protocol layer; and a physical layer. The distinctions between layers, however, is to be regarded in an illustrative rather than a restrictive sense, and is therefore does not to imply a particular preferred embodiment. TRANSACTION LAYER In one embodiment of the hub interface, the transaction layer supports the routing of separate transactions transmitted across the hub interface (which may consist of one or more packets.) For example, in one embodiment, the transaction layer of the hub interface generates transaction descriptors, which are included in the requests and data packets. The transaction descriptors may be used to support arbitration between queues within a hub agent (e.g., MCH), and/or to facilitate routing of requests and data packets through the hub interface. For instance, in one embodiment, the transaction descriptors support routing of completion packets back to the request-initiating agent based on initially supplied (within a request packet) routing information. The transaction descriptors also help to reduce or possibly minimize packet-decoding logic within the hub agents. In alternative embodiments, the transaction descriptors also provide the ability to distinguish the handling of requests based on their respective transaction attributes. For instance, the transaction attributes identified in the transaction descriptors may identify operations as Isochronous (i.e., operations that move fixed amounts of data on a regular basis; e.g., video or audio real time operations.) As a result, the operations, as identified by the transaction attributes, may be handled in accordance with a corresponding predetermined routing protocol in order to support a specific type of operation (e.g., isochronous.) In one embodiment, the transaction descriptors include two fields: a routing field and an attribute field. In alternative embodiments, more or less fields may be used to provide one or more of the functions of the transaction descriptors, without departing from the scope of the invention. In one embodiment, the routing field is a six-bit field used for packet routing, as shown below in Table 1. The size of the routing field, as well as the attribute field, may vary within the scope of the invention. Table 1 Routing Field of Transaction Descriptor 5 4 3 2 1 0 As shown in Table 1 , three bits of the routing field are used for the Hub ID which identifies the hub agent that initiated the transaction. In alternative embodiments, to provide a hub interface hierarchy exceeding 8, additional bits could be used in the routing field. For example, there may exist multiple hub interface hierarchies in a system, in which case the agent at the top of the hierarchies should be capable of routing completions back to the base of the hierarchy. In this context, "hierarchy" consists of multiple connected hub interface segments starting from a hub interface "root" agent (e.g., a MCH). For instance, computer system 100 may have only one hub interface hierarchy. Figure 1, however, illustrates an example of computer system 100 based on multiple hub interface hierarchies. In embodiments implementing only a one hub interface hierarchy, a default value of "000" may be used in the Hub ID field. The remaining three bits of the routing field may be used to identify internal pipes/queues within a hub interface agent. For example the I/O Control Hub may support internal USB (Universal Serial Bus) host controller traffic and Bus Mastering ID (BM- ED) traffic via separate "pipes." As such, the Pipe ID may be used communicate to the servicing agent (e.g., MCH 1 10) that traffic initiated by different "pipes" have different attributes, and may be handled in accordance with a predetermined protocol. If a hub interface agent does not implement separate internal pipes, it may use a default value of "000" in the Pipe ED field. In an alternative embodiment, the transaction descriptors further include an attribute field. In one embodiment, the attribute field is a three-bit value, which specifies how a transaction is to be handled when a target hub interface agent receives it. In some cases, the attribute field helps a system support demanding application workload, which relies on the movement, and processing of data with specific requirements or other differentiating characteristics. For example, the attribute field may support the isochronous movement of data between devices, as used by a few recently developed external busses. Such data movement requirements need to be maintained as data flows through the hub interface between I/O devices and the CPU/memory subsystem. In alternative embodiments, additional transaction attributes may include the ability to differentiate between "snooped" traffic where cache coherency is enforced by hardware (i.e., chipset) and "non-snooped" traffic that relies on software mechanisms to ensure data coherency in the system. Moreover, another possible attribute would be an "explicitly prefetchable" hint, to support a form of read caching and allow for more efficient use of the main memory bandwidth. Ordering Rules The transaction descriptors can also be used to support ordering rules between transactions transmitted across the hub interface. For example, in one embodiment, transactions with identical transaction descriptors are executed in strong order (i.e., first come - first serve.) Transactions having the same routing field but different attribute fields, however, may be reordered with respect to each other. For example, in one embodiment, isochronous transactions do not need to be strongly ordered with respect to asynchronous transactions. In addition, in one embodiment of the hub interface, data transmissions are permitted to make progress over requests, either in the same direction or the opposite direction. Read completions flowing in one direction are allowed to pass read requests flowing in the same direction. And, write requests are allowed to pass read requests flowing in the same direction. In alternative embodiments, however, the ordering rules for transactions travelling across the hub interface, may vary within the scope of the invention. For example, in one embodiment, the hub interface implements the ordering rules provided in Peripheral Component Interconnect (PCI) (Revision 2.2) to determine the flow of traffic across the hub interface in opposite directions. PROTOCOL LAYER In one embodiment, the hub interface uses a packet-based protocol with two types of packets: request and completion. A request packet is used for each hub interface transaction. Completion packets are used where required, for example, to return read data, or to acknowledge completion of certain types of write transactions (e.g., I/O writes and memory writes with requested completion). Completion packets are associated with their corresponding request packets by transaction descriptors and ordering, as previously discussed in the section on the Transaction Layer. In addition, in one embodiment, the hub interface uses an arbitration protocol that is symmetric and distributed. For example, each hub agent drives a request signal, which is observed by the other agent attached to the same interface. No grant signal is used, and agents determine ownership of the interface independently. Moreover, in one embodiment, no explicit framing signal is used. There is an implied relationship between the arbitration event that gives an agent ownership of the interface and the start of that agent's transmission. In alternative embodiment, framing signals could be used without departing from the scope of the invention. The end of a packet transmission occurs when a hub interface agent that owns the interface (e.g., is in the process of transmitting data), releases its control of the interface by de-asserting a request signal. In addition, in one embodiment, flow control is also accomplished by using a STOP signal to retry or disconnect packets, as is described in more detail below. Packet Definition In one embodiment of the hub interface, data is transferred at a multiple rate (e.g., lx, 4x, 8x) of the hub interface clock (HLCK), which in one embodiment is a common clock shared by the hub agents joined by the hub interface. The data is transmitted across a data signal path (PD) of the hub interface, which has an "interface width" of some power of two (e.g., 8, 16, 24, 32.) As a result, the hub interface may have varying data transfer granularities (i.e., transfer widths), depending upon the transfer rate and the width of the data signal path. For example, in the case of an eight-bit interface width in 4x mode, the transfer width is 32 bits per HLCK. As a result, by varying the transfer rate and/or the interface width of the data signal path, the transfer width (i.e., number of bytes transferred per HLCK) can be scaled. In addition, in one embodiment, packets may be larger than the transfer widths. As a result, the packets are transmitted in multiple sections (i.e., packet widths.) In one embodiment, the packets are divided into packet widths the size of double words (32 bits). In the case of a 32 bit transfer width, the bytes of a packet width are presented on the interface starting with the least significant byte (byte 0) and finishing with the most significant byte (byte 3), as shown below in Table 2. In the case of a 64 bit transfer width (e.g., a sixteen bit wide interface in 4x mode) the less significant double-word (packet width) is transferred on the lower bytes of the data signal (e.g., PD [0:7]) and the more significant double-word is transferred in parallel on the upper bytes of the data signal (e.g., PD [15:8]). The two examples are shown below in table 2. Table 2 Byte Transmission Order for 8 and 16 Bit Interface Widths HLC K PD[7:O] aππ- -.β 8 Bit Interface 16 Bit Interface pll3 The Protocol Layer of the hub interface is also responsible for framing the data. As such, the framing rules implemented by the hub interface define how to map one or more packet widths onto a set of transfer widths. To simplify the parsing of packets into packet widths, in one embodiment of the hub interface, the following three framing rules are implemented: a header section of a packet starts on the first byte of a transfer width; a data section of a packet (if present) starts on the first byte of a transfer width; and a packet occupies an integral number of transfer widths. Any available transfer widths not consumed by a packet may be filled with a bogus double word (DW) transmission, and will be ignored by the receiving hub agent. In alternative embodiments, more, less, and/or different framing rules may be used by the hub interface within the scope of the present invention. Table 3 and Table 4 set forth below, illustrate examples of the framing rules given above for the case of a 64 bit transfer width. Table 3 Request using 32 Bit Addressing and Containing Three Double-words of Data 4th Byte 3rd Byte 2nd Byte First Byte 4th Byte 3rd Byte 2nd Byte First Byte Transmitted Transmitted Transmitted Transmitted Transmitted Transmitted Transmitted Transmitted on PD[ 15 8] on PD[ 15 8] on PD[ 15 8] on PD[ 15 8] on PD[7 0] on PD[7 0] on PD[7 0] on PD[7 0] I Byte 7 I Byte 6 Byte 5 ! Byte 4 Byte 3 Byte 2 [ Byte 1 Byte 0 I Table 4 Request using 64 Bit Addressing and Containing Three Double-words of Data 4th Byte 3rd Byte 2nd Byte First Byte 4th Byte 3rd Byte 2nd Byte First Byte Transmitted Transmitted Transmitted Transmitted Transmitted Transmitted Transmitted Transmitted on PD[ 15 8] on PD[15 8] on PD[15 8] on PD[15 8] on PD[7 0] on PD[7 0] on PD[7 0] on PD[7 0] I Byte 7 I Byte 6 I Byte 5 I Byte 4 I Byte 3 I Byte 2 I Byte 1 I Byte 0 I Request Packets The packet header format for request packets, according to one embodiment, is shown below in Table 5 and Table 6. In the examples shown in Tables 5 and 6, the base header is one double-word, with one additional double-word required for 32 bit addressing, and two additional double-words required for the 64 bit addressing mode. The fields of the headers, as shown in Tables 5 & 6 are described below the tables. In alternative embodiments of the hub interface, the fields included in the header of the request packet may vary without departing from the scope of the invention. For example, the header may include additional field, less fields, or different fields in place of the fields shown below. Moreover, the encoding of the fields may also vary without departing from the scope of the invention. Table 5 Request Packet Header Format for 32 bit Addressing La-it Byte Transmitted First Byte Transmitted 31 30 29 28 27 26 25 24 123 22 21 20 19 18 17 l δ | l5 14 13 12 1 1 10 9 8 |7 6 5 4 3 2 1 0 Table 6 Request Packet Header Format for 64 bit Addressing Last Byte Transmitted First Byte Transmitted 31 30 29 28 27 26 25 24 J23 22 21 20 19 18 17 16 |l5 14 13 12 1 1 10 9 8 J7 6 5 4 3 2 1 0 Transaction Descriptor The Transaction Descriptor Routing and Attribute fields as previously described. rq/cp Request packets are identified with a '0' and completion packets with a ' 1 ' in this location. cr Completion required (T) or no completion required ('0'). r/w Read ('0') or Write (' 1 ')• This field indicates if data will be included with a completion (read) or a request (write). Address Format (af) The addressing format is either Implied ('0') or 32/64 bit CD- Lock (Ik) Rag to indicate that the request is part of a locked sequence. Requests and completions in a locked sequence will have this bit set. Hub agents, which do not comprehend lock, ignore this flag and will fill this field with '0'. Data Length The data length is given in double-words, encoded such that the number of double-words represented is one plus this number. Thus, "000000" represents one double-word. Space This field selects the destination space type for the request. In one embodiment, possible destination spaces include Memory ("00"), and IO ("01"). 1st DW BE Byte enables for the first double-word of any read or write request to Memory or IO. Byte enables are active low. If there is only one double-word for a request, this byte enable field is used. In one embodiment, it is illegal to issue a memory or IO read or write request with no bytes enabled. Last DW BE Byte enables for the last double-word of any read or write request. Byte enables are active low. If there is only one double-word for a request, this field must be inactive ("1111"). Byte enables may be discontiguous (e.g.: "0101"). This field is never used with special cycles since it overlaps the "Special Cycle Encoding" field. Addr[31:2] The 32 bit address is generated as it would be on PCI for same type of cycle. This double-word is included for the 32 and 64 bit addressing modes (but not for the implied addressing mode). Extended Address (ea) Indicates 32 bit addressing ('0') or 64 bit addressing (T). Config Type (ct) For configuration cycles only, this bit is used to indicate Type 0 ('0') or Type 1 (T) configuration cycle type. Because configuration cycles will always be performed with 32 bit addressing, this bit is overlapped with the "Extended Address" bit. Addr[63:32] Upper address bits for 64 bit addressing mode. This double-word is included for the 64 bit addressing mode. Completion Packets The header format for a completion packet, according to one embodiment, is shown below in Table 7. In one embodiment, the header is one double-word. The fields of the headers, as shown in Table 8 are described following the table. In alternative embodiments of the hub interface, however, the fields included in the header for a completion packet may vary without departing from the scope of the invention. For example, the header may include additional field, less fields, or different fields in place of the fields as described and shown below. Moreover, the encoding of the fields may also vary without departing from the scope of the invention. Table 7 Completion Packet Header Format Last Byte Transmitted First Byte Transmitted 31 30 29 28 27 26 25 24 J23 22 21 20 19 18 17 16 |l5 14 13 12 1 1 10 9 8 |7 6 5 4 3 2 1 0 Transaction Descriptor The Transaction Descriptor Routing and Attribute fields as previously discussed in the Transaction section. rq/cp Completion packets are identified with a ' 1 ' in this location. r/w Read ('0') or Write (T). This field indicates if data will be included with a completion (read) or a request (write). Lock (Ik) Flag to indicate that the completion is part of a locked sequence. Requests and completions in a locked sequence will have this bit set. Agents, which do not comprehend lock, ignore this flag and will fill this field with '0'. Data Length The data length is given in double-words, encoded such that the number of double- words represented is one plus this number. Thus, "000000" represents one double-word. Completion Status Indicates completion status using predetermined . Reserved All reserved bits are set to '0'. In one embodiment of hub interface, completions for memory reads may provide less than the full amount of data requested so long as the entire request is eventually completed. Likewise, completions for memory writes may indicate that less than the entire request has been completed. This might be done to satisfy a particular hub interface latency requirement for a particular platform. In addition, for a request that requires completion, the initiator, in one embodiment, retains information about the request, which may be stored in a buffer of the initiating hub agent. For example, this information may include the transaction descriptor, the size of the packet, lock status, routing information, etc. Furthermore, when receiving the completion(s), the initiator matches the completion(s) with the corresponding request. In the case of multiple completions, the initiator accumulates a count of the data completed for the original request until the original request is fully completed. Interface Arbitration and Packet Framing In one embodiment of the hub interface, when the interface is idle, the assertion of a request from either hub agent connected to the interface is considered an arbitration event. The first agent to request wins ownership of the interface. If agents request ownership simultaneously when the hub interface is idle, the least recently serviced hub agent wins. In one embodiment, all hub agents track the least recently serviced status (e.g., via a status flag of an internal register.) In alternative embodiment, alternative arbitration routines may be used within the scope of the present invention. Once a hub agent acquires the ownership of the interface, it will continue to own the interface until it completes its transaction, or until an allocated time bandwidth expires. For example, in one embodiment, a timeslice counter is provided in each hub agent to control bandwidth allocation and to limit an agent's interface ownership tenure. The time allotted to a hub agent (i.e., timeslice value) may be different or the same for hub interface agents attached to the same interface. The timeslice counter is started upon acquiring ownership of interface and counts hub interface base clock periods. In one embodiment, each hub agent is responsible for managing its own timeslice allocation. As such, in one embodiment, a timeslice value may be programmed via a hub interface command register for each interface in each hub agent. Figure 6 illustrates an example of arbitration for the hub interface between hub agent A and agent B and the transfer of two packets. The example illustrates arbitration out of an idle interface state, with the interface then returning to idle. Moreover, in the example illustrated, the interface is using a 4x data transfer mode with eight bit data signal (PD) path. Agent A, in the example illustrated in Figure 6, is the most recently serviced (MRS) agent. As a result, Agent A asserts its external request signal (RQA) and samples the state of the Agent B's request signal (RQB) on clock edge 1 (which is shown to be inactive) before starting packet transmission off the same edge. In one embodiment, there is a two clock delay before the transmitted data (i.e., data from Agent A) is available internally in the receiver (i.e., Agent B), starting from clock edge 3. The first packet consists of two double-words 602 and 604 and requires two base clocks to transmit in the 4x mode. The second packet is three double-words 606, 608, and 610, and so requires three base clocks in the 4x mode. Flow Control In one embodiment, packets may be retried or disconnected by a receiving agent due to lack of request queue space, data buffer space, or for other reasons. In one embodiment, Flow control is accomplished using a STOP signal. Figure 7 illustrates an example of the use of STOP signal. As illustrated, Agent A asserts its external request signal (RQA) and samples the state of the Agent B's request signal (RQB) on clock edge 1 (which is shown to be inactive) before starting packet transmission off the same edge (e.g., clock edge 1.) Following a two clock delay, the data transmitted from Agent A is available internally in the receiver at Agent B, starting from clock edge 3. In one embodiment, following receipt of data transmitted from Agent A, is the first opportunity for Agent B to enact flow control by asserting the STOP signal, as illustrated in Figure 7, at clock edge 4. In addition, when ownership of PD signal changes from one hub agent to another, ownership of the STOP signal will be also be exchanged following a predetermined number of clocks. Moreover, in one embodiment, the STOP signal is sampled on base clocks, which correspond to the final transfer of a packet width. For example, in a 4x mode (using an eight bit wide PD signal), the STOP signal is sampled each base clock. However, for a lx mode, the STOP signal is sampled each fourth clock (with the beginning of a transaction being used as a reference point). Following the reception of a STOP signal, the hub agent that receives the STOP signal determines whether it may retry sending additional packets. Figure 8 is a flow diagram describing the steps performed by a hub agent in determining whether it may retry sending a packet following receipt of a STOP signal, according to one embodiment. In step 802, a hub agent that is currently transmitting packets receives a STOP signal. In response, in step 804 the hub agent that receives the STOP signal determines if the other agent (which activated the STOP signal) is requesting ownership of the interface, by sampling the other hub agents request signal (e.g., RQB.). If the recipient of the STOP signal determines that the agent which sent the STOP signal is not requesting ownership of the interface, in step 806 the current owner of the interface may attempt to transmit a packet following recovery from the STOP. On the other hand, if it is determined that the agent which activated the STOP signal is requesting ownership, in step 808, the current owner determines if its timeslice has expired. If the timeslice for the current owner of the interface has expired, in step 810, the current owner releases ownership. If the timeslice for the current owner has not expired, the current owner may transmit a packet with an attribute that is different from the interrupted packet. More specifically, in step 812, the current owner determines if it has a packet with a attribute type that is different from any packets that have been retried in the present arbitration session (i.e., the period of the current owner's tenure), which needs to be transmitted. If the current owner does have a packet with a different attribute, in step 814 the current owner may attempt to transmit the packet. Otherwise, the current owner release ownership of the interface. PHYSICAL INTERFACE In one embodiment, the hub interface implements a physical interface that operates at a base frequency of either 66MHz or lOOMHz. Other frequencies may also be used. In addition, in one embodiment, the physical interface uses a source synchronous (SS) data transfer technique which can be quad-clocked to transfer data at 4X of the base hub interface clock. As a result, in an embodiment having an 8-bit data interface (e.g., PD) operating at a base frequency of 66MHz or lOOMHz, a bandwidth of 266 megabytes per second (MB/s) or 400MB/S may be achieved, respectively. Furthermore, in one embodiment, the hub interface supports a voltage operation of 1.8V, and is based on complementary metal-oxide semiconductor process (CMOS) signaling. In an alternative embodiments, however, the interface may operate at alternative frequencies and/or alternative sized data interfaces to provide varying bandwidths, and support alternative operating voltages, based on alternative signal processing, without departing from the scope of the invention. External Signals Definition Figure 9 illustrates the physical signal interface of the hub interface between two hub agents, according to one embodiment. As shown in Figure 9, the hub interface physical interface uses a bi-directional eight bit data bus (PD [7:0]) with a differential pair of source synchronous strobe signals (PSTRBN, PSTRBP) for data clocking. In an alternative embodiment, the interface can widened. For example, as shown in Figure 9, an additional eight bit data bus (PD [15:8]) can also be used along with an additional pair of pair of source synchronous strobe signals (PUSTRBN, PUSTRBP.) Moreover, in an alternative embodiment, unidirectional data signals could be used. In addition, one unidirectional arbitration signal connects each agent to the other (RQA, RQB), and a bi-directional STOP signal is used by the receiving agent to control data flow, as previously described. Additional interface signals include the system reset (Reset), common clock (HLCLK) and voltage reference signals (HLVREF). As well, signals for each hub agent (ZCOMP) to match its driver output impedance to the appropriate value to compensate for manufacturing and temperature variations are also included. The physical signals shown in the interface illustrated in Figure 9 are further described below in Table 8. In alternative embodiments of the hub interface, the signals included in the physical interface may vary without departing from the scope of the invention. For example, the physical interface may include more, less or different signals varying from the signals shown in Figure 9 and further described below in Table 8. Table 8 Hub Interface Signals for Eight Bit Agents ASTS = Actively Sustained Tri-State. " SS = Source Synchronous Mode Signal 3 CC = Common Clock Mode Signal 4 In one embodiment, Reset is a system-wide signal; it is an output from one component of the system and an input to the other component(s). Moreover, Reset is asynchronous with respect to HLCLK. Common Clock Transfer Mode Operation In one embodiment, many of the signals transmitted across the hub interface are transmitted in accordance with a common clock mode. More specifically, the timing of the signals that are transmitted via the common clock mode are referenced to a single clock (e.g., the hub interface clock.) In alternative embodiments, the signals may be tied to a system clock, exterior to the hub interface agents. Moreover, there may be more than one hub interface segment in a system, in which case different base clocks may be used for the different segments. For example, one component might implement both a 66MHz base hub interface and a lOOMHz base hub interface. Source Synchronous Transfer Mode Operation In one embodiment, the packets/data are transmitted using a source synchronous clock mode, which provides a technique for multiplying the data transfer rate of data. For example, in an embodiment using 4X source synchronous clocking mode with an eight bit data signal path, transmitting a double-word (i.e., four byte) requires only one hub interface clock cycle (HLCK.) Alternatively, transmitting a double word using IX source synchronous clocking mode on an eight bit data signal path would require a full hub interface clock cycle to complete. More specifically, in one embodiment of source synchronous transmission, strobes (e.g., PSTRBN/PSTRBP) are sent with a data transmission in accordance with a predetermined timing relationship between the strobes and the data. The strobes are thereafter used to latch the data into the receiving hub agent. More specifically, in one embodiment, the edges of the strobes PSTRBP /PSTRBN are used by the receiving hub agent to identify the presence and timing of data being transferred across the data signal paths. For example, as illustrated in the timing diagram of Figure 10, in one embodiment a first data transfer corresponds to the rising edge of PSTRBP and the falling edge of PSTRBN. A second data transfer corresponds to the rising edge of PSTRBN and the falling edge of PSTRBP. In addition, in one embodiment, as further shown in Figure 10, the transmit edges of the strobes PSTRBP/PSTRBN are positioned near the center of the data valid window. As a result, the receiving agent is given an input data sampling window to accommodate various system timing skews. Moreover, in one embodiment a minimum data valid before strobe edge (tDvb), and a minimum data valid after strobe edge (tDva) are also used by the receiving hub agent to identify and latch data being transmitted. Once the receiving hub agent latches the incoming data, the data is thereafter held for brief period to resynchronize the data with the hub interface clock (HLCK) before being passed along within the hub agent. Whereas many alterations and modifications of the present invention will no doubt become apparent to a person of ordinary skill in the art after having read the foregoing description, it is to be understood that any particular embodiment shown and described by way of illustration is in no way intended to be considered limiting. Therefore, references to details of various embodiments are not intended to limit the scope of the claims which in themselves recite only those features regarded as the invention.

Claims: CLAIMS What is claimed is 1 A system comprising a central processing unit (CPU), and a memory controller hub (MCH) coupled to the CPU, wherein the MCH includes a first interface controller operable to detect the presence of a first hub agent coupled to the MCH. 2 The system of claim 1 further comprising a first point to point link coupled between the first point to point link and the first interface controller 3. The system of claim 2 wherein the MCH further comprises: a second interface controller; a second point to point link coupled to the second interface controller; and a second hub agent coupled the second point to point link; wherein the second interface controller is operable to detect the presence of the second hub agent. 4 The system of claim 3 wherein the first hub agent is a network interface card and the second hub agent is a graphics accelerator. 5. The system of claim 2 wherein the first interface controller includes: a presence detect module; and control logic coupled to the presence detect module. 6. The system of claim 5 wherein the presence detect module monitors the hub interface to detect the presence of the first hub agent. 7. The system of claim 6 wherein the control logic responds to a poll request if the first hub agent is detected and does not respond to the poll request if the first hub agent is not detected. 8. The system of claim 5 wherein the first hub agent includes a presence acknowledge module for transmitting a presence acknowledge signal to the presence detect module via the point to point link. 9. The system of claim 8 wherein the presence detect module compπses a first resistor coupled to the hub interface and the presence acknowledge module comprises a second resistor coupled to the point to point link. 10. The system of claim 8 wherein the presence detect module comprises a first transistor coupled to the point to point link and the presence acknowledge module comprises a second transistor coupled to the hub interface. 1 1. The system of claim 2 further comprising an interface control hub (ICH) coupled to the MCH. 12. The system of claim 11 wherein the ICH comprises: a second interface controller; a second point to point link coupled to the second interface controller; and a second hub agent coupled the second point to point link; wherein the second interface controller is operable to detect the presence of a second hub agent coupled to the second interface controller. 13. The system of claim 12 wherein the ICH further includes a peripheral component interface (PCI) bridge coupled to a PCI bus. 14. The system of claim 11 wherein the components residing within the MCH are assigned component addresses within a first address range and the components residing within the ICH are assigned component addresses within a second address range. 15. A method of initializing a hub interface comprising: sampling a point to point link; determining whether a downstream agent is coupled to the point to point link; and transmitting a first signal if the downstream agent is detected; transmitting a second signal if the downstream agent is not detected. 16. The method of claim 15 further comprising: receiving the first signal at control logic if the downstream agent is detected; polling the first hub agent from a central processing unit (CPU); and polling the second hub agent from the CPU if the first hub agent responds to the poll. 17. The method of claim 15 further comprising: receiving the second signal at the control logic if the downstream agent is not detected; and disabling the hub interface at the first hub agent. 18. A computer system comprising: a first hub agent; and a point to point link coupled to the first hub agent, wherein the first hub agent samples the hub interface to detect the presence of a second hub agent upon initiation of the computer system. 19. The computer system of claim 18 wherein the first hub agent comprises: a presence detect module; and control logic coupled to the presence detect module. 20. The computer system of claim 19 wherein the control logic transmits a first signal to a central processing unit (CPU) if the second hub agent is detected and transmits a second signal to the CPU if the first hub agent is not detected.

Inventors: AJANOVIC JASMIN, GARCIA SERAFIN, HARRIMAN DAVID J, AJANOVIC, JASMIN, GARCIA, SERAFIN, HARRIMAN, DAVID, J.
Applicants: AJANOVIC JASMIN, GARCIA SERAFIN, HARRIMAN DAVID J, INTEL CORP, AJANOVIC, JASMIN, GARCIA, SERAFIN, HARRIMAN, DAVID, J., INTEL CORPORATION
Application year: 2000
Main classification: G06F13
IPC codes: G06F13