Abstract:
The present invention, a multiprocessor chip pervasive command interface, collects different types of pervasive commands into individual queues for each command type. As permitted by various grouping rules, valid commands are grouped together into one single command and placed on a functional interchip communications bus. This grouping of commands maximizes pervasive command bandwidth while the use of the functional bus minimizes the number of interchip connections.

Description:
BACKGROUND OF THE INVENTION 
   1. Technical Field 
   The invention relates to the field of multiprocessor systems, and more specifically placing commands on an inter-chip processor bus. 
   2. Description of Related Art 
   The basic structure of a conventional symmetric multi-processor computer system  10  is shown in  FIG. 1 . Computer system  10  has one or more processing units arranged in one or more processor groups; in the depicted system, there are four processing units  12   a ,  12   b ,  12   c  and  12   d  in processor group  14 . The processing units communicate with other components of system  10  via a system or interchip link bus  16 . Interchip link bus  16  is connected to one or more service processors  18   a ,  18   b , a system memory device  20 , and various peripheral devices  22 . A processor bridge  24  can optionally be used to interconnect additional processor groups. System  10  may also include firmware (not shown) which stores the system&#39;s basic input/output logic, and seeks out and loads an operating system from one of the peripherals whenever the computer system is first turned on (booted). 
   System memory device  20  (random access memory or RAM) stores program instructions and operand data used by the processing units, in a volatile (temporary) state. Peripherals  22  may be connected to interchip link bus  16  via, e.g., a peripheral component interconnect (PCI) local bus using a PCI host bridge. A PCI bridge provides a low latency path through which processing units  12   a ,  12   b ,  12   c  and  12   d  may access PCI devices mapped anywhere within bus memory or I/O address spaces. PCI host bridge  22  also provides a high bandwidth path to allow the PCI devices to access RAM  20 . Such PCI devices may include a network adapter, a small computer system interface (SCSI) adapter providing interconnection to a permanent storage device (i.e., a hard disk), and an expansion bus bridge such as an industry standard architecture (ISA) expansion bus for connection to input/output (I/O) devices including a keyboard, a graphics adapter connected to a display device, and a graphical pointing device (mouse) for use with the display device. 
   In a symmetric multi-processor (SMP) computer, all of the processing units  12   a ,  12   b ,  12   c  and  12   d  are generally identical, that is, they all use a common set or subset of instructions and protocols to operate, and generally have the same architecture. As shown with processing unit  12   a , each processing unit may include one or more processor cores  26   a ,  26   b  which carry out program instructions in order to operate the computer. An exemplary processor core includes the PowerPC™ processor marketed by International Business Machines Corp. which comprises a single integrated circuit superscalar microprocessor having various execution units, registers, buffers, memories, and other functional units, which are all formed by integrated circuitry. The processor cores may operate according to reduced instruction set computing (RISC) techniques, and may employ both pipelining and out-of-order execution of instructions to further improve the performance of the superscalar architecture. 
   Each processor core  12   a ,  12   b  includes an on-board (L1) cache (actually, separate instruction cache and data caches) implemented using high speed memory devices. Caches are commonly used to temporarily store values that might be repeatedly accessed by a processor, in order to speed up processing by avoiding the longer step of loading the values from system memory  20 . A processing unit can include another cache, such as a second level (L2) cache  28  which, along with a memory controller  30 , supports both of the L1 caches that are respectively part of cores  26   a  and  26   b . Additional cache levels may be provided, such as an L3 cache  32  which is accessible via interchip link bus  16 . Each cache level, from highest (L1) to lowest (L3) can successively store more information, but at a longer access penalty. For example, the on-board L1 caches in the processor cores might have a storage capacity of 128 kilobytes of memory, L2 cache  28  might have a storage capacity of 512 kilobytes, and L3 cache  32  might have a storage capacity of 2 megabytes. To facilitate repair/replacement of defective processing unit components, each processing unit  12   a ,  12   b ,  12   c ,  12   d  may be constructed in the form of a replaceable circuit board, pluggable module, or similar field replaceable unit (FRU), which can be easily swapped installed in or swapped out of system  10  in a modular fashion. A command unit is a generic term that includes, among others, processor cores, and the service processors (which may also be called flexible service processor). 
   It is axiomatic that the more parts there are to a device, the more opportunities there are to break one or more parts. Consequently, complex systems of microprocessors have relied on more commands being transported that are concerned, instead of with customer data, with error detection, system maintenance, reliability, accessibility and serviceability. Often these commands need be dispatched rapidly, in order to limit the impact of an error to certain parts of the system, and to rapidly reform and remedy the situation. 
   In systems where multiple microprocessors depend on a common bus to transmit commands, there is a problem that the limited bandwidth of the bus permits only simplex communication from a processor to one or more recipient processors. In other words, each processor needed to contend with other processors to use the interchip link bus. Moreover, a prior art single processor, with multiple commands, would need to queue such commands for iterative dispatch along the interchip link bus. It has become increasingly necessary to issue pervasive commands on the mainline or functional busses, rather than any the alternative slower specialized busses. 
   As the number of system-wide multi-chip pervasive functions grow, the ability to prioritize the additional interchip communication is important to handle such that the limited availability of the bus is maximized. Engineers now add a new function and require more inter-chip communication to accomplish it. The outcome is that chip I/O is increased, thus adding to chip size, complexity and as well as cost. 
   SUMMARY OF THE INVENTION 
   According to a preferred embodiment, the present invention, a multiprocessor chip pervasive command interface, collects different types of pervasive commands into individual queues for each command type. Each queue entry has a bit associated with it to indicate whether or not it is a valid command. As permitted by various grouping rules, valid commands are grouped together into one single command and placed on an interchip communications bus. The interchip communications bus has a frame when data may be placed on it. Upon the occurrence of that frame, the interface places all data from queues having a valid bit onto the interchip bus as a single command. The queues from which the valid commands are taken from are then advanced. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objectives and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
       FIG. 1  is the basic structure of a conventional symmetric multi-processor computer system; 
       FIG. 2  shows a multi-processor chip of the present invention in block form; 
       FIG. 3  shows a first embodiment physical configuration of the multi-processor chips in a symmetric multi-processor (SMP) arrangement on a circuit board; 
       FIG. 4  shows a set of queues for staging pervasive commands prior to offloading the commands to an interchip link, and in addition, the chip interface logic for reading the interchip link to identify pervasive commands addressed to a chip; and 
       FIG. 5  shows a flow diagram for assembling omnibus commands from pervasive controls for transmittal and reception of an interchip link or fabric bus of a SMP. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   One advantage shown by the present invention is to increase the number of commands that may simultaneously transit on an interchip link bus, particularly pervasive commands. 
   It is a further advantageous that the invention permit a programmed reversal of command priority, such that a functional command may be sent over the interchip link bus, while a pervasive command is held waiting. 
     FIG. 2  is a representative microprocessor or chip that may perform the functions of receiving and dispatching timing signals and occasionally recovering when soft errors occur. Chip  200  may have a first core processor  201   a  and a second core processor  201   b . Each core processor may be simply referred to as a core. A core processor may have multithreading capability, error detection and recovery functions, numerous general purpose registers (GPR) and special purpose registers (SPR). 
   Connectivity of first core  201   a  and second core  201   b  may be with the level 2 cache  203  or L2, and the non-cacheable unit  205  or NCU. NCU  205  may handle commands to store data received from a core onto a fabric bus  210  for storage to main memory. Such stores may be memory-mapped i/o. Access to memory that may be susceptible to frequent accesses later may be stored to the L2  203  in order to reduce latency of operations performed by a core. 
   L2  203  may similarly provide access to its contents via the fabric bus  210  which may interconnect to other chips on the same board, and also beyond the board upon which the chip  200  is placed. 
   A nearby, but off-chip level 3 cache or L3 may be provided. Controls governing access between the cores and the L3 are in L3 cache control  213 . Similarly, a memory controller  215 , and an I/O interface  217  may be provided on-chip to facilitate long-latency access to general RAM and to various peripheral devices, respectively. 
   Symmetric multi-processor (SMP) fabric controls  219 , is a special purpose device that mediates the contention for the fabric bus by the various attached devices, and provides for SMP topology configuration via expansion ports A, B, X, Y and Z. Five expansion ports are shown in the embodiment, however, it is understood that to achieve varying levels of complex multichip topologies, fewer or more expansion ports may be used. It is anticipated that five ports may provide 64 chips with rapid instruction, data and timing signals between and among them. 
   Pervasive controls  221  are circuits that exist both outside and mingled within the various processing blocks found on chip. Among the functions of pervasive controls is the providing of back-ups to the processor state on each core by providing redundant copies of various GPRs and SPRs of each core at convenient instruction boundaries of the each core processor. In addition pervasive controls may assist in the detection of errors and communication of such errors to an outside service processor for further action by, e.g. firmware. 
   Pervasive controls  221  are a gating point for redundant oscillators and other circuits which provide or receive derivative timing signals. It is appreciated that a fault, or other condition may remove one or more redundant oscillators from the configuration, and it is an object of the pervasive control to select the better timing signal (or at least one that is within tolerances) from among the redundant oscillators, and step-encoded signals that may arrive via the expansion ports. In addition, pervasive controls may form and issue commands through the interchip link bus or fabric bus, such as fabric maintenance operations, IO hotplug, and error indications, so called pervasive commands. The pervasive controls, however, share authority over the interchip link bus with other functional units of the chip, which may issue, from time to time functional commands, which may include memory reads, memory writes, cache-inhibited reads and cache-inhibited writes, among others. 
     FIG. 3  shows a first embodiment configuration of a symmetric multiprocessor using the chip of  FIG. 2  in the form of a processor node  300 . Processor node  300  may contain memory banks  301   a ,  301   b , i/o hubs  303 , service processors  305   a ,  305   b , ports or connectors  307   a  and  307   b  handling the A and B ports from each of the multichip modules (MCM)  321   a ,  321   b ,  321   c , and  321   d . Each multichip module may be identical in its hardware configuration, but configured by software to have varying topologies and functions as, e.g. between master and slave functions. Within a MCM may be found the chip  322  of  FIG. 2 , as well as level 3 cache memory  323   a  and  323   b . The processor node  300  may have a primary oscillator  331  and a secondary oscillator  333  that are each routed to each chip found on the processor node. Connections between the oscillators and functional units extend throughout the board and chips, but are not shown in  FIG. 3  in order to limit clutter. Similarly, it is understood that many convoluted interconnects exist between the ports and I/O hubs, among other components—though such interconnects are not shown here. 
   The chips within a node are connected by a synchronous coherency fabric. The separate nodes are connected by an asynchronous coherency fabric, i.e. each node may have its own logic oscillator pair sourced on the node. 
   The master-slave relationship among the nodes is established by service firmware at initial program load, which among other things, sets up various configuration registers in each processor chip. The configuration registers may exist as two different sets, indicating two alternate topologies for the SMP wherein a primary control register set organizes the nodes by one topology (e.g. using a first node as a ‘master’) and another topology (secondary control register set) may use a second node as a ‘master’. 
   A set of pervasive commands may permit the pervasive control of  FIG. 2  to accomplish its varied functions. Table 1 depicts some of the commands and further shows whether various attributes may be applicable to the pervasive command. Some commands are broadcast, i.e. all chips that snoop on the interchip link bus receive the commands. Otherwise a command may be point-to-point, i.e. directed to a particular chip or core processor. Regardless of whether a command is a pervasive command or a functional command, existing as a single command or multiplexed together with others, all commands that exist on the interchip link bus are said to transit the interchip link bus. 
   An omnibus command may be a pervasive command that is transmitted on an interchip link bus, or it may be several pervasive commands that are transmitted in a collective set of bit fields that are transmitted via the interchip link bus in consecutive bus beats. An omnibus command may include a Time-of-day (TOD) pervasive command. An omnibus command may include a miscellaneous command, wherein one or more pervasive commands are present, but none are a TOD pervasive command. A miscellaneous command can include any number of broadcast commands that can be fit into the data space of a single omnibus command, but can only include one point-to-point command. Conversely, a point-to-point command can be combined with any number of broadcast commands, but may not be combined with another point-to-point command. 
   Since a TOD command from a pervasive control may be 64 bits wide, there may not be enough bits available in an address request to support transmittal in a single command. Therefore a TOD command, e.g. “send TOD value”, may be split into two sub-commands, each sending 32 bits of TOD data at a time during an address bus beat of the fabric bus. Even though a TOD command is a broadcast command, in this embodiment it may not be combined with other commands since it requires the transmittal of a large amount of data and no other commands will fit into the same omnibus command. 
   Table 1 depicts each pervasive command and its attributes as a) broadcast among the chips; b) point-to-point to a single chip; c) able to combine with other broadcast commands; d) the number of fabric bus commands that may be required to transport the command. Each fabric bus command is 2 beats. 
   
     
       
             
           
             
             
             
             
             
           
         
             
               TABLE 1 
             
           
           
             
                 
             
             
               Pervasive command attributes 
             
           
        
         
             
                 
                 
               Point- 
                 
               Number of 
             
             
                 
               Broad- 
               To- 
               Able to 
               Bus 
             
             
               Command type 
               cast 
               Point 
               combine 
               Commands 
             
             
                 
             
             
               Time of Day 
               X 
                 
                 
               2 
             
             
               synchronization 
             
             
               In-memory trace 
               X 
                 
               X 
               1 
             
             
               (HTM) global 
             
             
               triggering 
             
             
               Trace array 
               X 
                 
               X 
               1 
             
             
               global triggering 
             
             
               Legacy Time Base 
               X 
                 
               X 
               1 
             
             
               enable 
             
             
               XSCOM Status 
                 
               X 
               X 
               1 
             
             
               reporting 
             
             
               Fabric 
               X 
                 
               X 
               1 
             
             
               Maintenance 
             
             
               Operations 
             
             
               Fabric and IO 
               X 
               X 
               X 
               1 
             
             
               Hotplug 
             
             
                 
             
           
        
       
     
   
   
     
       
             
           
             
             
             
           
         
             
               TABLE 2 
             
           
           
             
                 
             
             
               address bus beat 0 definition for a TOD command 
             
           
        
         
             
                 
               Bit 
               Description 
             
             
                 
                 
             
             
                 
                0:2 
               Reserved (‘000’) 
             
             
                 
                3 
               Bus beat: 
             
             
                 
                 
               ‘0’ = beat 0, the first 32 bits of TOD data will be 
             
             
                 
                 
               transmitted; 
             
             
                 
                 
               ‘1’ = beat 1, the second 32 bits of TOD data will be 
             
             
                 
                 
               transmitted 
             
             
                 
                4:35 
               TOD data is transmitted 
             
             
                 
               36 
               Reserved (‘0’) 
             
             
                 
               37 
               Point to Point command toggle: 
             
             
                 
                 
               ‘0’ = broadcast command: all chips must respond to 
             
             
                 
                 
               snoop request 
             
             
                 
                 
               ‘1’ = point to point command. Only chip selected with 
             
             
                 
                 
               bits 38:43 of address bus beat 0 should resend to 
             
             
                 
                 
               snoop request 
             
             
                 
               38:43 
               Point to point destination chip ID (if applicable) 
             
             
                 
               44:49 
               Fabric T-type. 0x3C for Pervasive Address Only 
             
             
                 
                 
               Commands 
             
             
                 
               50:52 
               Reserved (‘000’) 
             
             
                 
               53 
               Address bus beat 0 parity 
             
             
                 
                 
             
           
        
       
     
   
   
     
       
             
           
             
             
           
         
             
               TABLE 3 
             
           
           
             
                 
             
             
               address bus beat 1 definition for a TOD command 
             
           
        
         
             
               Bit 
               Description 
             
             
                 
             
             
                0:5 
               Source Chip ID 
             
             
                6:9 
               Command Source Unit - ‘0xA’ 
             
             
               10 
               Hotplug command valid 
             
             
               11 
               Fabric hotplug switch A/B registers 
             
             
               12:16 
               Reserved (‘00000’) 
             
             
               17:20 
               T-size - ‘0x8’ 
             
             
               21 
               Error indication command valid 
             
             
               22 
               Global HTM command valid 
             
             
               23 
               Global Trace command valid 
             
             
               24 
               Fabric hotplug command valid 
             
             
               25 
               XSCOM status command valid (this command may appear 
             
             
                 
               with all others except Fabric Hotplug) 
             
             
               26 
               TOD status command valid 
             
             
               27 
               Time base enable command valid 
             
             
               28:52 
               Reserved 
             
             
               53 
               Address Bus Beat 1 parity 
             
             
                 
             
           
        
       
     
   
     FIG. 4  shows a pervasive command interface or pervasive control to the interchip link, which may be a interchip link bus of the symmetric multiprocessor system. Such a pervasive control may, for example transmit a TOD register value by using the TOD command structure, as organized in tables 2 and 3. A macro or send block  441  may be active in transmitting a TOD command, which may include a TOD register value. The macro may also be known as tp_fbc_snd_pmac. The send block  441  may have TOD queue  443 , which may be two deep, for accepting commands for later dispatch along the interchip link bus  449 . Additional pervasive functions may have respective queues  443   b ,  443   c ,  443   d ,  443   e  and  443   f , for staging commands concerning In-memory trace (HTM) global triggering, trace array global triggering, and IO hotplug pervasive commands, among others. A macro or receive block  442  also known as, tp_fbc_rcv_pmc, may exist on the chip to receive such pervasive commands as well. 
   Associated with TOD queue  443  may be a valid bit  444  to indicate to a command arbiter  445  that a command is available and stored as contents to the queue. When a valid command is in the command queue a request may be sent to the ICL  449  that a command is waiting to be sent. The ICL  449  will respond with a request sent signal or grant and the command is placed into the interchip link. When command arbiter  445  delivers a valid command to pending command register  446 , a valid bit may be set  446   b.    
   Each chip has a tp_fbc_rcv_pmc macro or receive block  442  that snoops the fabric line for pervasive commands that are broadcast or point-to-point addressed to the chip. Each pervasive command has a unique command type, or ttype, that indicates to the snooping logic that it is a pervasive command. In this embodiment that ttype is 0x3C. 
   Referring to  FIG. 4  and  FIG. 5 ,  FIG. 5  shows flow diagram of steps that may occur when a TOD command or other pervasive command arrives at the send block  441 . Each queue may receive pervasive commands from various specialty macros of the pervasive control block of  FIG. 2 . Each queue may be a two position queue, and may initially be empty or without valid data. Thus each command may arrive and depart at a queue in typical first in first out fashion. The first step, once a queue obtains a command, is the embodiment examines (step  501 ) the command that is a Time-Of-Day (TOD) command on TOD queue  443 , and examines a next-lower-priority pervasive command (step  503 ) from a queue among queues  451   a ,  451   b ,  451   c ,  451   d  and  451   e , if a TOD command is not available. In many cases, a timeout (step  505 ) will not be applicable, since we may want pervasive commands to have high priority access to the interchip link bus. If such is the case, execution continues to determine (step  507 ) if a TOD command is among the one or more pervasive commands obtained. If the yes branch is taken, then the TOD command may be marked valid, so that it may be immediately susceptible to placing on the interchip communications link (step  511 ). If the no branch is taken, then the first examined pervasive command marked as valid is placed in the pending command register  446  for the next frame of the interchip communications link (ICL) (step  513 ). The data stored in the pending command register  446  is known as the pending command. Next it is decided if the secondary command (from step  503 ) is compatible with the command already queued for the ICL frame (step  515 ). If so, the command is placed (step  520 ) in the pending command register with the command already queued for the next ICL frame occurrence, and its queue entry is marked invalid. If not, the command is left in its command queue (step  519 ), wherein the queue may be any of the queues  443   b ,  443   c ,  443   d ,  443   e , and  443   f , and left marked valid so that it will be available to be added to the next command to be placed on the ICL. Note that the effort of marking valid, or marking invalid, may simply involve setting or resetting a latch associated with the data to be considered either valid or invalid. 
   A determination is made to see if another pervasive command has arrived at the head of a queue before a ICL frame has occurred (step  521 ). If so, and the new command is compatible with the commands already present in the pending ICL command, the additional command is added to the ICL command in queue (step  515 ). Each queue head is examined iteratively, until the last one is reached, and all compatible commands are either added to the pending ICL command or left valid for the next available command. When the ICL frame arrives, the pending command is loaded into the ICL and the fabric gives a “request sent” signal (step  531 ). 
   The interchip link bus transmits an initial set of data in bus beat  0  (step  535 ), and then a second set of date in bus beat  1  (step  537 ). Collectively, during the bus beats, the interchip link bus may transmit several pervasive commands at once. Once the bus beat  1  transmittal (step  537 ) is complete, the embodiment may advance (step  539 ) each queue that had its head vacated when the command formerly contained therein had been placed on the pending command register  446  of the ICL. 
   At the decision point to determine if a command is compatible with existing commands, table 1, shows the rules used. Moreover, if two commands of the same type arrive, the first of the two commands is not compatible with the second of the two commands, since the two commands are assigned the same bit fields for data transport. In addition, two commands that are point-to-point may not be compatible, for example, not all pervasive commands are compatible with a XSCOM status reporting. A fabric and IO hotplug command is not compatible with an XSCOM status reporting. 
   In addition to a TOD command, the present invention may transmit a miscellaneous command. Like the TOD command, the miscellaneous command may transport several pervasive commands among the bit fields of the miscellaneous command, provided that no two commands that are point-to-point are multiplexed together into the same omnibus command. The field structure of a pair of bus beats is shown in table 4 and table 5 which represent the condition of bits in a miscellaneous type of omnibus command. 
   
     
       
             
           
             
             
             
           
         
             
               TABLE 4 
             
           
           
             
                 
             
             
               Address bus beat 0 bit definition for Miscellaneous Command 
             
           
        
         
             
                 
               Bit 
               Description 
             
             
                 
                 
             
             
                 
                0:2 
               Reserved (‘000’) 
             
             
                 
                3 
               Core 0 malfunction alert 
             
             
                 
                4 
               Core 1 malfunction alert 
             
             
                 
                5 
               Recoverable interrupt 
             
             
                 
                6 
               Start HTM 
             
             
                 
                7 
               Stop HTM 
             
             
                 
                8 
               Pause HTM 
             
             
                 
                9 
               Freeze HTM 
             
             
                 
               10 
               Reset HTM 
             
             
                 
               12:21 
               HTM mark type, valid with Mare HTM, bit 11 of beat 0 
             
             
                 
               22:23 
               HTM mark source: 20 = core ID; 21 = thread ID 
             
             
                 
               24 
               HTM mark dropped 
             
             
                 
               25 
               Global trace start 
             
             
                 
               26 
               Global trace stop 
             
             
                 
               27 
               Global trace bank 
             
             
                 
               28 
               Global trigger 0 
             
             
                 
               29 
               Global trigger 1 
             
             
                 
               30 
               XSCOM Error 
             
             
                 
               31 
               XSCOM Address not acknowledged 
             
             
                 
               32 
               XSCOM command aborted 
             
             
                 
               33 
               XSCOM Command initiated by core 0 
             
             
                 
               34 
               XSCOM command initiated by core 1 
             
             
                 
               35 
               Time base enable set 
             
             
                 
               36 
               Timebase enable clear 
             
             
                 
               37 
               Point to point command: 
             
             
                 
                 
               ‘0’ = broadcast command: all chips respond to snoop 
             
             
                 
                 
               request; 
             
             
                 
                 
               ‘1’ = point to point command: only chip selected 
             
             
                 
                 
               with bits 38:43 of address bus beat 0 respond to 
             
             
                 
                 
               snoop request 
             
             
                 
               38:43 
               Point to point destination chip ID 
             
             
                 
               44:49 
               Fabric t-type = ‘0x3C’ for pervasive address only 
             
             
                 
                 
               commands 
             
             
                 
               50:52 
               Reserved (‘000’) 
             
             
                 
               53 
               Address bus beat 0 parity 
             
             
                 
                 
             
           
        
       
     
   
   
     
       
             
           
             
             
             
           
         
             
               TABLE 5 
             
           
           
             
                 
             
             
               Address bus beat 1 bit definition for Miscellaneous Command 
             
           
        
         
             
                 
               Bit 
               Description 
             
             
                 
                 
             
             
                 
                0:5 
               Source chip ID 
             
             
                 
                6:9 
               Command source unit, which may be ‘0xA’ 
             
             
                 
               10 
               Hotplug command valid 
             
             
                 
               11 
               Fabric Hotplug switch A and B registers 
             
             
                 
               12 
               Fabric initialize (broadcast mode only) 
             
             
                 
               13:16 
               Reserved (‘00000’) 
             
             
                 
               17:20 
               T-size, which is ‘0x8’ for all commands 
             
             
                 
               21 
               Error indication command valid 
             
             
                 
               22 
               Global HTM command valid 
             
             
                 
               23 
               Global HTM command valid 
             
             
                 
               24 
               Fabric Hotplug command valid 
             
             
                 
               25 
               XSCOM status command valid 
             
             
                 
               26 
               TOD command valid 
             
             
                 
               27 
               Timebase enable command valid 
             
             
                 
               28:52 
               Reserved 
             
             
                 
               53 
               Address bus beat 1 parity 
             
             
                 
                 
             
           
        
       
     
   
   The purpose of the timeout loop (step  506 ), for those cases where a timeout may be set to operate, is to provide a time during which the lower priority functional commands may also access the interchip link bus, without being pre-empted by pervasive commands. A determiniation that pervasive commands are lower priority, as compared to functional commands, causes the yes branch of the timeout applicable (step  505 ) to be taken. During the times that that the flow of  FIG. 5  experiences a timeout loop, another routine may seize the interchip link bus and dispatch functional commands. Giving priority to the dispatch of functional commands over pervasive commands is called applying a timeout. 
   It is important to note that while the present invention has been described in the context of a fully functioning data processing system, those of ordinary skill in the art will appreciate that the processes of the present invention are capable of being distributed in the form of a computer readable medium of instructions and a variety of forms and that the present invention applies equally regardless of the particular type of signal bearing media actually used to carry out the distribution. Examples of computer readable media include recordable-type media, such as a floppy disk, a hard disk drive, a RAM, CD-ROMs, DVD-ROMs, and transmission-type media, such as digital and analog communications links, wired or wireless communications links using transmission forms, such as, for example, radio frequency and light wave transmissions. The computer readable media may take the form of coded formats that are decoded for actual use in a particular data processing system. 
   The description of the present invention has been presented for purposes of illustration and description, and is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. The embodiment was chosen and described in order to best explain the principles of the invention, the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.