Abstract:
A method and apparatus to prevent invalid data from propagating into devices connected to a PCI tristate bus is provided. The method and apparatus utilize the PCI bus control signals to monitor the bus transaction&#39;s mode (e.g., as a bus target or as a bus master), type (e.g., read, write), and status (e.g., ongoing bus transaction). Using these information, control the opening and closing of a window gate hardware to allow valid data to propagate into a device connected to the PCI tristate bus and to prevent invalid data from propagating into the device.

Description:
FIELD OF THE INVENTION 
     The invention generally relates generally to network controllers in computer systems, and more particularly to a controller coupled to the Peripheral Component Interconnect (PCI) Local Bus. 
     BACKGROUND OF THE INVENTION 
     In the area of digital systems, the task of accommodating increasing bus traffic continues to pose a challenge. The primary bottle-neck in most bus transactions appears to be the system bus. The system bus is a bottle-neck primarily because many devices share the same bus and must contend for its resources. 
     The PCI bus is a high performance, 32-bit or 64-bit bus with multiplexed address and data lines which can accommodate multiple high performance peripherals. The PCI Bus supports burst modes in which a bus transaction may involve an address phase followed by one or more data phases in tandem with a command phase followed by one or more byte enable phases. As such, an external device may require the use of the bus for multiple clock cycles during a bus transaction which can exacerbate bottle-neck problems associated with a system bus. Reference is now made to FIG. 1 which illustrates an overview of a computer system that utilizes the PCI bus. In FIG. 1, computer system 100 comprises host CPU 101, host memory 102, peripheral hardware controller 103, and bridge device 105. Peripheral hardware controller 103 is coupled to host CPU 101 and host memory 102 through PCI bus 104. More particularly, peripheral hardware controller 103 provides an interface between PCI bus 104 and external devices such as disk drivers, display monitors, parallel data port, local area network, wide area network, or the like. 
     In general, host CPU 101 and external devices may take turns controlling PCI bus 104 in carrying out transactions such as read and write transactions. While a device which takes control of PCI bus 104 to initiate the transaction is known as a &#34;bus master&#34; device, a device at the other end of the transaction is known as a &#34;bus target&#34; (or &#34;slave&#34;) device. Information that are involved in bus transactions between devices include data, address, commands, byte enables, and identification of bus master and bus target device. 
     While a bus may be synchronous or asynchronous, PCI bus is a synchronous bus. In other words, information flowing from the bus master device to the target device and vice versa are synchronized to a system clock such that a bus transaction must take place in an integral number of synchronized clock cycles. In carrying out bus transactions, bus protocols must be followed. These protocols consists mainly of bus mastership, requests for read or write transactions, and acknowledgment of such requests. PCI bus protocols can be found in &#34;The PCI Local Bus Specification Rev 2.1&#34;, published by the PCI Special Interest Group, P.O. Box 14070, Portland, Ore. 97214 and incorporated herein by reference. 
     The PCI bus can be tristated which means that the bus is floated to indicate that it is available for use. When a bus is floated, it generally indicates to devices connected to it that it is available for usage. However, depending on the threshold voltage of a device connected to the bus, the logic value associated with a tristated PCI bus may inadvertently trigger an undesired state in the device connected to the bus. In the Prior Art, to prevent the propagation of invalid data values into a device, flip-flops or latches are implemented inside the device to hold known and unknown data propagated from the bus until it can be determined whether the data is valid or not. Having additional flip-flops or latches for this purpose is not cost efficient. 
     Hence, there is a need for an apparatus, system, and method to prevent invalid data from propagating into a device connected to the bus without incurring unnecessary added costs. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention provides an apparatus, system, and method to prevent invalid data from propagating into a device connected to the bus without incurring unnecessary added costs. 
     The present invention meets the above need with an apparatus to control information transfer in and out of a device connected to a computer bus. In general, the apparatus utilizes the PCI bus control signals to monitor the bus transaction&#39;s mode (e.g., as a bus target or as a bus master), type (e.g., read, write), and status (e.g., ongoing bus transaction). Using these information, control the opening and closing of a window gate hardware to allow valid data to propagate into a device connected to the PCI tristate bus and to prevent invalid data from propagating into the device. 
     The apparatus comprises a window hardware circuit, an address decoder, a target mode window closer, a master mode window closer, a target window controller, a fast opener circuit, and a master window controller. 
     The window hardware circuit is coupled to the computer bus. The window hardware circuit allowing address, data, command, byte enable, and device identification information to pass through as outputs to the device in response to a first and a second control signal. The address decoder is coupled to the window hardware circuit and the computer bus. The address decoder generates a signal indicating whether an address output from the window hardware circuit is valid. The target mode window closer is coupled to the computer bus. The target mode window closer also receives a signal indicating whether a transaction is a write or read transaction. In response to these inputs, the target mode window closer generates a first internal stop signal when the transaction is complete and when the transaction is a read transaction. 
     The master mode window closer is coupled to the computer bus. The master mode window closer also receives a signal indicating whether there is an active bus master and a signal indicating whether there is another device requesting control of the bus. The master mode window closer generates a second internal stop signal when there is an active bus master and when there is another device requesting control of the bus. 
     The target window controller is coupled to the computer bus, the address decoder, the target mode window closer, and the master mode window closer. The target window controller generates a third internal stop signal. The fast opener circuit is coupled to the computer bus, the target window controller, and the window hardware. The fast opener circuit generates the first control signal. The master window controller is coupled to the computer bus and the window hardware. The master window controller also receives the signal indicating whether there is an active bus master. In response to the inputs, the master window controller generates the second control signal. 
     All the features and advantages of the present invention will become apparent from the following detailed description of its preferred embodiment whose description should be taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Prior Art FIG. 1 is a block diagram illustrating a typical computer system that uses PCI bus in the computer system. 
     FIG. 2 is a block diagram illustrating a computer system implementing the present invention. 
     FIG. 3 is a block diagram of the network controller in the computer system of FIG. 2. 
     FIG. 4 is a block diagram illustrating a portion of the host bus interface in the network controller of FIG. 3 in accordance to the present invention. 
     FIG. 5 is a gate-level schematic of the fast opener circuit in FIG. 4. 
     FIG. 6 is a gate-level schematic of the target window controller in FIG. 4. 
     FIG. 7 is a gate-level schematic of the target mode window closer in FIG. 4. 
     FIG. 8 is a gate-level schematic of the master mode window closer in FIG. 4. 
     FIG. 9 is a gate-level schematic of the window hardware circuit in FIG. 4. 
     FIG. 10 is a gate-level schematic of the master window controller in FIG. 4. 
     FIG. 11 is a gate-level schematic of the address range decoder in FIG. 4. 
     FIG. 11A is a gate-level schematic of the exclusive NOR-gate cluster 1101 of FIG. 11. 
     FIG. 11B is a gate-level schematic of the exlcusive NOR-gate cluster 1102 of FIG. 11. 
     FIG. 11C is a gate-level schematic of the AND-gate cluster 1107 of FIG. 11. 
     FIG. 12 is a timing diagram for the start of a target read transaction. 
     FIG. 13 is a timing diagram for the end of a target transaction. 
     FIG. 14 is a timing diagram for the start of a master read transaction. 
     FIG. 15 is a timing diagram for the end of a master write transaction. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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 obvious to one skilled in the art that the present invention may be practiced without these specific details. In other instances well known methods, procedures, components, and circuits have not been described in detail as not to unnecessarily obscure aspects of the present invention. 
     Reference is now made to FIG. 2 illustrating a block diagram of computer system 200 utilizing aspects of the present invention. Computer system 200 comprises a host processor system chip set which includes host processor 202, cache memory 204, bridge/memory controller 222, and dynamic random access memory 216. Host processor 202 may be any one of a number of commercially available microprocessors such as those marketed by Intel® (Santa Clara, Calif. and Motorola® (Schaumburg, Ill. PCI bus 234 couples bridge-memory controller 222 to a number of external devices such as local area network controller 224, hard disk controller 226, audio controller 218, video graphics array 230, and other expansion bus interfaces 228. Video graphics array 230 may drive display device 232 such as a monitor or a liquid crystal display device. 
     Bridge/memory controller 222 is used to directly access any external device coupled to PCI bus 234. Generally, such external devices are mapped in memory or I/O address spaces. Bridge/memory controller 222 also performs data buffering/posting and PCI bus central functions (e.g., arbitration). While bridge/memory controller 222 may be considered as a PCI bus master, any of the external devices coupled to PCI bus 234 may also act as a bus master. 
     Although the present invention is practiced in a PCI bus environment, it is to be understood that the invention is applicable to any type of synchronous system bus with appropriate protocols. Furthermore, while the present invention has applicability in network controllers such as LAN controllers and the like, the best mode, it is to be understood that the invention is applicable to any peripheral controllers such as disk controllers, audio controllers, graphics controllers etc. 
     FIG. 3 is an overall block diagram of network controller 324. Host bus interface 334 connects network controller 324 to PCI bus 234. In addition to performing address decoding to access I/O devices and memory locations, host bus interface responds to applicable CPU commands and controls. The present invention resides inside host bus interface 334. Embedded ARM processor 338 is a local processor which helps to offload central processor 202 in terms of processing. ARM processor 338 helps to direct data traffic inside network controller 324. ARM processor 338 is also used to assemble data that comes through into the desired format. In asynchronous modes, such function is called segmentation and reassembly. 
     Network controller interface 340 provides the interface to communications networks to which network controller 324 is connected. List manager 342 controls the transfer of data to and from internal static random access memory (SRAM) 336. When network data signals come in from communications networks, list manager 342 formats and stores the data into internal SRAM 336. List manager 342 also communicates with ARM processor 338 about the occurrence of certain activities. In response, ARM processor 338 executes instruction codes stored in local SRAM 346 to perform predetermined tasks. Memory controller 344 is connected to an external dynamic random access memory (DRAM) (not shown). Memory controller 344 retrieves from the external DRAM additional instruction codes, which when executed by ARM processor 338 carry out additional tasks. 
     In accordance with the present invention, various PCI bus control signals and other internal signals are monitored to determine whether information should be allowed to propagate through to network controller 324. Referring now to FIG. 4 which is a block diagram of host bus interface 334 comprising window hardware 488, fast opener 480, target window controller 482, target mode window closer 484, master mode window closer 486, master window controller 490, and address range decoder 492. Host bus interface 334 also has a target state machine (not shown) and a master state machine (not shown) which generates some internal signals. As shown in FIG. 4, while all the upper-case signals are coming of the PCI bus, all the lower-case signals are internal signals. 
     Window hardware 488 receives as inputs address &amp; data signals AD (32 bits), bus command &amp; byte enables signal C/BE (4 bits), and identification select signal IDSEL (1 bit). Window hardware 488 also receives as inputs control signals fast --  enable and pci --  window --  open. Based on the state of control signals fast --  enable and pci --  window open, window hardware 488 determines whether or not to allow signals AD, C/BE, and IDSEL to pass through as signals wad (windowed address &amp; data), wcbe (windowed command &amp; byte enable), and widsel (windowed identification select) respectively. Hence, window hardware 488 acts as a gate to allow valid data through and prevent invalid data from propagating into network controller 324. Signals fast --  enable and pci --  window --  open are provided by fast opener circuit 480 and master window controller 490 respectively. 
     Fast opener circuit 480 receives as inputs bus signals FRAME, CLK, RESET, as well as control signal pci --  tx --  termin from target window controller 482. In general, signal FRAME indicates to fast opener circuit 480 the occurrence of a bus transaction on the PCI bus which prompts fast opener to generate signal fast --  enable to window hardware 488 to open its window to allow the input data to pass through as output. On the other hand, when signal pci --  tx --  termin is de-asserted, it triggers fast opener circuit 480 which in turn signals window hardware 488 to close its window and not let its inputs pass through as output. 
     Target window controller 482 receives as inputs target ready signal TRDY, bus transaction signal FRAME, address-not-valid signal addr --  vld --  n, clock signal CLK, signal RESET, and device select signal DEVSEL. Address-not-valid signal addr --  vld --  n, which is generated by address range decoder 492, indicates whether the address received is valid. Device select signal DEVSEL indicates that a device connected to the bus has been selected and as such, it can be used to indicate whether the transaction is valid. Target window controller 482 also receives as inputs control signal Trmin --  stop from target mode window closer 484 and control signal Trmin --  stop --  master from master mode window closer 486. Target window controller 482 examines the aforementioned input signals to determine when to de-assert signal pci --  tx --  termin to close the gate. Target window controller 482 de-asserts signal pic --  tx --  termin in a few scenarios such as when the transaction is complete, when the data received is determined to be invalid, when there is a read transaction and protocols require that the bus be floated in the middle of the transaction to allow data to be sent over the bus, etc. 
     Target mode window closer 484 receives as inputs stop signal STOP, clock signal CLK, reset signal RESET, and indicator signal writenotread. Signal writenotread indicates whether the transaction is a target write or target read. Stop signal STOP indicates that the end of the current data cycle and the start of a different transaction at the next clock cycle. Based on these input signals, target mode window closer 484 determines, for example, whether the transaction is complete or whether the transaction is a read to determine whether to assert signal Trmin --  stop. In general, signal Trmin --  stop is asserted when the device (e.g., network controller 224) is a target, and the transaction is terminated by the assertion of STOP signal. 
     Master mode window closer 486 receives as inputs master --  active signal, stop signal STOP, and signal force --  slave --  retry. When asserted, master --  active signal indicates that a selected device (e.g., network controller 224) intends to be a master of the bus and it is ready to initiate a transaction. However, master --  active signal does not indicate whether the selected device has successfully gained control of the bus. Signal force --  slave --  retry indicates that there is now a bus master and all other devices have to retry their transactions later. Based on these input signals, master mode window closer 486 determines if there is an active bus master and if there is any other device contending for control of the bus. If network controller 224 is not active and there is a device wanting control of the bus, master mode window closer 486 asserts signal Trmin --  stop --  master. In general, signal Trmin --  stop --  master is asserted when the device is a master. 
     Master window controller 490 receives as inputs master --  active signal, target ready signal TRDY, clock signal CLK, and reset signal RESET. Based on these input signals, master window controller 490 determines whether network controller 224 is a master of the bus. If it is determined that network controller 224 is a master of the bus, master window controller 490 asserts signal pci --  window --  open to window hardware 488 to open the window. 
     Address range decoder 492 receives window address &amp; data signal wad, window command &amp; byte enable signal wcbe, and window identification select signal widsel. Address range decoder 492 also receives signals base0[31:m] and base1[31:n] which indicate the address range of the memory address space and the I/O address space. These signals base0[31:m] and signals base1[31:n] where 0≦m≦31 and 0≦n≦31 are bits in a configuration register. More particularly, m and n are bit values of the upper most significant bit within the 32 bits of address space. The implementor of the device determines what these values should be. For example, for an address range of 32 words, the lower five bits are excluded from the address comparator. In other words, only bits [31:6] of the incoming address is compared against base0[31:6] and base1[31:6] which is part of the configuration register. Address range decoder 492 decodes window address &amp; data signal wad, window command &amp; byte enable signal wcbe, and window identification select signal widsel and check them against the address ranges of the memory and I/O space. If the address is from either the memory space or the I/O space, address range decoder 492 asserts signal addr --  vld --  n LOW. Address range decoder 492 also asserts sign addr --  vld --  n LOW if a valid configuration register is selected. 
     Referring now to FIG. 5 illustrating a gate-level diagram of fast opener circuit 480. As shown in FIG. 5, fast opener circuit 480 includes flip-flop 501, OR-gate 502, and OR-gate 503. Flip-flop 501 receives bus transaction indicator signal FRAME as input. Flip-flop 501 is clocked by system clock signal CLK. Flip-flop 501 can be reset by RESET signal. OR-gate 502 receives as one input the Q-bar output of flip-flop 501 and as a second input bus transaction indicator signal FRAME. Since signal FRAME is asserted LOW when there is a bus transaction and is HIGH when there is not, OR-gate 502 outputs a HIGH signal when there is a transition from LOW-to-HIGH, when there is a transition from HIGH-to-LOW, and when signal FRAME stays HIGH. As such, by inverting the output of OR-gate 502, the output of OR-gate 502 is always HIGH when there is a transaction on PCI bus 234 between two consecutive clock cycles. The output of OR-gate 502 together with signal pci --  tx --  termin are provided as input to OR-gate 503. Accordingly, OR-gate 503 outputs a HIGH fast --  enable signal when there is a transaction on PCI bus 234 between two consecutive clock cycles unless signal pci --  tx --  termin is asserted LOW to indicate that the transaction is terminated. 
     FIG. 6 illustrates a gate-level diagram of target window controller 482. As shown, target window controller 482 includes OR-gate 601, AND-gate 602, NOR-gate 603, multiplexer 604, and flip-flop 605. OR-gate 601 receives as inputs TRDY signal, DEVSEL signal, and inverted FRAME signal. Accordingly, when any or all of the following scenario occurs: TRDY signal is asserted HIGH, or DEVSEL signal is asserted HIGH, or FRAME signal is asserted LOW, OR-gate 601 provides a HIGH output signal. In so doing, OR-gate 601 indicates whether a bus target is ready for a transaction, whether a device connected to the bus has been selected, or whether a bus transaction is occurring. The output of OR-gate 601 is provided as an input to AND-gate 602. AND-gate 602 also receives signals Trmin --  stop --  master and Trmin --  stop as other inputs. As such, AND-gate 602 will output a LOW signal when any of its input signal is LOW. More particularly, AND-gate 602 outputs a LOW signal when either signal Trmin --  stop --  master or signal Trmin --  stop is LOW indicating that the bus transaction is terminated. NOR-gate 603 receives as input bus transaction signal FRAME and address valid signal addr --  vld --  n. Because bus transaction signal FRAME is asserted LOW when there is a bus transaction and address valid signal addr --  vld --  n is asserted LOW when the address data (for a selected device) is verified by address range decoder 692 to be valid, NOR-gate 603 provides a HIGH output only when there is a bus transaction and the address data is valid. 
     The outputs of NOR-gate 603 and AND-gate 602 are supplied as inputs to multiplexer 604. Multiplexer 604 is controlled by signal pci --  tx --  termin. Signal pci --  tx --  termin is the output of flip-flop 605 which receives the output of multiplexer 604 and is clocked by system clock signal CLK. Flip-flop 605 can be reset by RESET signal. In so doing, signal pci --  tx --  termin is LOW when there is no bus transaction, the address data for the selected device is not valid, or the bus transaction is terminated. 
     Reference is now made to FIG. 7 illustrating a gate-level diagram of target mode window closer 484. Target mode window closer 484 includes OR-gate 701 and flip-flop 702. OR-gate 701 receives as inputs writenotread signal indicating whether the current bus transaction is a read or a write transaction and STOP signal indicating that the current bus transaction is in its last clock cycle. The output of OR-gate 701 is provided as input to flip-flop 702 which is clocked by system clock signal CLK and reset by RESET signal. Flip-flop 702 provides signal Trmin --  stop as its output. In so doing, target mode window carries out the following logic: if the bus transaction is a write transaction (a HIGH writenotread signal), the window is open for the entire bus transaction. Otherwise if the bus transaction is a read transaction (a LOW writenotread), the window needs to be closed starting at the next clock cycle so that the PCI bus can be floated for data to be read. Also, if the current bus transaction is in its last clock cycle (a LOW STOP signal), the window needs to be closed starting at the next clock cycle. When the window needs to be closed, signal Trmin --  stop is asserted LOW. 
     FIG. 8 illustrates a gate-level diagram of master mode window closer 486 which consists of an OR-gate 801 and a NAND-gate 802. OR-gate 801 receives as inputs master --  active signal and force --  slave --  retry signal. OR-gate 801 assures that a HIGH output is provided if master --  active signal is HIGH indicating that there is a bus master is ready to initiate a transaction or if force --  slave --  retry signal is HIGH indicating that other devices seeking to gain control of the bus mastership. The output cf OR-gate 801 is provided as an input to NAND-gate 802. NAND-gate 802 receives an inverted STOP signal as a second input. Hence, NAND-gate 802 outputs a LOW Trmin --  stop --  master signal when the current device is a bus master and the current bus transaction is in its last clock cycle. 
     FIG. 9 illustrates a gate level diagram of window hardware 488. Window hardware 488 consists of OR-gate 901 and NAND-gates 902-939. OR-gate 901 receives as inputs signals fast --  enable and pci --  window --  open. OR-gate 901 assures that a HIGH output is provided if either fast --  enable signal or pci --  window --  open is HIGH indicating that the window should be open. The output of OR-gate 901 is provided as an input to NAND-gates 902-938. NAND-gates 902-934 receive inverted address &amp; data AD signal bits as other inputs. As such, NAND-gates 902-934 allow the address &amp; data AD signals through as their output when either fast --  enable signal or pci --  window --  open signal is HIGH. Similarly, NAND-gates 935-938 receive inverted command &amp; byte enable CBE signal bits as other inputs. As such, NAND-gate 935-938 allow the address &amp; data AD signals through as their output when either fast --  enable signal or pci --  window --  open signal is HIGH. NAND-gate 939 receives as inputs fast --  signal and the invert of device identification select signal IDSEL. NAND-gate 903 allows signal IDSEL to pass through when fast --  enable signal is HIGH. 
     FIG. 10 illustrates a gate-level diagram of master window controller 490 which simply consists of OR-gate 1001. OR-gate 1001 receives as inputs master --  active signal and the invert of target ready signal TRDY. OR-gate 1001 outputs signal pci --  window --  open. As long as master --  active signal is HIGH indicating the current device is a bus master or target ready signal TRDY is LOW indicating that a bus target is ready, the window should be open. Hence, pci --  window --  open is asserted HIGH in that case. 
     FIG. 11 illustrates a gate-level diagram of address range decoder 492. Address range decoder 492 consists of exclusive NOR-gate clusters 1101-1105, AND-gate 1106, and AND-gate clusters 1107-1108, and OR-gate 1109. As shown in FIG. 11A, exclusive NOR-gate cluster 1101 consists of four NOR-gates (1120-1123). Each exclusive NOR-gate in NOR-gate cluster 1101 receives as inputs a different bit from window command &amp; byte enable signal wcbe and a different bit from a predetermined encoded value indicating the configuration register selected. Each exclusive NOR-gate in cluster 1101 compares the values of the two inputs and outputs a HIGH value only when the two input values are equal. In so doing, exclusive NOR-gate cluster 1101 acts as a comparator that signals when window command &amp; byte enable signal wcbe carries a command signal for a configuration register. Referring back to FIG. 11, the 4-bit output of exclusive NOR-gate cluster 1101 is provided as input to AND-gate 1106. Window identification select signal widsel is also provided as an input to AND-gate 1106. When both signal widsel is asserted HIGH indicating a device connected to the PCI bus has been selected and the 4-bit output from exclusive NOR-gate is HIGH (i.e., all four bits are HIGH), AND-gate 1106 outputs a HIGH value to NOR-gate 1109 which in turns asserts a LOW addr --  vld --  n signal to indicate that a valid configuration register is selected. 
     FIG. 11B illustrates, as an example, exclusive NOR-gate cluster 1102. In general, there are (m+1) exclusive NOR-gates in exclusive NOR-gate cluster 1102 where 0≦m≦31. For FIG. 11B, however, it is assumed that m is equal to 31. Hence, exclusive NOR-gate cluster 1102 consists of 32 exclusive NOR-gates (1124-1156). Each exclusive NOR-gate in exclusive NOR-gate cluster 1102 receives as inputs a different bit from window address &amp; data signal wad and a different bit from address range base0[31:m] from a configuration register. Each exclusive NOR-gate in cluster 1102 compares the values of the two inputs and outputs a HIGH value only when the two input values are equal. In so doing, exclusive NOR-gate cluster 1102 acts as a comparator that signals when window address &amp; data signal wad carries an address for a device connected to the PCI bus that is in the device&#39;s memory address space. Referring back to FIG. 11, exclusive NOR-gate cluster 1103 consists of 4 exclusive NOR-gates connected in a identical fashion as NOR-gates 1120-1123 in FIG. 11A. Each exclusive NOR-gate in cluster 1103 receives a different bit from window command &amp; byte enable signal wcbe and a predetermined encoded value indicating the types of memory operations (e.g., read or write). Each exclusive NOR-gate in cluster 1103 compares the values of the two inputs and outputs a value one (1) only when the two input values are equal. In so doing, exclusive NOR-gate cluster 1103 acts as a comparator that signals when window command &amp; byte enable signal wcbe carries a command signal for the device. The outputs of exclusive NOR-gate clusters 1102 and 1103 are provided as inputs to AND-gate cluster 1107. AND-gate cluster 1107 consists of a number of AND-gates arranged in several levels to serve as an AND logcial function. FIG. 11C illustrates, as an example, AND-gate cluster 1107. In general, (m+1+4) inputs are provided as inputs to AND-gate cluster 1107. For FIG. 11C, however, it is assumed for the sake of convenience that m is equal to 31. Hence, there are 36 inputs of AND-gate cluster 1107 supplied to AND-gates 1110-1115 wherein each AND-gate receives 6 inputs. The outputs of AND-gates 1110-1115 are provided as inputs to AND-gates 1116-1118. The outputs of AND-gates 1116-1118 are provided as inputs to AND-gate 1119 which outputs a single signal that acts as the output of AND-gate cluster 1107. As such the AND-gates in AND-gate cluster 1107 combine to function as one big AND-gate. AND-gate cluster 1107 is well-known in the art. 
     Referring now back to FIG. 11, AND-gate cluster 1107 provides its output to NOR-gate 1109. When all inputs of AND-gate 1107 are HIGH indicating that the current command involves a valid operations and the current address is valid, NOR-gate 1109 in turns asserts a LOW addr --  vld --  n signal. 
     Exclusive NOR-gate 1104 cluster consists of (n+1) exclusive NOR-gates connected in an similar fashion as NOR-gates 1124-1156 in FIG. 11B. Accordingly, for brevity and clarity, exclusive NOR-gate 1104 is not discussed further. Each exclusive NOR-gate in exclusive NOR-gate cluster 1104 receives as inputs a different bit from window address &amp; data signal wad and a different bit from address range base1[31:n]. Each exclusive NOR-gate in cluster 1104 compares the values of the two inputs an outputs a HIGH value only when the two input values are equal. In so doing, exclusive NOR-gate 1104 acts as a comparator that signals when window address &amp; data signal wad carries an address for the device connected to the PCI bus that is in the device&#39;s I/O address space. Exclusive NOR-gate cluster 1105 consists of 4 exclusive NOR-gates connected in an identical fashion as NOR-gates 1120-1123 in FIG. 11A. Each exclusive NOR-gate in exclusive NOR-gate cluster 1105 receives as inputs a different bit from window command &amp; byte enable signal wcbe and a predetermined encoded value indicating the types of I/O operations (e.g., read or write). Each exclusive NOR-gate in cluster 1105 compares the values of the two inputs an outputs a value one (1) only when the two input values are zeros. In so doing, exclusive NOR-gate 1105 acts as a comparator that signals when window command &amp; byte enable signal wcbe carries a command signal for an I/O device. The outputs of exclusive NOR-gate clusters 1104 and 1105 are provided as inputs to AND-gate cluster 1108. AND-gate cluster 1108 is similar to AND-gate cluster 1107 shown in FIG. 11C except that (n+1+4) inputs are provided as inputs to AND-gate cluster 1108 where 0≦n≦31. Accordingly, for brevity and clarity, AND-gate cluster 1108 is not discussed any further. AND-gate cluster 1108 provides its output to NOR-gate 1109. When both inputs of AND-gate 1107 are HIGH indicating that the current command involves a I/O operations and the current I/O address is valid, NOR-gate 1109 in turns asserts a LOW addr --  vld --  n signal. 
     Reference is now made to FIG. 12 illustrating a timing diagram for the start of a target read transaction. As shown in FIG. 12, prior to time T1, in response to an asserted LOW FRAME signal, fast opener circuit 480 asserts a HIGH fast --  enable signal which triggers window hardware 488 to open its gate to allow the address from address &amp; data signal AD to pass through as window address &amp; data signal WAD. Similarly, the HIGH fast --  enable signal triggers window hardware 488 to open its gate to allow command &amp; byte enable signal CBE to pass through to become window command &amp; byte enable signal wcbe. When signal addr --  vld --  n is subsequently asserted LOW indicating that the address is verified to be a valid address. When a LOW signal addr --  vld --  n is coupled with a LOW FRAME signal, they trigger target window controller 482 to assert signal pic --  tx --  termin HIGH. A HIGH signal pic --  tx --  termin indicates that there is a valid transaction and hence, fast --  enable signal is to remain HIGH. During time T2, sufficient information is gathered to determine whether the current transaction is a read transaction. The transaction is a target transaction because target ready signal TRDY is asserted LOW prior to time T2 and initiator ready signal IRDY is kept HIGH all through. By time T3, the command has been decoded to determine that the current transaction is a read transaction and the window should be closed after the bus has been floated for one clock cycle. For this reason, signal pic --  tx --  termin is asserted LOW at time T3 which triggers the fast --  enable signal to go LOW. 
     FIG. 13 illustrates a timing diagram for the end of a target transaction. As shown in FIG. 13, the transaction is a target transaction because target ready signal TRDY is asserted LOW all the way until time T6. Up until time T5, fast opener circuit 480 asserts a HIGH fast --  enable signal which triggers window hardware 488 to open its gate to allow the address from address &amp; data signal AD to pass through as window address &amp; data signal WAD. Since this is near the end of a transaction, there is no more command or byte enable information sent. When FRAME signal goes HIGH prior to time T4 indicating there is no transaction on the bus, initiator ready signal IRDY goes LOW to indicate that a master is ready for a transaction. In response, pci --  tx --  term is asserted LOW at time T5 which triggers fast --  enable signal to go LOW to close the window gate at time T6. Near time T6, signal IRDY goes HIGH to indicate that the bus master has completed the transaction. 
     FIG. 14 illustrates a timing diagram for the start of a master read transaction initiated by network controller 324. As shown in FIG. 14, starting at time T7, signal master --  active is asserted HIGH to indicate that network controller 224 intends to be a master of the bus and is getting ready to initiate a transaction. Subsequent to this, FRAME signal is asserted LOW to indicate that a transaction is occurring on the PCI bus. In response to the asserted LOW FRAME signal, fast opener circuit 480 asserts a HIGH fast --  enable signal which triggers window hardware 488 to open its gate to allow the address from address &amp; data signal AD to pass through as window address &amp; data signal WAD. Similarly, the HIGH fast --  enable signal triggers window hardware 488 to open its gate to allow command &amp; byte enable signal CBE to pass through to become window command &amp; byte enable signal wcbe. Because the current transaction is a read transaction, in response to signal Trmin --  stop being asserted LOW, fast --  enable signal goes LOW just prior to time T8 to close the window gate starting at the next clock cycle. Subsequent to time T8, signal DEVSEL is asserted LOW to indicate a device from which information is to be retrieved. Signal initiator ready IRDY is then asserted LOW to signify that the bus master is ready for a transaction (e.g., retrieve data from a target). Just prior to time T9, target ready signal TRDY is asserted LOW to indicate that the bus target is now ready for data transfer. Enable --  adcbe --  in is derrived from signal pci --  window --  open and fast --  enable as shown in FIG. 9. As can be seen in FIG. 14, enable --  adcbe --  in is first turned on HIGH then pulled LOW during the time signal FRAME is LOW. Signal internalAD[31:0] shows &#34;xxxx&#34; information propagting in from the periphery of the chip, but window hardware circuit 488 prevents these information from propagating inside as can be seen on signal WAD[31:0] and WCBE[3:0]. 
     FIG. 15 illustrates a timing diagram for the end of a burst write, master transaction of network controller 324. As shown in FIG. 15, prior to time T10, signal master --  active is asserted HIGH to indicate that network controller 224 intends to be a master of the bus and is getting ready to terminate the current transaction. Subsequent to this, target ready signal TRDY is asserted HIGH to indicate that the target is not ready for data retrieval. Additionally, STOP signal is asserted LOW to indicate the end of the current transaction cycle. When signal STOP is asserted LOW, it triggers signal termin --  stop --  master to assert an immediate LOW as can be seen from the logic in FIG. 8 followed by signal termin --  stop being triggered to be asserted LOW for the next clock as can be seen from the logic in FIG. 7. These two signals trigger signal pic --  tx --  termin to be de-asserted and keep window hardware 488 closed at the end of the transaction. 
     The preferred embodiment of the present invention, a window mechanism to prevent invalid data from propagating into a device connected to a PCI tristate bus, is thus described. While the present invention has been described in particular embodiments, the present invention should not be construed as limited by such embodiments, but rather construed according to the below claims.