MULTICOPY QUEUE STRUCTURE WITH SEARCHABLE CACHE AREA

Abstract: An arrangement and a method of maintaining a count of the number of copies of a frame that have been transmitted from a network switch uses a cache memory to store the number of copies of a frame to be transmitted from the network switch. A queue is used to queue entries that indicate the transmission of a copy of a frame, and these queued entries are released to a buffer manager. Whenever a transmission is made of a copy of this frame, and the buffer manager examines the entry indicating this transmission after the entry leaves the queue, the buffer manager searches for a corresponding entry in the cache memory and changes the copy number (i.e., the count of the number of transmissions) of that frame in the cache memory.

Description: MULTICOPY QUEUE STRUCTURE WITH SEARCHABLE CACHE AREA FIELD OF THE INVENTION The present invention relates to the field of communications, and more particularly, to method and apparatus for transmitting multiple copies of data and maintaining a count of the number of copies that have been transmitted. BACKGROUND ART In many network systems, such as packet switched networks (e.g., Ethernet networks), it is often desirable for a single end station to send the same data to multiple other end stations on the network. For example, in a conventional e-mail system, a user may desire to send the same e-mail message to four different users connected to the e-mail system. In a packet switched network, a switch makes the forwarding decision when it receives frames of data from an end station through a port of the switch. If the frame is to be transmitted to a number of end stations, the switch must make the forwarding decision to forward the frame to the correct ports. One concern in such systems is to ensure that the correct number of copies of the frame are transmitted. This concern is especially difficult to satisfy if the switch has a number of ports through which the information can be transmitted. For example, if copies of the frame are being transmitted simultaneously from three different ports of the switch, maintaining an accurate count of the number of copies that have been transmitted is a problem. It is also important to maintain an accurate count of the number of copies that have been transmitted so that the switch is made aware of when resources used to temporarily hold the data may be reused to store other data. If this information is not provided on a timely basis, there may be unnecessary congestion and contention for the resources to store the data. SUMMARY OF THE INVENTION There is a need for an arrangement and a method of maintaining a count of the number of copies of a data item, such as a frame, that have been transmitted from a device, such as a network switch of a packet switched network. This and other needs are met by an embodiment of the present invention which provides an arrangement for maintaining a count of the copies of a frame that are transmitted from a network switch, comprising a multicopy queue, a searchable memory, and a buffer manager. The multicopy queue receives and queues entries, each entry having a frame identifier and a copy number. Each copy number is either a positive or negative value, a copy number with a positive value representing the total number of copies of that frame to be transmitted, and a copy number with a negative value representing a transmission of one of the copies of that frame that has occurred. The memory is configured to receive and store the entries from the multicopy queue that have positive copy numbers. The buffer manager examines the entries after they exit the multicopy queue. When an entry from the multicopy queue contains a negative copy number (indicating that a copy of the has been transmitted), the buffer manager searches the first memory for an entry having the same frame identifier as the entry with the negative copy number. The buffer manager decrements the copy number of the entry in the memory if the copy number of the entry in the memory is greater than one, and deletes the entry in the memory if the copy number is equal to one. An advantage of this embodiment of the present invention is that an accurate count of the number of transmissions is made, even in a multiport switch environment in which the copies can be simultaneously transmitted from different port. The count is maintained for each multiple copy frame in a memory, such as a cache memory. Whenever a transmission is made of a copy of this frame, the buffer manager changes the copy number (i.e., the count of the number of transmissions) of that frame. The use of a queue to queue entries that indicate the transmission of a copy of a frame provides the buffer manager the opportunity to recognize each transmission of a copy, and adjust the count in the memory accordingly. This is because the queue signals the transmission of a copy to the buffer manager in a serial fashion so that all the transmissions will be recognized, even if they occur simultaneously. The earlier stated needs are also met by another embodiment of the present invention which provides a memory arrangement comprising a first memory configured to store entries, each entry having a data pointer that points to data stored in a second memory and an associated data item. A second memory stores the data at locations in the second memory pointed to by the data pointer. The locations in the second memory are configured to store the associated data item with the data, so that the second memory is an extension of the first memory to store the associated data items. One of the advantages provided by the above embodiment is that the second memory may serve as a directly addressable extension of the first memory, on an as needed basis. The data items, which are associated with data and normally stored in the first memory, are storeable in the second memory with the data itself. This arrangement therefore provides a larger capacity memory for storing the data items, but only when needed, so that the first memory may be kept small and relatively fast. Another embodiment of the invention provides an arrangement for maintaining a count of a number of copies of a data item to be transmitted from a device and includes a first memory in which entries are stored and a manager device. Each entry that is stored in d e first memory corresponds to a different data item of which multiple copies are to be transmitted from the device. Each entry has a pointer which points to a location in a second memory in which the data item is stored and a copy number which indicates how many copies of the data item remain to be transmitted. The manager device, adjusts the copy number in one of the entries stored in the first memory when a copy of the data item with the same pointer as that entry in the first memory is transmitted from the device. In another embodiment of the present invention, a multiport switch arrangement for a packet switched network is provided, the switch being configured to transmit multiple copies of a single frame. The switch arrangement includes a multicopy queue configured to receive and queue entries, each entry having a frame pointer and a copy number. Each copy number has either a positive or negative value. A copy number with a positive value represents the total number of copies of that frame to be transmitted, and a copy number with a negative value represents a transmission of one of the copies of that frame that has occurred. A searchable first memory receives and stores the entries from the multicopy queue that have positive copy numbers. A buffer manager examines the entries exiting the multicopy queue. When an entry from the multicopy queue contains a negative copy number, the buffer manager searches the first memory for an entry having the same frame pointer as the entry with the negative copy number. The buffer manager decrements the copy number of the entry in the first memory having that frame pointer if the copy number of that entry in the first memory is greater than one,. If the copy number is equal to one, the buffer manager deletes that entry in the first memory. The switch arrangement of the present invention maintains a count of the number of transmissions of a copy of a frame by queuing the indications of a transmission and changing the copy number stored in a memory when the indication of the transmission exits the queue. By queuing the negative copy numbers and providing them one at a time for processing by the buffer manager, an accurate count of the transmitted copies of a frame may be maintained, even when there are simultaneous transmissions of copies through multiple ports. The earlier stated needs are also satisfied by the present invention which provides a method of maintaining a count of the number of transmissions of a frame from a network switch. This method includes loading a searchable first memory with entries, each entry comprising a frame identifier and a copy number indicating the number of copies of that frame that remain to be transmitted from the network switch. The first memory is searched, when a frame is transmitted, for a frame that has the same frame identifier as the transmitted frame. The copy number of the entry in the first memory is decremented when the search of the first memory locates an entry with the same frame identifier as the transmitted frame. The foregoing and other features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a block diagram of a packet switched system constructed in accordance with an embodiment of the present invention. Figure 2 is a block diagram of a multiport switch constructed in accordance with an embodiment of the present invention and used in the packet switched system of Figure 1. Figure 3 is a schematic depiction of a switch subsystem of the multiport switch of Figure 3, constructed in accordance with an embodiment of the present invention. Figure 4 is a block diagram of a single output queue of the switch subsystem of Figure 4, constructed in accordance with an embodiment of the present invention. Figure 5 is a detail of a first type of output queue in accordance with an embodiment of the present invention. Figure 6 is a detail of a second type of output queue in accordance with an embodiment of the present invention. Figure 7 is a detail of an overflow area of the external memory, configured in accordance with an embodiment of the present invention. Figure 8 is a block diagram of a linked list data structure employed in the present invention. Figure 9 schematically depicts a frame buffer header format in accordance with an embodiment of the present invention. Figure 10 is a detail of the multicopy, reclaim and free buffer pool area of the switch subsystem of Figure 4, constructed in accordance with an embodiment of the present invention. Figure 11 is block diagram of a free buffer pool structure constructed in accordance with an embodiment of the present invention. Figure 12 is a block diagram of a multicopy queue configured in accordance with an embodiment of the present invention. Figure 13 is a schematic representation of the multicopy cache constructed in accordance with an embodiment of the present invention. Figure 14 is a block diagram of a queuing block of the buffer manager of the switch subsystem and a port vector FIFO, constructed in accordance with an embodiment of the present invention. DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS The present invention will be described with the example of a switch in a packet switched network, such as an Ethernet (IEEE 802.3) network. It will become apparent, however, that the present invention is also applicable to other packet switched systems, as described in detail below, as well as to other types of systems in general. Figure 1 is a block diagram of an exemplary system in which the present invention may be advantageously employed. The exemplary system 10 is a packet switched network, such as an Ethernet network. The packet switched network includes an integrated multiport switch (IMS) 12 that enables communication of data packets between network stations. The network may include network stations having different configurations, for example twenty-four (24) 10 megabit per second (M\bps) network stations 14 that send and receive data at a network data rate of 10 M\bps, and two 100 M\bps network stations 22 that send and receive data packets at a network speed of 100 M\bps. Hence, the switch 12 selectively forwards data packets received from the network stations 14 or 22 to the appropriate destination based upon Ethernet protocol. According to the disclosed embodiment, the 10 M\bps network stations 14 send and receive data packets to and from the switch 12 via a media 17 and according to half-duplex Ethernet protocol. The Ethernet protocol ISO/IEC 8802-3 (ANSI/IEEE Std. 802.3, 1993 Ed.) defines a half-duplex media access mechanism that permits all stations 14 to access the network channel with equality. Traffic in a half-duplex environment is not distinguished or prioritized over the medium 17. Rather, each station 14 includes an Ethernet interface card that uses carrier-sense multiple access with collision detection (CSMA/CD) to listen for traffic on the media. The absence of network traffic is detected by sensing a deassertion of a receive carrier on the media. Any station 14 having data to send will attempt to access the channel by waiting a predetermined time after the deassertion of a receive carrier on the media, known as the interpacket gap interval (IPG). If a plurality of stations 14 have data to send on the network, each of the stations will attempt to transmit in response to the sensed deassertion of the receive carrier on the media and after the IPG interval, resulting in a collision. Hence, the transmitting station will monitor the media to determine if there has been a collision due to another station sending data at the same time. If a collision is detected, both stations stop, wait a random amount of time, and retry transmission. The 100 M\bps network stations 22 preferably operate in full-duplex mode according to the proposed Ethernet standard IEEE 802.3x Full-Duplex with Flow Control - Working Draft (0.3). The full-duplex environment provides a two-way, point-to-point communication link between each 100 M\bps network station 22 and the switch 12, where the switch 12 and the respective stations 22 can simultaneously transmit and receive data packets without collisions. The 100 M\bps network stations 22 each are coupled to network media 17 via 100 M\bps physical (PHY) devices 20 of type 100 Base-TX, 100 Base-T4, or 100 Base-FX. The switch 12 includes a media independent interface (Mil) 24 that provides a connection to the physical devices 20. The 100 M\bps network stations 22 may be implemented as servers or routers for connection to other networks. As shown in Figure 1, the network 10 includes a series of switch transceivers 26 that perform time division multiplexing and time division demultiplexing for data packets transmitted between the switch 12 and the 10 M\bps stations 14. A magnetic transformer module 19 maintains the signal waveform shapes on the media 17. The switch 12 includes a transceiver interface 18 that transmits and receives data packets to and from each switch transceiver 16 using a time-division multiplexed protocol across a single serial non-return to zero (NRZ) interface 23. The switch transceiver 16 receives packets from the serial NRZ interface 23, demultiplexes the received packets, and outputs the packets to the appropriate end station 14 via the network media 17. According to the disclosed embodiment, each switch transceiver 16 has four independent 10 M\bps twisted-pair ports and uses 4:1 multiplexing across the serial NRZ interface enabling a four-fold reduction in the number of PINs required by the switch 12. The switch 12 contains a decision making engine, switching engine, buffer memory interface, configuration/control/status registers, management counters, and MAC (media access control) protocol interface to support the routing of data packets between the Ethernet ports serving the network stations 14 and 22. The switch 12 also includes enhanced functionality to make intelligent switching decisions, and to provide statistical network information in the form of management information base (MIB) objects to an external management entity, described below. The switch 12 also includes interfaces to enable external storage of packet data and switching logic in order to minimize the chip size of the switch 12. For example, the switch 12 includes a synchronous dynamic RAM (SDRAM) interface 34 that provides access to an external memory 36 for storage of received frame data, memory structures, and MIB counter information. The memory 36 may be an 80, 100 or 120 MHz synchronous DRAM having a memory size of 2 or 4 Mb. The switch 12 also includes a management port 30 that enables an external management entity to control overall operations of the switch 12 by a management MAC interface 32. The switch 12 also includes a PCI interface 26 enabling access by the management entity via a PCI host and bridge 28. Alternatively, the PCI host and bridge 28 may serve as an expansion bus for a plurality of switch devices 12. The switch 12 includes an internal decision making engine (Figure 2) that selectively transmits data packets received from one source to at least one destination station. The internal decision making engine may be substituted with an external rules checker. The switch 12 includes an external rules checker interface (ERCI) 40 that allows use of an external rules checker 42 to make frame forwarding decisions in place of the internal decision making engine. Hence, frame forwarding decisions can be made either by the internal switching engine or the external rules checker 42. The switch 12 also includes an LED interface 44 that clocks out the status of conditions per port and drives LED external logic 46. The LED external logic 46, in turn, drives LED display elements 48 that are human readable. An oscillator 38 provides a 40 MHz clock input for the system functions of the switch 12. Figure 2 is a block diagram of the integrated multiport switch (IMS) 12 of Figure 1. The switch 12 includes twenty-four (24) 10 M\bps media access control (MAC) ports 50 for sending and receiving data packets in half-duplex between the respective 10 M\bps network stations 14 (ports 1-24), and two 100 M\bps MAC ports 53 for sending and receiving data packets in full-duplex between the respective 100 M\bps network stations (ports 25, 26). As described above, the management interface 30 also operates according to MAC layer protocol (port 0). Each of the MAC ports 50, 53 and 30 has a receive first in-first out (FIFO) buffer 52 and transmit FIFO 54. Data packets from a network station are received by the corresponding MAC port and stored in the corresponding receive FIFO 52. The received data packet is output from the corresponding receive FIFO 52 to the external memory interface 34 for storage in the external memory 36. The header of the received packet is also forwarded to a decision making engine, either internal rules checker 58 or the external rules checker interface 40, to determine which MAC ports will output the data packet. Specifically, the packet header is forwarded to the internal rules checker 58 or the external rules checker interface 40, depending on whether the switch 12 is configured to operate using the internal rules checker 58 or the external rules checker 42. The internal rules checker 58 and external rules checker 42 provide the decision making logic for determining the destination MAC port for a given data packet. The decision making engine may thus output a given data packet to either a single port, multiple ports, or all ports (i.e., broadcast). For example, each data packet includes a header having source and destination address, where the decision making engine may identify the appropriate output MAC port based upon the destination address. Alternatively, the destination address may correspond to a virtual address that the appropriate decision making engine identifies as corresponding to a plurality of network stations. Alternatively, the received data packet may include a VLAN (virtual LAN) tagged frame according to IEEE 802. Id protocol that specifies another network (via a router at one of the 100 M\bps stations 22) or a prescribed group of stations. Hence, either the internal rules checker 58 or the external rules checker 42 via the interface 40 will decide whether a frame temporarily stored in the buffer memory 36 should be output to a single MAC port or multiple MAC ports. Use of the external rules checker 42 provides advantages such as increased capacity, a random- based ordering in the decision queue that enables frame forwarding decisions to be made before the frame is completely buffered to external memory, and enables decisions to be made in an order independent from the order in which the frames were received by the switch 12. The decision making engine (i.e., internal rules checker 58 or the external rules checker 42) outputs a forwarding decision to a switch subsystem 56 in the form of a port vector identifying each MAC port that should receive the data packet. The port vector from the rules checker includes the address location storing the data packet in the external memory 36, and the identification of the MAC ports to receive the data packet for transmission (e.g., MAC ports 0-26). The switch subsystem 56 fetches the data packet identified in the port vector from the external memory 36 via the external memory interface 34, and supplies the retrieved data packet to the appropriate transmit FIFO 54 of the identified ports. Additional interfaces provide management and control information. For example, a management data interface 59 enables the switch 12 to exchange control and status information with the switch transceivers 16 and the 100 M\bps physical devices 20 according to the Mil management specification (IEEE 802.3u). For example, the management data interface 59 outputs a management data clock (MDC) providing a timing reference on the bidirectional management data IO (MDIO) signal path. The PCI interface 26 is a 32-bit PCI revision 2.1 compliant slave interface for access by the PCI host processor 28 to internal IMS status and configuration registers 60, and access external memory 36. The PCI interface 26 can also serve as an expansion bus for multiple switch devices. The management port 30 interfaces to an external MAC engine through a standard seven-wire inverted serial GPSI interface, enabling a host controller access to the switch 12 via a standard MAC layer protocol. Figure 3 depicts the switch subsystem 56 of Figure 2 in more detail according to an exemplary embodiment of the present invention. Other elements of the multiport switch 12 of Figure 2 are reproduced in Figure 3 to illustrate the connections of the switch subsystem 56 to these other elements. The switch subsystem 56 contains the core switching engine for receiving and forwarding frames. The main functional blocks used to implement the switching engine include: a port vector FIFO 70, a buffer manager 72, a plurality of port output queues 74, a management port output queue 75, an expansion bus port output queue 77, a free buffer pool 104, a multicopy queue 90, a multicopy cache 96 and a reclaim queue 98. The operation and structure of these functional blocks will be described in more detail, but a brief overview of the switch subsystem 56 of Figure 3 is first presented to provide context to the later discussion of the individual elements. There are two basic types of frames that enter the multiport switch 12 from the ports: unicopy frames and multicopy frames. A unicopy frame is a frame that is received at a port which is to be transmitted by the multiport switch 12 to only one other port. By contrast, a multicopy frame is a frame that is received at one port for transmission to more than one port. In Figure 3, each port is represented by a separate MAC 50, having its own receive FIFO 52 and transmit FIFO 54. Frames, whether unicopy or multicopy, are received by the internal MAC engines 50. When the frame packet is received at the port, it is placed in the receive FIFO 52. Each frame has a header, which is provided to a rules checker, either the internal rules checker 58 or the external rules checker 42. The rules checker 42 or 58, based on d e information in the header, determines from where the frame packet will be cast, i.e., through which port or ports will the frame packet be transmitted. At the same time as the rules checker 42 or 58 is making its forwarding determination, the buffer manager 72 obtains a free buffer pointer from the free buffer pool 104. This free buffer pointer is the location in external memory 36 at which the frame will be stored by the receive FIFO 52. Once the free buffer pointer is obtained from the free buffer pool 104 by the buffer manager 72, the buffer pointed to by the free buffer pointer is no longer considered free. The frame data is transferred over data bus 80 from the receive FIFO 52 to the external memory 36 in a direct memory access (DMA) transaction. The frame is stored in the location pointed to by the free buffer pointer obtained from the free buffer pool 104, although a number of other buffers may be used to store a frame, as will be described. In addition to the header data, the rules checker 42 or 58 also receives the free buffer pointer from the buffer manager 72. This free buffer pointer is now referred to as a frame pointer since it points to the memory location in the external memory 36 where the frame is stored. The rules checker 42 or 58 uses the header information to make the forwarding decision and generate a forwarding instruction in the form of a "port vector". In the exemplary illustrated embodiment, the port vector is a 28-bit vector with a bit set for each output port to which the frame should be forwarded. Assume for this overview example that the received frame is a unicopy frame. Accordingly, only one bit is set in the port vector generated by the rules checker 42 or 58. The bit that is set in the port vector corresponds to a particular one of the ports. The rules checker 42 or 58 places the port vector and die frame pointer (as well as a control opcode and a VLAN index) into the port vector FIFO 70. The port vector is examined by the port vector FIFO 70 to determines into which particular output queue 74 (or queues) the frame pointer associated with the port vector should be input. The port vector FIFO 70 places the frame pointer into the top of the appropriate output queue 74. This queues the transmission of the frame. At some point in time, the frame pointer reaches the bottom of the output queue 74 after passing through the output queue 74. The buffer manager 72 takes the frame pointer when it arrives at the bottom of the output queue 74 and passes the frame pointer to the appropriate transmit FIFO 54 of the correct port via frame pointer read bus 86. This schedules the transmission of the frame. The frame data is read in a DMA transaction from the location in external memory 36 pointed to by the frame pointer, is placed in the appropriate transmit FIFO 54 and then transmitted. A multicopy transmission is similar to the unicopy transmission, except that the port vector has multiple bits set, designating the multiple ports from which the frame will be transmitted. The frame pointer is placed into each of the appropriate output queues 74 and transmitted from the corresponding transmit FIFOs 54. The buffer manager 72 uses the special control queues, i.e., the free buffer pool 104, the multicopy queue 90, and the reclaim queue 98, and the multicopy cache 96 to manage the process of allocating buffers to store received frames and retrieving buffers for re-use once the frame has been transmitted to its designated output port(s). The buffer manager 72 also maintains "overflow" regions in external memory 36 for the output queues 74 and the control queues 104, 90 and 98, as will be described in more detail later. With this operational overview serving as background, the individual sections and various aspects of the switch subsystem 56 will now be discussed in more detail. The first of these aspects that will be described is the structure of the various output queues 74 of the present invention. In addition to the output queues 74 designated for the 10 Mb/s output ports and the 100 Mb/s output ports, an output queue 75 is provided for the management port 30, and an output queue 77 is provided for the expansion port 26. These output queues 75, 77 have the same external structure as the output queues 74, but different internal configurations, as will be described. Figure 4 is a block diagram of the external structure of an output queue 74 in accordance with an embodiment of the present invention. As is apparent from Figure 4, the output queue 74 of the present invention has a three-part configuration. For highest performance it is preferable to keep all of the queuing structure on the chip (referring to the multiport switch 12), but the real estate on a chip is very expensive. This presents a dilemma when the chip is designed to switch, and needs to queue, a large number of entries. The present invention solves this dilemma by providing a single output queue that includes a high performance, low capacity section that is on-chip, and an overflow area that is off-chip. The overflow area allows the queue to serve as a large capacity queue as needed, albeit with a relatively lower performance than the on-chip portion. A single logical output queue 74 of the present invention, according to the embodiment of Figure 4, has three physical sections. These include an output queue write side 76, an output queue read side 78, and an output queue overflow area (generally designated as 110) located in the external memory 36. Access to the external memory 36 for all of the output queues 74 is through the external memory interface 34, as described earlier. The present invention takes advantage of the bursting nature of current external memories, so that the data (e.g., frame pointers) is sent on and off die chip to the overflow queue area 110 in bursts over the bus 84 connecting the chip 12 to the external memory 36. The output queue write side 76 and the output queue read side 78 are located on the chip 12. The write side 76 and the read side 78 are considered to be small, expensive resources. By contrast, the overflow area 110, forming the third part of the output queue 74, is large and inexpensive. The write side 76 and the read side 78 provide high performance, while the path through the overflow area provides a low-performance, large capacity path. In operation, the output queue write side 76 receives an entry. In the exemplary embodiment of a multiport switch 12 according to die present invention, the entry is a frame pointer that points to the first buffer in external memory in which the first 256 bytes of a frame are stored. It should be apparent to those of skill in the art, however, that the output queue structure 74 is not limited to frame pointers as entries, but is widely applicable to queue other types of entries, both in multiport switches and in other technologies. After the entry flows through and reaches the bottom of the output queue write side 76, control logic associated with the output queue 74 makes a decision as to what to do with the entry. If there is space in the output queue read side 78, and the overflow area 110 for that output queue 74 is empty, then one or more entries are passed directly from the output queue write side 76 to the output queue read side. This passing of the entry or entries directly from the write side 76 to the read side 78 is performed entirely on the chip 12, and is tiierefore a low-latency, fast flow-through of an entry. If the output queue read side 78 is full, and there is at least a burst-size amount of data (e.g., 16 bytes worth of entries) in the output queue write side 76, then the data is written in a burst fashion into the overflow area 110 for that output queue 74. If the output queue read side 78 is full, but there is not yet a burst-size amount of data in the output queue write side 76, then the entry remains in the output queue write side and nothing further is done. Eventually, the output queue read side 78 will empty, and when d e output queue read side 78 has enough space to accommodate a burst-size amount of data, and there is data in the overflow area 110, a burst of data is provided from the overflow area 110 into the output queue read side 78. In the output queue structure, the read side 78 is acting most like a traditional queue, because it is from diis portion that entries are taken, one by one. The output queue write side 76 mostly serves a collection function to assemble the data into bursts for writing to the external memory 36. Hence, the present invention transforms single events (placing an entry into the output queue 74) into a burst event. The write side 76 allows an accumulation of data to dien burst, if necessary, to the overflow area 110 in the external memory 36. The overflow area 110 provides inexpensive storage in times of congestion, rather than dedicating expensive chip resources to a function that will only be needed on relatively rare occasions. Even though the present invention utilizes an overflow area 110 that is off-chip, the accessing of this area 110 is performed in a manner that is efficient, by bursting a number of bytes of information at a time. This is in contrast to conventional queuing structures in which single entries are written and read to and from the queue. During operation, if there is a lot of entries arriving at the output queue 74, these entries are placed into the overflow area 110 to avoid overflowing the on-chip queue 78. Hence, the discarding of frames is largely prevented with the queue structure of the present invention. Also, the total amount of memory dedicated to die overflow areas 110 may be readily changed by changing the size of the external memory 36. Furthermore, the sizes of the individual specific overflow areas 110 are programmable to customize the queue sizes, without impacting the performance of the output queues 74. Typically, a queue is an ordered structure with a first-in, first-out arrangement. In some types of queues, however, such as the reclaim queue 98 and the free buffer pool 104, the order of entries does not matter. If it is possible to send data directly from die write side 100 to the read side 102, the present invention permits information to be directly sent d is route, bypassing the overflow area for the queue. This is permitted even if there is information in the associated overflow area, as long as the information is not order-sensitive. For example, the reclamation of buffers is not order-sensitive, since any order in which the buffers are eventually returned to the free list in the free buffer pool 104 after the buffer is no longer needed to store the frame is acceptable. Hence, in order to avoid incurring the bandwidth of a write to the overflow area 110 for the reclaim queue 98 in the external memory 36 when the data is not order-sensitive, the information is passed directly from the write side 100 to the read side 102, assuming the read side 102 has room for more entries. The reclaim queue 98 is an example of a type of queue that queues data which is not order-sensitive. However, tiiere are many other types of data in different applications that are also not order-sensitive, so that this feature of the present invention finds utility in queues that queue these other types of data. In the multiport switch of an exemplary embodiment of the present invention as depicted in Figures 1 and 2, diere are twenty-eight output queues (each associated with an output port): twenty-four for the 10 Mb/s user ports, two for the 100 Mb/s server ports, one for the management port and one for the expansion bus port. The output queues 74, 75 and 77 provide temporary storage for frame pointers when they are queued for transmission. Queuing takes the form of the port vector FIFO 70 writing frame pointers into the various output queues 74, 75 and 77 indicated in a forwarding port vector. In certain preferred embodiments of the invention, the various output queues 74, 75 and 77 contain several or all of the following fields: unicopy bit, frame pointer, control opcode or control signals, and VLAN (virtual local area network) index. The unicopy bit flags a frame which is to be forwarded to only one output port. The frame pointer points to the frame in external memory 36. The control opcode identifies special information about the frame (i.e., newly learned frame, etc.). The control signals use information from the control opcode to indicate how d e ports will handle frames before transmission. The VLAN index provides die reference to a VLAN tag which should be inserted (if necessary) into the outgoing frame. However, these fields are exemplary only, as the present invention is applicable to other output queues with different types of fields. The internal structure of an exemplary embodiment of a first type of output queue 74, the 10 Mb/s port output queue, is depicted in Figure 5. The 10 Mb/s output queues 74 hold entries for frames to be forwarded to d e 10 Mb/s ports. The output queue write sides 76 for these queues hold thirty-two entries and die output queue read sides 78 hold sixteen entries in the exemplary illustrated embodiment, although other sizes are contemplated and within the scope of the invention. Each entry in a 10 Mb/s output queue 74 comprises a unicopy bit and a frame pointer (14 bits). In the exemplary embodiment of d e multiport switch of the present invention, the VLAN index is not necessary because there is no VLAN tagging on 10 Mb/s ports. The internal structure of an exemplary embodiment of a second type of output queue 74, the 100 Mb/s port output queue, is depicted in Figure 6. The 100 Mb/s port output queues hold entries for frames to be forwarded to d e 100 Mb/s ports. The output queue write side 76 holds sixty-four entries in this type of output queue, and the output queue read side holds sixteen entries. Each entry comprises a VLAN index, a partial control opcode (bits 4-0), a unicopy bit and a frame pointer. An exemplary map of the external memory 36 is depicted in Figure 7. The overall capacity of the external memory 36 may be, for example, 4 Mb, although other capacity memories are employed in different embodiments. The use of an external memory 36 for the overflow areas according to the present invention permits increasing or decreasing d e size of the output queues by simply changing the external memory. This is an advantage over systems in which die queue structure is entirely on the chip, as the overall queuing capacity is set at manufacture of the chip. To satisfy the storage requirements of the switch 12, an exemplary embodiment of the external memory 36 allocates space for the following areas: free buffer pool overflow 120, reclaim queue overflow 122, multicopy queue overflow 124, management port output queue overflow 126, individual output queue overflows 128 for each of the 10 Mb/s and 100 Mb/s destination ports, expansion bus port output queue overflow 130, the MIB counters 132, and die global frame buffer pool 134. The BASE Address for the entire memory region is programmable in a memory base address register among the registers 60 on the chip. The BASE Address for each area in the external memory map is programmable in the register set. No length register is required, d e length for a given area being equal to the area from that area's BASE Address to die BASE Address of the next area in the mapping. Since the length (and therefore capacity) of each of the individual overflow areas is programmable, the overall capacity of each queue is programmable. This feature of the present invention permits customization of the switch to provide particular output queues wid increased capacity, as needed. The following overflow areas store entries that do not fit into die control queues on die chip 12 are therefore placed into the external memory 36. The free buffer pool overflow area 120 stores the address pointers to currently unused buffers in the global frame buffer pool 134. The reclaim queue overflow area 122 stores frame pointers to linked-list chains that are no longer needed. The multicopy queue overflow area 124 stores frame pointers with copy numbers ">=1" (for queued frame pointers) and frame pointers with copy numbers "-1" (for successfully transmitted frames) The following overflow areas store entries for output queues which do not fit on-chip. The management port output queue overflow area 126 stores frame pointers awaiting transmission to die management port. Output queue overflow areas 128 store frame pointers awaiting transmission to the appropriate 10 Mb/s port or 100 Mb/s port. The expansion bus port output queue overflow area 130 stores frame pointers awaiting transmission to the expansion bus port. The MIB counter region 132 contains all the per port statistics which are updated periodically by die switch 12. The switch 12 maintains 8-bit and 16-bit counters on-chip for storing MIB statistics. The switch 12 updates d e 32-bit or 64-bit MIB counters in external memory 36 with the frequency required to prevent loss of MIB data. The global frame buffer pool 134 contains buffers in linked-lists which store received frame data. At any given time, diese linked lists contain valid frame data, obsolete buffers which will be returned by d e buffer manager 72 to the free buffer pool 104 or are owned by the PCI host processor 28. Referring now to Figure 8, frame data received from any MAC port or the PCI bus is stored in external memory 36 in a linked-list data structure format in an exemplary embodiment of die present invention. The buffers 140 used to create the linked-list are 256 bytes in length, although other sized buffer lengths are employed in different embodiments of d e invention. Address pointers to each of diese buffers 140 are stored by the free buffer pool 104 in the switch 12. As a frame is received at one of the ports of die switch 12, the buffer manager 72 requests address pointers from the free buffer pool 104 for linking buffers 140 to store the frame. The address pointer to the first buffer in external memory 36 that stores d e frame becomes the frame pointer for that frame. The frame pointer is used in the switch subsystem 56 for queuing frames to be transmitted. The buffers 140 are chained together by address pointers in each buffer header 142 diat indicate die location of the next buffer in memory. The buffer headers 142 also contain other information about the frame data contained in the buffer 140. The first buffer's header is 12 bytes, as depicted in the exemplary buffer header format of Figure 9a. Each subsequent buffer's header is 4 bytes, as depicted in Figure 9b. The external memory bursts are 2 banks x 16-bytes long, so actual frame storage capacity in each buffer is 256B-16B = 240B. As depicted in Figures 9a and 9b, die first and subsequent buffer header formats contain die following fields: Buffer Format Bit: indicates what buffer format is in use. A one indicates die first buffer's format, which is 12 bytes in length. A zero indicates a subsequent buffer's format, which is 4 bytes. It is used for each of the remaining buffers when chaining buffers. E Bit (End of Frame Marker): indicates this is the last buffer for a frame. When the E bit is set, there are no more buffers in the chain. C Bit (CRC Error Detected): indicates a CRC error was detected by die receiver. When d e C Bit is detected, die transmit function will purposely transmit an inverted CRC. L Bit (Alignment Error): indicates a Frame Alignment Error was detected (along widi a CRC Error) in the receive frame. O Bit (Receive FIFO Overflow): indicates die receive FIFO overflowed and the data in the buffer may not be valid. Buffer Length: the total number of bytes which are valid in the data field of die buffer beginning with the first byte after the buffer header. This length should not include die Offset Byte value. Next Buffer Pointer: the pointer to the next buffer. The next buffer pointer is not valid when die E Bit is set. Offset Byte Count: indicates where the first byte of die frame starts in the frame data section of die buffer. An offset of zero means the data will begin at the first byte after the buffer header 142. An offset of zero indicates frame data will begin at die byte following the 16 th byte in the buffer. For nonzero values of offset, frame data will begin following 16B + Offset from the beginning of the buffer. The transmit function will skip over the number of bytes indicated in d e offset Byte field. P Bit (Port Type): indicates the port type of the incoming receive frame. A zero indicates a 10 Mb/s port and a one indicates a 100 Mb/s port. This bit is used by the host 28 in conjunction with the time stamp field when it programs the switch 12 to forward frames to the expansion bus before the frame is completely received and buffered to external memory 36. T Bit: indicates the received frame type: tagged or untagged. A one indicates a tagged frame and die VLAN Identifier field contains the received VLAN ID. A zero indicates an untagged frame and die VLAN ID is not valid. Receive Port Number: the number of d e port from which die frame was received. VLAN Identifier: the VLAN ID received from a "tagged" port. If the frame was received from an untagged port, this field is invalid. R Bit (Recalculate CRC): indicates die CRC needs to be stripped and recalculated at die transmit function. The switch 12 sets this bit when a tagged frame is received. In addition, if die host 28 modifies a frame's contents, the host 28 should set diis bit. When the switch 12 transmits a frame, it will examine this bit to determine whether to transmit the existing CRC or strip and recalculate die CRC. A Bit (Append CRC): indicates diat there is no CRC at d e end of die frame data. The host can create a frame in memory (without a CRC) then set diis bit. The switch 12 will generate and append a CRC when transmitting the frame. If the A Bit is set, d e frame length should not include CRC. F Bit (Format Bit): identifies die Frame Length/Time Stamp field. A zero indicates die field is die time stamp of the incoming frame. A one indicates the field is the frame length of the received frame. Frame length time stamp: dependent on F Bit. IF F Bit is cleared, this field represents the time stamp from the beginning of the received frame. The time stamp has a resolution of lμs. If the F Bit is set, indicates die total length of the received frame including CRC and any received VLAN Tag. When a frame is received, die switch 12 marks d is field widi d e time stamp (from the timer register). If the host 28 has programmed the switch 12 to forward expansion bus frames before the frame has been completely received, it can use d e time stamp (along with the speed of the receive port) to gauge how much data it can fetch from external memory 36 without over-reading die frame data. Once the entire frame has been received, the switch 12 writes the frame length into this field and sets the F Bit. Copy number: used to indicate d e number of copies successfully queued for transmission by the port vector FIFO 70. This field is used to store the copy number for a frame pointer if the buffer manager 72 needs to make space in the multicopy cache 96 for new entries. Figure 10 is a detailed depiction of some of the elements of the switch subsystem 56 of Figure 3. These elements are used to provide the buffers for storage of frames, and to reclaim these buffers and make them available for use again once die buffers are no longer needed for storage of the frame. As described earlier, each output queue 74, 75 (except output queue 77) passes frame pointers to the buffer manager 72, which schedules transmission of the frames pointed to by die frame pointers. The buffer manager 72 controls the following functions: 1) managing the internal busses of the switch 12; 2) facilitating queuing/dequeuing frame pointers to/from the output queues 74; 3) managing the control queues 90, 98 in order to locate and return buffers to die free buffer pool 104; 4) controlling the flow of data to and from the external memory 36; and 5) maintaining the memory structures, including MIBs and overflow areas. The buffer manager 72 contains a scheduler function for allocating all accesses to external memory 36. These accesses include 1) writing received frame data to memory buffers 140, 2) reading frame data from memory buffers 140 for transmission and 3) maintaining (i.e., writing and reading) die frame pointers in each of the overflow areas for the output queues 74 and control queues 90, 98 and 4) updating MIB counters. After the buffer manager 72 has copied a given frame pointer to all the appropriate output queue(s) 74, 75, the port vector FIFO 70 calculates the number of copies (the "copy number") and places die frame pointer and d e copy number into the write side 92 of d e multicopy queue 90. The copy number may be "0", indicating d at d e frame should not be forwarded, a "1", indicating a unicopy transmission, or a number ">1", indicating a multicopy transmission. These three cases are described below. When die copy number is "0", which means that the frame pointer has a null forwarding port vector with no bits set, the port vector FIFO 70 passes the frame pointer directly to die write side 100 of the reclaim queue 98. When die buffer manager 72 services the reclaim queue 98, as will be described, die buffer manager 72 breaks down the linked-list chain of buffers and returns die address pointer for each "free" buffer to the write side 106 of die free buffer pool 104. When the copy number is "1", a unicopy transmission, the port vector FIFO 70 copies die frame pointer, control signals/control opcode and d e VLAN index to die output queue 74 of d e appropriate port. The port vector FIFO 70 sets the unicopy bit in the output queue 74 (see Figures 5 and 6) to indicate diat tiiis is a single transmission. When the buffer manager 72 reads die frame pointer and die unicopy bit from the output queue 74 of the port, it schedules the transmission as discussed previously. The buffer manager 72 uses die frame pointer to locate the first buffer in external memory 36 in which the frame is stored. The buffer manager 72 reads die buffer header from diis first buffer, captures data from the first buffer and places d is data in d e appropriate MAC transmit FIFO 54. The links to subsequent buffers, assuming that the frame spans multiple buffers, provides the buffer manager 72 with die address to find and transmit all of the buffers in the chain for that frame. Once the data has been placed in the FIFO 54 for transmission, the buffer becomes obsolete and is returned to die free buffer pool 104 for eventual re-assignment to store data of anodier frame. When the copy number is greater dian 1, the port vector FIFO 70 copies the frame pointer, VLAN index and control signals/control opcode to each of die appropriate output queues 74. (When referring to queues 74, reference is also made to queues 75, 77). The port vector FIFO 70 clears the unicopy bit for the appropriate frame pointers in the output queues 74 and places the frame pointer widi a copy number of ">1" into the write side 92 of the multicopy queue 90. Whenever die buffer manager 72 reads a frame pointer and a cleared unicopy bit from one of die output queues 74, die buffer manager 72 schedules die transmission of the frame, but also checks the multicopy cache 96 for an entry with a frame pointer having a copy number of "1". If a frame pointer with a copy number of "1" is found in the multicopy cache 96, dien the buffer manager 72 schedules the frame for transmission and reclaims the buffers during transmission in the same manner as in the unicopy transmission of a frame. However, if the frame pointer is not in the multicopy cache 96 or the copy number of the frame pointer in the multicopy cache 96 is greater than 1, then die buffer manager 72 transmits the frame but does not reclaim die buffers. After successful transmission, the buffer manager 72 places a copy of the frame pointer, along with a copy number of "-1" into the write side 92 of die multicopy queue 90. Each time a multicopy frame is transmitted, die buffer manager 72 places a copy of the frame pointer into the multicopy queue 90, provided die buffer manager 72 did not find die frame pointer in the multicopy cache 96 with a copy number of "1". Hence, at any given time, the multicopy queue 90 may contain the frame pointer with a copy number that is > "1" and/or several copies of the same frame pointer, each with a copy number of -1. The buffer manager 72 constantly services the multicopy queue 90 and die multicopy cache 96 in order to reclaim obsolete buffers. When it services the multicopy queue 90 and reads a frame pointer with a copy number ">1", die buffer manager 72 attempts to place this new entry (frame pointer and copy number) into die multicopy cache 96. If d e multicopy cache 96 is full, the buffer manager 72 makes space for the new frame pointer. The buffer manager 72 reads an "older" multicopy cache entry, updates the copy number for this entry in its buffer header in external memory 36, d en clears the entry from the multicopy cache 96. As room becomes available in the multicopy cache 96, the buffer manager 72 is able to place the new entry from the multicopy queue 90 into die multicopy cache 96. When die buffer manager 72 services the multicopy queue 90 and reads a frame pointer with a copy number of "-1", it searches the multicopy cache 96, looking for a matching frame pointer address widi a copy number ">=1" to decrement or delete. If the buffer manager 72 finds a frame pointer match, the buffer manager 72 will: 1) decrement the multicopy cache's frame pointer if the copy number is ">1" or 2) delete die multicopy cache's frame pointer/copy number entry and place the frame pointer into the reclaim queue 98 if d e copy number is "1". If the buffer manager 72 does not find a matching frame pointer, the buffer manager 72 searches the frame pointer's buffer header in external memory 36 (see Figure 9) for the copy number. If die copy number in memory is "1", the buffer manager 72 places the frame pointer into the reclaim queue 98. If die copy number in memory is ">1", the buffer manager 72 places the frame pointer with diis copy number into the multicopy cache 96, then decrements die copy number. The buffer manager 72 constantly services the reclaim queue 98 by reading frame pointers, then "walking" the linked-list chain to return buffers to the free buffer pool 104. This activity only returns buffers for frames that had null port vectors and were queued to the reclaim queue by the port vector FIFO 70, or frames with a multicopy forwarding vector and have completed transmissions of all of die copies. Buffers linked for unicopy frames are returned directly to die free buffer pool 104 when the frame is transmitted, as described above. If die port vector FIFO 70 is not able to place a frame pointer for a unicopy forwarding vector into an output queue 74, because diat output queue 74 and its overflow area 110 in external memory 36 are full, the frame is discarded. The frame pointer is returned to the reclaim queue 98 and die discarding of die frame is noted by the management resources of the switch. If the port vector FIFO 70 is not able to place one or more frame pointers for a multicopy forwarding vector, because one or more of die output queues 74 and dieir overflow areas 110 in external memory 36 are full, the frame is only forwarded to die output queues with available space and die copy number placed into the multicopy queue 90 will only reflect the successfully placed frame pointers. The non-placement of the frame pointer is noted by die switch management resources for each of the ports for which the frame pointer could not be queued. If die port vector FIFO 70 is not able to place any of the frame pointers for a multicopy forwarding vector, because all of the output queues 74 and tiieir overflow areas 110 in external memory 36 are full, the frame pointer is passed to die reclaim queue 98, and die switch management resources are duly notified. The multicopy queue 90 is a high priority queue used by die buffer manager 72 to keep track of how many transmissions must be completed of a particular multicopy frame before all buffers (i.e., address pointers) used to store die frame can be returned to die free buffer pool 104. The write side 92 and read side 94 of diis output queue hold 64 and 16 entries, respectively. The multicopy queue 90 feeds the multicopy cache 96, which is used by d e buffer manager 72 to determine when to reclaim buffers. The multicopy queue internal structure is depicted in Figure 12. The port vector FIFO 70 places a copy of a frame's frame pointer and copy number which is ">1", based on die number of frame pointers it successfully placed in the output queues 74, into the multicopy queue 90. If a particular port's output queue 74 is full, d e port vector FIFO 70 cannot place a copy of the frame pointer into the output queue 74; hence it cannot include this as a successful event in determining the copy number. Each time the buffer manager 72 reads an output queue frame pointer and finds die unicopy bit is "0", (i.e., a multicopy), it checks die multicopy cache for the frame pointer with a copy number of "1", which indicates diat d is is the last transmission. If this match is found, the buffer manager 72 transmits the frame and reclaims the buffers in the same manner as in d e unicopy transmission, by providing die obsolete buffers to the free buffer pool 104 after the transmission of the contents of each buffer. If the match is not found, die buffer manager 72 transmits the multicopy frame and places a copy of the frame pointer with a copy number of "-1" into the multicopy queue 90. When a host has finished using a multicopy frame pointer for a frame which was queued to die expansion bus output queue 75 or die management port output queue 77 (through the PCI interface 26), the host writes a copy of die frame pointer with a copy number of "-1" into the multicopy queue through a frame pointer register. This register is one of the registers depicted in die block of registers 60 in Figure 2. Similar to the output queues 74, the multicopy queue 90 is structured with an input path and an output path. The input path, or write side, allows the port vector FIFO 70 and buffer manager to place frame pointers/copy numbers into the multicopy queue 90. The output path, or read side, allows die multicopy queue 90 to place frame pointers/copy numbers into the multicopy cache 96. Additional storage for frame pointers/copy numbers, teπned the multicopy queue overflow 124, is provided in external memory 36. When frame pointers/copy numbers are written into an empty multicopy queue 90, they pass from the write side 92 to die read side 94 until die read side 94 is full. Additional frame pointers copy numbers written to the write side 92 of the multicopy queue 90 are placed into the multicopy queue overflow area 124 in external memory 36. Once the read side 94 of die multicopy queue 90 and its overflow area 124 are full, additional frame pointers/copy numbers placed into die multicopy queue begin to fill the write side 92. The ordering of die frame pointers passing through the multicopy queue 90 is maintained, such diat when space clears in the multicopy queue read side 94, frame pointers/copy numbers are moved from the multicopy queue overflow area 124 to the multicopy queue read side 94 and from die multicopy queue write side 92 to the multicopy queue overflow area 124. The multicopy cache 96 is similar to die multicopy queue 90 but provides a searchable region for scanning frame pointers/copy numbers. The multicopy cache 96 holds up to 256 entries. The buffer manager 72 reads a frame pointer from the multicopy queue 90 and either places it into the multicopy cache 96 or processes it, depending on whedier the copy number is ">1" or "-1". In addition, each time the buffer manager 72 reads a frame pointer from the read side 78 of an output queue 74, the buffer manager 72 schedules d e transmission. If die unicopy bit is "0" (meaning a multicopy frame), the buffer manager 72 scans the multicopy cache 96 for the frame pointer with a copy number of "1", which indicates this is the last transmission of this frame. If there is a match, the buffer manager 72 removes the entry and returns buffers to the free buffer pool during frame transmission. If there is not a match, the buffer manager 72 places the frame pointer with a copy number of "-1" into the multicopy queue 90 upon completion of die transmission. Periodically, d e buffer manager 72 services the multicopy queue 90 by reading a frame pointer/copy number and placing it into the multicopy cache 96 or processing it. This is done independently of frame transmission. Two cases follow depending on whedier die buffer manager reads a frame pointer with a copy number ">1" or "-1": 1) The buffer manager 72 reads a frame pointer widi a copy number ">1" from the multicopy queue 90. Provided there is room in the multicopy cache 96, it writes a new entry. If die multicopy cache 96 is full, the buffer manager 72 must clear space in the cache 96. This is done by reading one of die older frame pointer/copy numbers from the multicopy cache 96, updating diat frame pointer's buffer header in external memory 36 with the copy number in the multicopy cache 96, then deleting diis cache entry. Once there is space, the new frame pointer/copy number is written into the multicopy cache 96. 2) The buffer manager 72 reads a frame pointer with a copy number "-1" from the multicopy queue 90. The buffer manager 72 searches the multicopy cache 96 for a matching frame pointer with a copy number ">=1". Two cases follow, depending on whedier or not d e buffer manager 72 finds a frame pointer match in the multicopy cache 96: a) The buffer manager 72 finds a frame pointer match. If the multicopy cache 96 entry's copy number is "1", the buffer manager 72 deletes die multicopy cache entry and places the frame pointer in the reclaim queue 98. If die cache entry's copy number is ">1", the buffer manager 72 decrements die copy number by "1". b) The buffer manager 72 does not find a frame pointer match in die multicopy cache 96. This means that the matching frame pointer has previously been moved to die buffer header of die frame's linked-list chain in external memory 36. The buffer manager 72 must go to the buffer header and read die copy number. If this value (in memory) is "1", the frame is no longer needed and the buffer manager 72 places the frame pointer in the reclaim queue 98. If this value (in memory) is ">1", the buffer manager 72 places a copy of die frame pointer/copy number (that was in external memory 36) into the multicopy cache 96 and decrements die copy number by "1". If the multicopy cache 96 is full, d e buffer manager clears space by moving one of the older frame pointer/copy numbers into external memory 36. The reclaim queue 98 holds the frame pointers to the linked-list chains which are no longer needed. The buffer manager 72 writes a frame pointer to the reclaim queue when it services the multicopy cache and discovers that a frame pointer's copy number is "1" (i.e., die last transmission of the frame has completed successfully). Additionally, die port vector FIFO 70 writes frame pointers to the reclaim queue 98 under die following conditions: 1) a frame pointer's port vector is null or 2) the frame pointer could not be queued because all of d e forwarding vector's output queues were full. Finally, the host writes a frame pointer to the reclaim queue 98 (using a frame pointer register) when it has finished using a unicopy frame which was queued to die expansion bus output queue 77 or die management port output queue 75. When die buffer manager 72 processes reclaim queue entries, it walks a frame pointer's linked- list chain to return each buffer to the free buffer pool 104. The internal structure of the reclaim queue structure is not depicted, but contains only the frame pointers (14 bits) in the exemplary embodiment of die invention. The reclaim queue write side 100 holds 64 entries and the reclaim queue write side 102 side holds 16 entries. Similar to the output queues 74, die reclaim queue 98 is structured with an input path and an output path. The input padi, or write side 100, allows die buffer manager 72 to place frame pointers in the reclaim queue 98. The output path, or read side 102, allows the buffer manager 72 to read a frame pointer and return all associated buffers to the free buffer pool 104. Additional storage for frame pointers is provided in die reclaim queue overflow area 122 provided in external memory 36. When frame pointers are written into an empty reclaim queue 98, they pass from the write side 100 to die read side 102 until die read side 102 is full. Additional frame pointers written to the write side 100 of die reclaim queue 98 are placed into die reclaim queue overflow area 122 in external memory 36. Once the read side 102 and overflow area 122 of the reclaim queue 98 are full, additional frame pointers placed into the reclaim queue 98 begin to fill the write side 100. Figure 11 depicts an exemplary embodiment of die internal structure of the free buffer pool 104. The free buffer pool 104 is a FIFO that contains address pointers to all free buffers 140 in external memory 36. When frames are received, the buffer manager 72 captures available address pointers from the free buffer pool 104 to store incoming data. The buffer manager 72 also allocates address pointers from the free buffer pool 104 to the host processor 28 (when requested). The host can request or return address pointers to the free buffer pool 104 by reading or writing a free buffer pool register among the registers 60 in direct input/output space. The write side 106 and die read side 108 of the free buffer pool 104 each holds 64 entries in an exemplary embodiment of die invention. The free buffer pool 104 is structured widi an input path and an output path (similar to the output queues 74). The input padi, or write side 106, allows die buffer manager 72 or host 28 to place address pointers into the free buffer pool 104. The output path, or read side 108 of die free buffer pool 104 allows the buffer manager 72 to provide address pointers to the host 28 or pull address pointers from the pool 104 for storing receive frame data. Additional storage for available address pointers, the free buffer pool overflow area 120, is provided in external memory 36, as described earlier. Upon start-up of the switch 12, d e free buffer pool generates address pointers from the read side 108. As frames come in, the free list in the free buffer pool 104 is read. If diere are not enough buffer pointers in the write side 106 to handle die traffic demands, d e overflow area 120 is accessed to obtain more buffer pointers. Certain embodiments of die present invention provide an advantageous arrangement and method of providing die buffer pointers upon start-up of the switch 12. When die switch 12 first powers up, it is not required for the overflow area 120 in external memory 36 to contain buffer pointers. Instead, die buffer pointers are created on die fly. The switch 12 on power up could generate and place into the overflow area 120 die buffer pointers, but there may be 16,000 or 32,000 such pointers, and this would slow up die powering on procedure of the switch 12. The present invention takes advantage of die fact diat on power up, all of die buffers are free, and d e identities of diese buffers are known. Therefore, the buffer pointers are generated as diey are needed after power up, using a counter 180, as depicted in Figure 10. The free list count generator 180 is connected to the input of a multiplexer 182. Since the free list in d e free buffer pool 104 is empty on startup, the free list counter 180 generates the buffer pointers. Once the free list reaches the highest count, it will not generate any more buffer pointers. When a frame packet is received in the switch 12, die frame packet is broken up into fixed length buffers. Typically, frames vary in size. The buffers are 256 bytes in size and the data portion of a buffer is 240 bytes. Following transmission of the contents of a buffer, the buffer pointers are put into the reclaim queue 98 or, if the buffer chain can be walked, directly into the free list of the free buffer pool 104. During operation of the switch 12, any address pointers returned to the free buffer pool 104 pass from the write side 106 to the read side 108. If the read side 108 becomes full, additional address pointers are passed to die overflow area 120. Once the read side 108 and the overflow area 120 are full, additional address pointers placed into die free buffer pool 104 will begin to fill the write side 106 of die pool 104 again. Figure 13 depicts a schematic representation of the internal arrangement of the multicopy cache 96 in accordance widi an embodiment of the present invention. As briefly discussed earlier, die time order of die entries to the multicopy cache 96 is maintained. In the present invention, this maintaining of a time order is not done by time stamping, as in the prior art, but by physical ordering in a memory. The multicopy cache 96 of the present invention also avoids die use of validity bits, and instead encodes validity, as will be discussed. Referring to Figure 13, the multicopy cache 96 is configured as a four-way set-associative memory. An entry into the multicopy cache 96 includes a frame pointer and its copy number, as explained earlier. The lowest six bits of the frame pointer determine die row in die set-associative cache 96 in which the entry will be stored. In the illustrated embodiment of die invention, tiiere are sixty-four rows in die cache 96, altiiough otiier numbers of rows are not limited if die cache size is made larger. The set-associative cache 96 is divided into four columns, each of which can be searched in parallel. When the buffer manager 72 stores an entry into the cache 96, the entry always enters the first column, the uppermost (51:39) bits of the row indicated by the six least significant bits of the frame pointer. The row is read, all of d e entries are shifted to the right by 13 bits, and the row is written back. The entry that is actually written into die cache 96 includes die upper eight bits of the frame pointer that form an address tag, and d e five-bit copy number associated widi die frame pointer. When the entry is read out of die cache 96, die frame pointer is re-formed widi die address tag and die bits that index d e row number of the cache 96. The oldest entry in the cache 96 is removed from the cache 96 if the row is full and a new entry to d e row is written. As described earlier with respect to the buffer headers 142, the copy number associated widi the frame pointer that is removed is written into the buffer header 142 of die frame in external memory pointed to by die removed frame pointer. Hence, die frames (i.e., the buffers 140) stored in external memory 36 serve as an overflow area for the multicopy cache 96 to store copy numbers. One of die advantageous features of die present invention is that tiiere is no separate valid bit in the set-associative cache 96. When the copy number is 00000, then the buffer manager 72 knows that die entry is no longer valid and removes the entry from die cache 96. This simplifies the organization of the cache. Another advantage of the cache 96 of the present invention is d at it allows a very fast search to be performed, since the buffer manager 72 needs only to examine a single row, already determined by die frame pointer that has exited the multicopy queue 90. The four entries in the row are examined in parallel, further increasing the speed of die search. Although described as a four-way set-associative memory, this is exemplary only as the memory can be n-way set-associative without departing from die scope of the invention. From the above description, it should be understood d at die present invention maintains a time order (age) of die cache entries by physical positioning of the entries in the cache on a row basis. In otiier words, die physical position of the entry in the cache is an indication of the relative age of an entry. The aging of an entry is performed by the physical re-ordering of die entries in die memory. Certain embodiments of die present invention provide customization of the latency of frames switched by die switch 12, on a port by port basis. Referring to Figure 14, the port vector FIFO 70 examines the programmed switch mode of the receive port to determine when to place die frame pointer and associated information into the appropriate output queue 74 of the transmit port. For a first mode (low latency mode), die port vector FIFO 70 has no restrictions on when to place the frame pointer into the output queue(s) 74. For a second mode (intermediate latency mode), the port vector FIFO 70 places die frame pointer into the output queue(s) 74 only after 64 bytes of the frame have been received. For a tiiird mode (high latency mode), the port vector FIFO 70 places the frame pointer into the output queue(s) 70 only after the frame has been completely received. There are some special cases which alter the timing of when the port vector FIFO 70 passes frame pointers to the output queues 74: 1) frames forwarding from a first or second mode 10 Mb/s port to a 100 Mb/s port; 2) frames forwarding to die management port 30 and 3) frames forwarding to die expansion bus port. In case 1), the 10 Mb/s port to 100 Mb/s port rate mismatch forces the forwarding mode to be the third, high latency mode. In case 2), all frames passed to die management port are third mode frames. In case 3), any frame forwarding to die expansion bus port uses the switch mode of the expansion bus port 26. When a multicopy port vector contains one of the special case ports, the queuing of the frame pointers for the entire port vector becomes d at of d e longest latency switch mode represented in the port vector. For example, if a frame is received by a first or a second mode port, but its multicopy forwarding port vector contains the management port 30, the switch mode is die tiiird mode. In this situation, a copy of the frame pointer is placed into all the output queues 74 only after the frame has been completely received. The switch modes will now be described in more detail. The switch mode that applies to die input (i.e., receive) port determines forwarding latency (how soon die switch 12 will forward a frame once it begins receiving the frame) and die ability to reduce fragment error propagation to output ports. The second, intermediate latency mode is the default for each port; however, the switch mode is programmable on a per-port basis in the registers 60. In all of the three modes, frame data received at the receive FIFO 52 of an internal MAC port is forwarded to a buffer 140 in the external memory 52 as soon as possible. At approximately the same time, the rules checker 42 or 58, receives the destination address and source address, die receive port number, the frame pointer and some additional information, then performs the appropriate lookup. Once the lookup is completed, the rules checker 42 or 58 returns the frame pointer and d e forwarding port vector to d e port vector FIFO 70. The port vector FIFO 70 places the frame pointer in the write side 76 of die output queues 74 for the output port(s) identified in die port vector. The receive port's switch mode defines die latency between when the port vector FIFO 70 receives die port vector (and die frame pointer) and places the frame pointer into the output queue(s) 74. This is described for the three modes below. Once the frame pointer passes to the read side 78 of die output queues 74, d e buffer manager 72 reads die frame pointer and schedules transmission. The buffer manager 72 begins moving frame data from the address specified by the frame pointer. Once the transmit FIFO 54 of the MAC port has been primed to its start point (and assuming the medium is available for transmission of data), frame transmission commences. The first mode is designed to provide die lowest latency. Frames are received and forwarded at line-rate speed. In this first mode, tiiere is no network error protection because a frame is queued for transmission before it can be determined whether the frame is a fragment (i.e., <64 bytes in length) or contains a CRC error. In the first mode, frame reception may not complete before frame transmission at the output port(s) commences. If a receive frame terminates in a runt or with an invalid CRC, the receive MAC marks the buffer header 142 in external memory 36 to indicate these conditions. The transmit MAC guarantees that if transmission commences on a frame which later terminates as a runt or with an invalid CRC, die MAC will generate a bad CRC. If die transmit MAC has not started a frame's transmission and the buffer header 142 indicates die frame terminated in a runt or an invalid CRC, die buffer manager 72 will not forward the frame to the output port. The second mode provides low latency for forwarding frames and some network error protection. Frames are received and forwarded after sixty-four or more bytes have been received. This allows the switch 12 to filter (i.e., not forward) fragments of frame; however, it does not completely filter CRC error frames that are greater than sixty-four bytes. In the second mode, frame pointers for frames which achieve the sixty-four byte threshold at die receive MAC are queued to die appropriate output queue(s) 74. Frames which fail to achieve the minimum sixty-four byte threshold are deleted and their frame pointers are not placed in output queue(s) 74. If a receive frame greater than or equal to sixty-four bytes terminates with an invalid CRC, die receive MAC marks the buffer header 142 in external memory 36 to indicate this condition. If transmission has commenced on a frame greater than or equal to sixty-four bytes which later terminates with an invalid CRC, d e transmit MAC will complete the transmission with a bad CRC. If die transmit MAC has not started a frame transmission and d e buffer header 142 indicates the frame (greater than or equal to sixty-four bytes) terminated in an invalid CRC, die buffer manager 72 returns die frame pointer to the reclaim queue 98 (for a unicopy forward) or the multicopy queue 96 (for a multicopy forward) without forwarding to the output port(s) 74. The third mode is a store-and-forward mode diat provides the highest level of network error protection among the three modes, but has a higher forwarding latency. Frames are completely received before the switch 12 forwarding diem to output ports. In this mode, die switch 12 screens out all fragments and CRC error frames before forwarding. In the third mode, once a valid frame completes successfully at die receiver (i.e., greater tiian or equal to sixty-four bytes with a valid CRC), die frame pointer is queued to die appropriate output queue(s) 74. Frames which terminate in a receive error (invalid CRC, runt (>64 bytes) etc.) are deleted and tiieir frame pointers are not placed in output queue(s) 74. The port vector FIFO 70 makes the decision to put die port vector into an output queue 74, in dependence on d e selected mode of the receive port and d e amount of data that has been received. In the embodiment described above, tiiere are three thresholds, altiiough there are different numbers of thresholds in other embodiments. In die exemplary embodiment, diese thresholds are: 1) receiving n bytes (e.g. 6 bytes) where n<64 bytes; 2) receiving 64 bytes; and 3) receiving all of the frame. The present invention forwards frames to the output queues 74 based on thresholds. The port vector FIFO 70 re-orders the sequence of transmission based on amount of type of data received and die mode in which the port is programmed. Although the exemplary embodiment makes forwarding decisions based on d e amount of received data, otiier embodiments of d e invention make forwarding decisions based on otiier factors, such as the types of data received. In implementing the forwarding scheme of die present invention, the buffer manager 72 maintains a table 160 in a cache memory (CAM) 161 that associates a frame pointer with a receive port. Every time the port vector FIFO 70 receives a new port vector and frame pointer from the rules checker 42 or 58, it makes an association to determine whether die receive port has finished receiving a frame, and if not, how much of die frame has already been received. The port vector FIFO 70 does not receive any information regarding die identity of d e receive port from the rules checker 42 or 58. The only information the port vector receives that provides any identification of die port are the frame pointers. The port vector FIFO 70 queries the address table 160 with the frame pointer. Either the address table returns the receive port if the frame is still being received, or die address table 160 cannot find die frame pointer which means that the frame has already been received. Once the frame is completely received, die frame pointer is moved out of die address table 160. This means that the third tiireshold (the frame complete) is met. Accordingly, the frame pointer may be dropped into the output queue 74 immediately. If die address table 160 returns the receive port, the port vector FIFO 70 puts the frame pointer and associated information into a holding area 162 and begins monitoring two signals from that receive port. These two signals flag one of three events. The first event is flagged when die port receives n bytes. At that point, if that port is in the first mode, the port vector FIFO 70 starts processing the frame pointer by sending it to die appropriate output queue 74. If die receive port is not in the first mode, the port vector FIFO 70 waits until it receives the a signal indicating occurrence of the second event. If this port is in the second mode, then the port vector FIFO 70 releases the frame pointer from the holding area 162 to enter the proper output queue 74. Finally, if the receive port is in the third mode, tiien the port vector FIFO 70 awaits receipt of the flag indicating that the frame is complete. Every receive port (reference numeral 164 in Figure 14) maintains this flag, and provides this information to the port vector FIFO 70. It is up to the port vector FIFO 70 to determine the port associated widi the frame pointer. The port vector FIFO 70 maintains the information identifying die mode each port is in. In summary, upon receiving a frame pointer, the port vector FIFO 70 first queries the address table 160 of the buffer manager 72 to determine the receive port, determines die mode for that receive port, and dien monitors the flags from that receive port and releases the frame pointer according to die mode and die flags. Although the present invention has been described and illustrated in detail, it is to be clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of die present invention being limited only by die terms of the appended claims.

Claims: CLAIMS WHAT IS CLAIMED IS: 1. An arrangement for maintaining a count of the copies of a frame that are transmitted from a network switch, comprising: a multicopy queue configured to receive and queue entries, each entry having a frame identifier and a copy number, each copy number bemg either a positive or negative value, a copy number with a positive value representing the total number of copies of that frame to be transmitted, a copy number wid a negative value representing a transmission of one of the copies of that frame that has occurred; a searchable first memory configured to receive and store the entries from the multicopy queue that have positive copy numbers; and a buffer manager configured to examine the entries exiting the multicopy queue, and when an entry from the multicopy queue contains a negative copy number, search the first memory for an entry having the same frame identifier as the entry with the negative copy number, and decrement the copy number of the entry in the first memory having that frame identifier if d e copy number of that entry in the first memory is greater than one, and delete tiiat entry in the first memory if the copy number is equal to one. 2. The arrangement of Claim 1, wherein the frame identifiers are frame pointers tiiat point to a location in a second memory where the frame is stored. 3. The arrangement of Claim 2, wherein the first memory is a cache memory. 4. The arrangement of Claim 2, further comprising the second memory which is configured to store the frames in buffers. 5. The arrangement of Claim 4, wherein the buffers in the second memory have buffer headers widi a copy number area in which a copy number for the frame stored in that buffer is storeable. 6. The arrangement of Claim 5, wherein the buffer manager is further configured to write an entry with a positive copy number exiting from die multicopy queue into the first memory, and if die first memory is full prior to writing the entry with the positive copy number, remove an oldest entry from the first memory and write the copy number of that entry into the copy number area of the buffer header of die buffer that stores the frame pointed to by the frame pointer in the entry removed by die buffer manager. 7. The arrangement of Claim 6, wherein the buffer manager is further configured to retrieve the copy number from the copy number area of the buffer that stores the frame pointed to by die frame pointer in the entry containing the negative copy number, when the search of the first memory by the buffer manager does not locate in the first memory an entry having the same identifier as the entry with the negative copy number. 8. A memory arrangement comprising: a first memory configured to store entries, each entry having a data pointer that points to data stored in a second memory and an associated data item; a second memory configured to store the data at locations in the second memory pointed to by the data pointer, wherein the locations in the second memory are configured to store die associated data item with the data, such tiiat the second memory is an extension of the first memory to store the associated data items. 9. The memory arrangement of Claim 8, wherein the first memory is a searchable cache, the data is a frame of data, die data pointer is a frame pointer, and die associated data item is a copy number indicating die number of copies of the frame are to be transmitted from a device. 10. The memory arrangement of Claim 9, further comprising a buffer manager configured to examine entries, and when an entry contains a negative copy number, search the cache for an entry having the same frame pointer as the entry widi d e negative copy number, and decrement the copy number of the entry in the cache having that frame pointer if the copy number of that entry in the cache is greater than one, and delete d at entry in the cache if the copy number is equal to one. 11. The memory arrangement of Claim 10, wherein the buffer manager is further configured to write an entry with a positive copy number into the cache, and if die cache is full prior to writing the entry with the positive copy number, remove an oldest entry from the cache and write the copy number of that entry into a copy number area at the location in the second memory that stores d e frame pointed to by die frame pointer in the entry removed from the cache by the buffer manager. 12. The memory arrangement of Claim 11, wherein the buffer manager is further configured to retrieve the copy number from the copy number area at the location that stores the frame pointed to by the frame pointer in die entry containing the negative copy number, when the search of the cache by the buffer manager does not locate in the cache an entry having the same identifier as the entry with the negative copy number. 13. An arrangement for maintaining a count of a number of copies of a data item to be transmitted from a device, comprising: a first memory in which entries are stored, each entry stored in the first memory corresponding to a different data item of which multiple copies are to be transmitted from a device, each entry having a pointer which points to a location in a second memory in which the data item is stored and a copy number which indicates how many copies of die data item remain to be transmitted; and a manager device configured to adjust die copy number in one of the entries stored in the first memory when a copy of the data item witi the same pointer as that entry in the first memory is transmitted from die device. 14. The arrangement of Claim 13, wherein the manager device is further configured to receive and examine entries in the device, wherein the entries in the device include die entries to be stored in the first memory, and entries diat include said pointer and a negative copy number which indicates tiiat one copy of the data item pointed to by the pointer has been transmitted from the device, and to adjust die copy number of one of the entries stored in die first memory when the manager device receives an entry widi a negative copy and a pointer that matches the pointer of that entry stored in the first memory. 15. The arrangement of Claim 14, wherein die device is a network switch, the manager device is a buffer manager of the multiport switch, the first memory is a cache memory, the data item is a frame of data, and die pointer is a frame pointer. 16. A multiport switch arrangement for a packet switched network, the switch configured to transmit multiple copies of a single frame, the switch arrangement comprising: a multicopy queue configured to receive and queue entries, each entry having a frame pointer and a copy number, each copy number being either a positive or negative value, a copy number with a positive value representing the total number of copies of that frame to be transmitted, a copy number widi a negative value representing a transmission of one of d e copies of that frame that has occurred; a searchable first memory configured to receive and store the entries from the multicopy queue that have positive copy numbers; and a buffer manager configured to examine the entries exiting the multicopy queue, and when an entry from the multicopy queue contains a negative copy number, search the first memory for an entry having the same frame pointer as the entry with the negative copy number, and decrement the copy number of the entry in the first memory having tiiat frame pointer if the copy number of that entry in the first memory is greater than one, and delete that entry in the first memory if the copy number is equal to one. 17. The switch arrangement of Claim 16, wherein the frame pointer points to a location in a second memory where the frame is stored. 18. The switch arrangement of Claim 17, further comprising the second memory which is configured to store the frames in buffers. 19. The switch arrangement of Claim 18, wherein the buffers in the second memory have buffer headers widi a copy number area in which a copy number for the frame stored in that buffer is storeable. 20. The switch arrangement of Claim 19, wherein the buffer manager is further configured to write an entry with a positive copy number exiting from the multicopy queue into the first memory, and if die first memory is full prior to writing the entry with the positive copy number, remove an oldest entry from the first memory and write the copy number of diat entry into the copy number area of the buffer header of d e buffer that stores the frame pointed to by the frame pointer in the entry removed by d e buffer manager. 21. The switch arrangement of Claim 20, wherein the buffer manager is further configured to retrieve d e copy number from die copy number area of die buffer that stores the frame pointed to by die frame pointer in the entry containing the negative copy number, when the search of the first memory by the buffer manager does not locate in die first memory an entry having die same identifier as die entry with the negative copy number. 22. A method of maintaining a count of the number of transmissions of a frame from a network switch, comprising the steps of: loading a searchable first memory with an entries, with each entry comprising a frame identifier and a copy number indicating die number of copies of that frame that remain to be transmitted from the network switch; searching the first memory for an entry, when a frame is transmitted, that has die same frame identifier as the transmitted frame; and decrementing die copy number of the entry in the first memory when the search of die first memory locates an entry with d e same frame identifier as the transmitted frame. 23. The method of Claim 22, further comprising the step of: loading a multicopy queue with entries, each entry having a frame pointer and a copy number, each copy number being either a positive or negative value, a copy number with a positive value representing the total number of copies of that frame to be transmitted, a copy number with a negative value representing a transmission of one of the copies of tiiat frame that has occurred; and wherein die step of loading die first memory with entries includes loading only those entries exiting from the multicopy queue that have a positive copy number. 24. The method of Claim 23, further comprising the step of determining whedier the first memory is full of entries prior to writing the entry with the positive copy number, and if die first memory is full, removing an oldest entry from the first memory and writing the copy number of that entry into a copy number area of a location in a second memory that stores the frame pointed to by die frame pointer in the removed entry. 25. The method of Claim 24, further comprising the steps of retrieving the copy number from the copy number area of the location that stores the frame pointed to by die frame pointer in the entry containing the negative copy number, when the search of the first memory does not locate in the first memory an entry having the same pointer as the entry with die negative copy number.

Inventors: ERIMLI BAHADIR, RUNALDUE THOMAS J, ERIMLI, BAHADIR, RUNALDUE, THOMAS, J.
Applicants: ADVANCED MICRO DEVICES INC, ADVANCED MICRO DEVICES, INC.
Application year: 1998
Main classification: H04L12
IPC codes: G06F5, H04L12, H04L13, H04Q3