METHOD AND APPARATUS FOR RECLAIMING BUFFERS

Abstract: A method and arrangement for reclaiming buffers used to store frames, following the transmission of a frame, checks to determine whether the transmission of a frame is the last copy of that frame to be transmitted. If it is the last copy, or the only copy, the buffers storing that frame are reclaimed for reuse after the contents of each buffer are transmitted. If the frame is not the last copy, or it cannot be determined whether it is the last copy, then the frame is transmitted and a count of the number of transmitted copies of that frame is decremented. Once all of the copies have been transmitted, the buffer reclaiming process is initiated.

Description: METHOD AND APPARATUS FOR RECLAIMING BUFFERS FIELD OF THE INVENTION The present invention relates to the field of data storage, and more particularly, to method and apparatus for reclaiming buffers used to store data to allow the buffers to be re-used to store different data. BACKGROUND ART In many systems in which is received and forwarded, such as packet switched networks (e.g., Ethernet networks), memory buffers are used to temporarily store data prior to forwarding of the data. For example, in a packet switched network, a frame of data may be received at a network switch, which will forward the frame to an appropriate port, and is loaded into individual buffers as the data is received at the switch. It is common for the size of the frame to be larger than the capacity of an individual buffer, so that multiple buffers to be used to store a single frame of data. Buffering the data as it is received allows robust error checking on the frame to be performed, and also permits rate matching between transmitting and receiving ports, among other features. When a frame of data arrives at a switch, only buffers that are currently available (or "free") are used to store the data, so that data is not overwritten before the data is transmitted. Once the data has been transmitted, or is otherwise no longer needed, the buffers storing d e data may be reused to store data of a new frame. Also, it is possible for buffers to be set aside for storing data of an incoming frame, but error checking or an insufficient amount of data received may cause the frame to be discarded. In this circumstance as well, the buffers should be returned for reuse. For purposes of efficiency, and to assure that buffers are available for storage of incoming frames to the extent possible, the return of the buffers for reuse should be performed as quickly as possible after they are no longer needed. In systems allowing for multiple buffers to store a single frame, after the entire frame is transmitted, the switch will recognize the transmission of the frame and begin the process of reclaiming the individual buffers that store the chain. If multiple copies of the same frame are transmitted, then the switch waits until all of the copies of the frame are transmitted before beginning the process of reclaiming the buffers that stored the frame. Hence, the same reclaiming procedure would be used whetiier a single copy of the frame is transmitted, or multiple copies of the frame are transmitted. Another concern in systems using buffers to store data in a memory is the generation of the pointers to store those buffers. The buffer pointers, which point to the locations in memory where the buffers are located, may be generated upon startup of the system and loaded into a buffer pool, from where they are retrieved when the buffers are used to store data. After reclaiming the buffers following their use, the buffer pointers are returned to the buffer pool. Where the system employs a large number of buffers, this initial generation and loading of buffer pointers into the buffer pool upon startup of the system is a relatively time-consuming process. SUMMARY OF THE INVENTION There is a need for an arrangement and a meϋ od of reclaiming buffers in a fast manner upon the transmission of data from buffers in a memory so that the buffers can be available for reuse to store other data. This and other needs are met by the present invention which provides an arrangement for reclaiming buffers that store frames for a network switch in a packet switched network. The arrangement comprises a reclaim queue configured to queue frame pointers that point to a location in a buffer memory of a first buffer in which a frame is stored. A searchable first memory stores a count of the number of copies of a frame to be transmitted from the network switch. A free buffer pool is configured to queue buffer pointers that point to locations of buffers in a buffer memory. A buffer manager receives a frame pointer of a frame for transmission of a copy of that frame and searches the first memory to determine if transmission of this copy would be die last copy of that frame to be transmitted. If this transmission would be the last copy, the buffer manager causes the transmission of that last copy of that frame from the network switch and places the buffer pointer of each buffer that stores the frame into the free buffer pool after the transmission of the contents of that buffer. One of the advantages of the present invention is that, when it is possible to determine that the next copy to be transmitted of a frame is the last copy that is to be transmitted, the buffers which store the frame are reclaimed after the contents of each buffer are transmitted. This returns the buffers much faster for reuse than waiting until an entire frame has been transmitted, then determining that no more copies of that frame are to be sent, and only then begin reclaiming the buffers that stored the frame. .The earlier stated need and others are met by another embodiment of the present invention which provides a method of reclaiming buffers, which store a frame, following transmission of the frame from a network switch. In the method of the invention, an attempt is made to determine if a copy of the frame to be transmitted is the last copy of the frame to be transmitted. If it is determined that the copy of the frame to be transmitted is the last copy of the frame to be transmitted, the buffers that store the frame are reclaimed after the contents of each buffer is transmitted from the network switch. However, if it is determined that the copy of the frame to be transmitted is not the last copy of the frame to be transmitted, an indication of the transmission of the copy of the frame is queued after the frame is transmitted from the network switch. Also, if it is not determinable whether the copy of the frame to be transmitted is the last copy of the " frame to be transmitted, an indication of the transmission of the copy of the frame is queued after the frame is transmitted from the network switch. One advantage of this embodiment of the present invention is that an attempt is made to determine the status of a transmission of a frame, i.e., whether it is the last copy. If it can be determined, then a speeding up of the reclaiming process is achieved by reclaiming the buffers immediately after the contents of each buffer are transmitted. This is allowed since it is known that the data in the buffers will not be needed to send another copy of that frame. However, when it is not the last copy, or if it cannot be determined that it is the last copy, then an indication of the transmission is queued. This indication can be used later in keeping a count of the number of copies that have been transmitted. Hence, under certain circumstances, present invention speeds up the reclaiming process. In certain embodiments of the invention, it is also determined whether the only a single copy of the frame is to be sent, or multiple copies. With such embodiments, a speeding up process for reclaiming buffers when during the transmission of the single copy is possible, since it is known that the contents of the buffers will not be needed for transmission of other copies of that frame. Hence, the reclaiming of the buffers of a single copy of a frame may be performed in the same manner as the last copy of a multiple copy frame (when it is known to be the last copy of the multiple copy frame). There is also a need for an arrangement and method of generating buffer pointers upon system startup that is less time-consuming upon startup. This and other needs are met by the present invention which provides an arrangement for making buffers available in a memory for storing data. This arrangement includes a free buffer pool configured to store a plurality of buffer pointers. Each buffer pointer points to a different location in memory at which an associated free buffer, which is a buffer currently available for storing data, is located. Reclaim circuitry is provided that returns buffer pointers of buffers that have become available for storing data to the free buffer pool. A counter generates buffer pointers upon startup of the arrangement to provide free buffers available for storage of data. A multiplexer has a first input coupled to the output of the free buffer pool and a second input coupled to the output of the counter, and a control input that selects between the first and second inputs. An advantage of this embodiment of the present invention is that upon startup of the system, the buffer pointers are generated as needed, rather than generated and loaded into the free buffer pool before they can be used. When there are a great many buffer pointers that can be generated, such as 32,000 buffer pointers, waiting for the generation and loading of all of these pointers prior to beginning the use of the buffers for storing data is wasteful of time. With the present invention, the buffers pointed to by die buffer pointers generated by the counter can be immediately used for data storage, without waiting for all of the buffer pointers to be generated. Eventually, when all of the buffer pointers are generated during operation of the switch, then the counter is no longer needed and the multiplexer switches so that all of the buffer pointers are retrieved from the free buffer pool. 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 widi 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 wim 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 wim an embodiment of the present invention. Figure 12 is a block diagram of a multicopy queue configured in accordance widi 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 odier 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 Ed emet protocol. The Ethernet protocol ISO/TEC 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 widi 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 die 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 d e 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 anotiier station sending data at the same time. If a collision is detected, bodi 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 Ediernet standard IEEE 802.3x Full-Duplex widi 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 tiiat 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 otiier networks. As shown in Figure 1, me network 10 includes a series of switch transceivers 26 that perform time division multiplexing and time division demultiplexing for data packets transmitted between die switch 12 and ti e 10 M\bps stations 14. A magnetic transformer module 19 maintains the signal waveform shapes on d e 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 d e serial NRZ interface 23, demultiplexes the received packets, and outputs the packets to the appropriate end station 14 via me network media 17. According to me 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 die routing of data packets between die Ediernet 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 mat 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 widi 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 eidier by the internal switching engine or the external rules checker 42. The switch 12 also includes an LED interface 44 d at 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 me 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, ei ier internal rules checker 58 or the external rules checker interface 40, to determine which MAC ports will output the data packet. Specifically, die packet header is forwarded to the internal rules checker 58 or the external rules checker interface 40, depending on whedier die switch 12 is configured to operate using the internal rules checker 58 or die external rules checker 42. The internal rules checker 58 and external rules checker 42 provide the decision making logic for determining die destination MAC port for a given data packet. The decision making engine may thus output a given data packet to eidier 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, die 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 iat specifies anod er network (via a router at one of me 100 M\bps stations 22) or a prescribed group of stations. Hence, eidier d e internal rules checker 58 or the external rules checker 42 via d e interface 40 will decide whedier 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 die frames were received by the switch 12. The decision making engine (i.e., internal rules checker 58 or d e 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 me address location storing the data packet in the external memory 36, and the identification of d e MAC ports to receive the data packet for transmission (e.g., MAC ports 0-26). The switch subsystem 56 fetches me data packet identified in the port vector from d e 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 die bidirectional management data 10 (MDIO) signal padi. 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. Od er elements of the multiport switch 12 of Figure 2 are reproduced in Figure 3 to illustrate the connections of die switch subsystem 56 to these odier 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 diese 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 me 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, whedier unicopy or multicopy, are received by the internal MAC engines 50. When the frame packet is received at die port, it is placed in me receive FIFO 52. Each frame has a header, which is provided to a rules checker, eidier the internal rules checker 58 or the external rules checker 42. The rules checker 42 or 58, based on the information in the header, determines from where ie frame packet will be cast, i.e., tiirough which port or ports will me 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 d e location in external memory 36 at which die frame will be stored by the receive FIFO 52. Once me free buffer pointer is obtained from me free buffer pool 104 by the buffer manager 72, the buffer pointed to by e 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 wi a bit set for each output port to which the frame should be forwarded. Assume for this overview example ti at 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 mat is set in the port vector corresponds to a particular one of the ports. The rules checker 42 or 58 places d e port vector and the 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 widi 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 ie transmission of the frame. At some point in time, e 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 d e frame pointer to the appropriate transmit FIFO 54 of the correct port via frame pointer read bus 86. This schedules d e 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 dien transmitted. A multicopy transmission is similar to the unicopy transmission, except that the port vector has multiple bits set, designating me 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, me multicopy queue 90, and die reclaim queue 98, and die multicopy cache 96 to manage me process of allocating buffers to store received frames and retrieving buffers for re-use once die 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. Wi 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 me 10 Mb/s output ports and the 100 Mb/s output ports, an output queue 75 is provided for me 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 widi 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 me 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 me bursting nature of current external memories, so that the data (e.g., frame pointers) is sent on and off die chip to me 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 d e output queue 74, is large and inexpensive. The write side 76 and the read side 78 provide high performance, while the pa 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 me 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 odier types of entries, both in multiport switches and in odier technologies. After the entry flows through and reaches me bottom of the output queue write side 76, control logic associated widi 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 die output queue read side. This passing of the entry or entries directly from the write side 76 to die read side 78 is performed entirely on the chip 12, and is therefore a low-latency, fast flow-through of an entry. If me output queue read side 78 is full, and diere is at least a burst-size amount of data (e.g., 16 bytes worth of entries) in the output queue write side 76, en die data is written in a burst fashion into the overflow area 110 for that output queue 74. If die 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, men the entry remains in the output queue write side and nod ing further is done. Eventually, d e output queue read side 78 will empty, and when die 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 die overflow area 110 into die 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 this 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 then 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 mough 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 me overflow area 110 to avoid overflowing die on-chip queue 78. Hence, the discarding of frames is largely prevented widi the queue structure of the present invention. Also, the total amount of memory dedicated to the 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 wid a first-in, first-out arrangement. In some types of queues, however, such as the reclaim queue 98 and me free buffer pool 104, the order of entries does not matter. If it is possible to send data directly from the write side 100 to the read side 102, the present invention permits information to be directly sent mis route, bypassing the overflow area for the queue. This is permitted even if mere 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 die buffers are eventually returned to me free list in me free buffer pool 104 after the buffer is no longer needed to " store the frame is acceptable. Hence, in order to avoid incurring me bandwidth of a write to die 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, there are many odier types of data in different applications diat are also not order-sensitive, so mat this feature of the present invention finds utility in queues that queue these other types of data. In die multiport switch of an exemplary embodiment of the present invention as depicted in Figures 1 and 2, there are twenty-eight output queues (each associated widi 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 me invention, the various output queues 74, 75 and 77 contain several or all of die 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 me 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 the ports will handle frames before transmission. The VLAN index provides the 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 odier output queues wim 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 the 10 Mb/s ports. The output queue write sides 76 for these queues hold fliirty-two entries and the output queue read sides 78 hold sixteen entries in the exemplary illustrated embodiment, although odier sizes are contemplated and witiiin 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 the 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 the 100 Mb/s ports. The output queue write side 76 holds sixty-four entries in this type of output queue, and die 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 odier 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 the size of the output queues by simply changing the external memory. This is an advantage over systems in which the 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 me 10 Mb/s and 100 Mb/s destination ports, expansion bus port output queue overflow 130, the MIB counters 132, and me 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 die external memory map is programmable in the register set. No length register is required, the length for a given area being equal to the area from that area's BASE Address to me BASE Address of the next area in the mapping. Since the length (and therefore capacity) of each of me 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 with increased capacity, as needed. The following overflow areas store entries that do not fit into the control queues on the chip 12 are dierefore 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 the 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 the switch 12. The switch 12 maintains 8 -bit and 16-bit counters on-chip for storing MIB statistics. The switch 12 updates the 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, these linked lists contain valid frame data, obsolete buffers which will be returned by the buffer manager 72 to me 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 the 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 the invention. Address pointers to each of these 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, me 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 the 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 that indicate die location of die 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, the first and subsequent buffer header formats contain the 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 the receiver. When the C Bit is detected, the transmit function will purposely transmit an inverted CRC. L Bit (Alignment Error): indicates a Frame Alignment Error was detected (along with a CRC Error) in the receive frame. O Bit (Receive FIFO Overflow): indicates me receive FIFO overflowed and the data in the buffer may not be valid. Buffer Length: me total number of bytes which are valid in the data field of the buffer beginning wim the first byte after the buffer header. This length should not include die Offset Byte value. Next Buffer Pointer: the pointer to me next buffer. The next buffer pointer is not valid when me E Bit is set. Offset Byte Count: indicates where the first byte of die frame starts in the frame data section of the 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 the byte following me 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 die 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 wim 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 the VLAN ID is not valid. Receive Port Number: the number of the port from which the 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 the CRC needs to be stripped and recalculated at the 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 this 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 me CRC. A Bit (Append CRC): indicates that there is no CRC at me end of die frame data. The host can create a frame in memory (without a CRC) dien set mis bit. The switch 12 will generate and append a CRC when transmitting the frame. If the A Bit is set, the frame length should not include CRC. F Bit (Format Bit): identifies the Frame Length/Time Stamp field. A zero indicates the field is the time stamp of the incoming frame. A one indicates me field is die 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 the total length of the received frame including CRC and any received VLAN Tag. When a frame is received, me switch 12 marks this field wim the time stamp (from the timer register). If the host 28 has programmed die switch 12 to forward expansion bus frames before the frame has been completely received, it can use the time stamp (along with the speed of d e 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 mis field and sets the F Bit. Copy number: used to indicate the number of copies successfully queued for transmission by the port vector FIFO 70. This field is used to store me 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 mem available for use again once the 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 the 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 the free buffer pool 104; 4) controlling me flow of data to and from the external memory 36; and 5) maintaining me 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) the 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 me frame pointer and the copy number into the write side 92 of the multicopy queue 90. The copy number may be "0", indicating mat the 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 the copy number is "0", which means that the frame pointer has a null forwarding port vector widi no bits set, the port vector FIFO 70 passes the frame pointer directly to the write side 100 of me reclaim queue 98. When the buffer manager 72 services the reclaim queue 98, as will be described, the buffer manager 72 breaks down the linked-list chain of buffers and returns the address pointer for each "free" buffer to the write side 106 of the free buffer pool 104. When the copy number is "1", a unicopy transmission, the port vector FIFO 70 copies the frame pointer, control signals/control opcode and the VLAN index to the output queue 74 of me appropriate port. The port vector FIFO 70 sets the unicopy bit in the output queue 74 (see Figures 5 and 6) to indicate that this is a single transmission. When the buffer manager 72 reads the frame pointer and the unicopy bit from the output queue 74 of the port, it schedules me transmission as discussed previously. The buffer manager 72 uses the frame pointer to locate the first buffer in external memory 36 in which the frame is stored. The buffer manager 72 reads the buffer header from mis first buffer, captures data from the first buffer and places mis data in the appropriate MAC transmit FIFO 54. The links to subsequent buffers, assuming that the frame spans multiple buffers, provides the buffer manager 72 with the address to find and transmit all of the buffers in the chain for mat frame. Once the data has been placed in the FIFO 54 for transmission, the buffer becomes obsolete and is returned to me free buffer pool 104 for eventual re-assignment to store data of another frame. When the copy number is greater than 1, the port vector FIFO 70 copies the frame pointer, VLAN index and control signals/control opcode to each of the 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 me frame pointer with 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 the output queues 74, the buffer manager 72 schedules the 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 wi i a copy number of "1" is found in die multicopy cache 96, then the buffer manager 72 schedules the frame for transmission and reclaims die 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 the buffer manager 72 transmits the frame but does not reclaim me 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 the multicopy queue 90. Each time a multicopy frame is transmitted, me buffer manager 72 places a copy of the frame pointer into the multicopy queue 90, provided me buffer manager 72 did not find the 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 the 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", the buffer manager 72 attempts to place this new entry (frame pointer and copy number) into the multicopy cache 96. If the 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 die copy number for diis entry in its buffer header in external memory 36, then 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 the multicopy cache 96. When the 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 wim a copy number ">=1" to decrement or delete. If me 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 me multicopy cache's frame pointer/copy number entry and place me frame pointer into the reclaim queue 98 if the 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 the copy number in memory is "1", the buffer manager 72 places the frame pointer into the reclaim queue 98. If the copy number in memory is ">1", the buffer manager 72 places the frame pointer with this copy number into the multicopy cache 96, then decrements the copy number. The buffer manager 72 constantly services die reclaim queue 98 by reading frame pointers, then "walking" e linked-list chain to return buffers to me free buffer pool 104. This activity only returns buffers for frames that had null port vectors and were queued to me reclaim queue by the port vector FIFO 70, or frames with a multicopy forwarding vector and have completed transmissions of all of the copies. Buffers linked for unicopy frames are returned directly to me free buffer pool 104 when the frame is transmitted, as described above. If the port vector FIFO 70 is not able to place a frame pointer for a unicopy forwarding vector into an output queue 74, because that 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 me 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 the output queues 74 and their overflow areas 110 in external memory 36 are full, the frame is only forwarded to die output queues widi available space and the 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 the switch management resources for each of the ports for which the frame pointer could not be queued. If the port vector FIFO 70 is not able to place any of me frame pointers for a multicopy forwarding vector, because all of the output queues 74 and dieir overflow areas 110 in external memory 36 are full, the frame pointer is passed to die reclaim queue 98, and the switch management resources are duly notified. The multicopy queue 90 is a high priority queue used by the 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 me frame can be returned to die free buffer pool 104. The write side 92 and read side 94 of this output queue hold 64 and 16 entries, respectively. The multicopy queue 90 feeds the multicopy cache 96, which is used by die 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 the 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, the 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 the unicopy bit is "0", (i.e., a multicopy), it checks the multicopy cache for the frame pointer with a copy number of "1", which indicates that mis is die last transmission. If is match is found, die buffer manager 72 transmits e frame and reclaims die buffers in the same manner as in die unicopy transmission, by providing me obsolete buffers to the free buffer pool 104 after the transmission of the contents of each buffer. If the match is not found, the buffer manager 72 transmits the multicopy frame and places a copy of the frame pointer widi 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 the expansion bus output queue 75 or the management port output queue 77 (through the PCI interface 26), the host writes a copy of the 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 padi and an output padi. The input padi, 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 the multicopy queue 90 to place frame pointers/copy numbers into the multicopy cache 96. Additional storage for frame pointers/copy numbers, termed 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 the read side 94 until the 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 the multicopy queue 90 and its overflow area 124 are full, additional frame pointers/copy numbers placed into me multicopy queue begin to fill the write side 92. The ordering of me frame pointers passing through the multicopy queue 90 is maintained, such diat when space clears in die multicopy queue read side 94, frame pointers/copy numbers are moved from die multicopy queue overflow area 124 to die 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 me 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 eidier 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 die transmission. If the 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 me 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 the transmission. Periodically, me 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 whether the buffer manager reads a frame pointer with a copy number ">1" or "-1": 1) The buffer manager 72 reads a frame pointer with a copy number ">1" from the multicopy queue 90. Provided there is room in the multicopy cache 96, it writes a new entry. If me multicopy cache 96 is full, the buffer manager 72 must clear space in the cache 96. This is done by reading one of the 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 this cache entry. Once mere 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 wi a copy number ">=1". Two cases follow, depending on whedier or not the 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 die frame pointer in the reclaim queue 98. If the cache entry's copy number is ">1", the buffer manager 72 decrements the copy number by "1". b) The buffer manager 72 does not find a frame pointer match in the multicopy cache 96. This means that the matching frame pointer has previously been moved to the buffer header of the 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 die 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 the frame pointer/copy number (that was in external memory 36) into the multicopy cache 96 and decrements the copy number by "1". If the multicopy cache 96 is full, the buffer manager clears space by moving one of the older frame pointer/copy numbers into external memory 36. The reclaim queue 98 holds me 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., the last transmission of the frame has completed successfully). Additionally, the port vector FIFO 70 writes frame pointers to the reclaim queue 98 under the following conditions: 1) a frame pointer's port vector is null or 2) the frame pointer could not be queued because all of die forwarding vector's output queues were full. Finally, die 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 the expansion bus output queue 77 or the management port output queue 75. When me 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 me frame pointers (14 bits) in the exemplary embodiment of the 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, the reclaim queue 98 is structured with an input padi and an output padi. The input path, or write side 100, allows the 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 the 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 the read side 102 until me read side 102 is full. Additional frame pointers written to the write side 100 of die reclaim queue 98 are placed into the 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 die reclaim queue 98 begin to fill the write side 100. Figure 11 depicts an exemplary embodiment of die internal structure of me 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 the invention. The free buffer pool 104 is structured with an input padi and an output path (similar to the output queues 74). The input path, or write side 106, allows the buffer manager 72 or host 28 to place address pointers into the free buffer pool 104. The output padi, or read side 108 of die free buffer pool 104 allows the buffer manager 72 to provide address pointers to die 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, the free buffer pool generates address pointers from me 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 the traffic demands, the overflow area 120 is accessed to obtain more buffer pointers. Certain embodiments of the present invention provide an advantageous arrangement and method of providing the buffer pointers upon start-up of the switch 12. When the switch 12 first powers up, it is not required for the overflow area 120 in external memory 36 to contain buffer pointers. Instead, me buffer pointers are created on the fly. The switch 12 on power up could generate and place into me overflow area 120 the buffer pointers, but there may be 16,000 or 32,000 such pointers, and this would slow up the powering on procedure of the switch 12. The present invention takes advantage of the fact that on power up, all of me buffers are free, and the identities of these buffers are known. Therefore, e 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 die input of a multiplexer 182. Since the free list in the free buffer pool 104 is empty on startup, the free list counter 180 generates the buffer pointers. Once the free list reaches die highest count, it will not generate any more buffer pointers. When a frame packet is received in the switch 12, the frame packet is broken up into fixed lengdi 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 die switch 12, any address pointers returned to die free buffer pool 104 pass from the write side 106 to the read side 108. If die read side 108 becomes full, additional address pointers are passed to me overflow area 120. Once the read side 108 and the overflow area 120 are full, additional address pointers placed into the free buffer pool 104 will begin to fill the write side 106 of the 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 die 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 the 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 me frame pointer determine the row in me set-associative cache 96 in which the entry will be stored. In the illustrated embodiment of the invention, there are sixty-four rows in the cache 96, although other 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 die cache 96, die entry always enters me first column, the uppermost (51:39) bits of the row indicated by die six least significant bits of the frame pointer. The row is read, all of ie entries are shifted to me right by 13 bits, and die row is written back. The entry that is actually written into the cache 96 includes die upper eight bits of the frame pointer mat form an address tag, and the five-bit copy number associated with the frame pointer. When die entry is read out of die cache 96, the frame pointer is re-formed with the address tag and the bits that index the 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 the row is written. As described earlier with respect to die buffer headers 142, the copy number associated with the frame pointer that is removed is written into die buffer header 142 of the frame in external memory pointed to by the removed frame pointer. Hence, the 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 there is no separate valid bit in die set-associative cache 96. When the copy number is 00000, then the buffer manager 72 knows that the entry is no longer valid and removes the entry from the cache 96. This simplifies the organization of die cache. Anod er advantage of d e cache 96 of die present invention is mat it allows a very fast search to be performed, since the buffer manager 72 needs only to examine a single row, already determined by the frame pointer that has exited die multicopy queue 90. The four entries in the row are examined in parallel, further increasing me speed of me 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 the scope of the invention. From the above description, it should be understood that the 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 other words, the physical position of die entry in the cache is an indication of die relative age of an entry. The aging of an entry is performed by die physical re-ordering of die entries in die memory. Certain embodiments of the present invention provide customization of the latency of frames switched by die switch 12, on a port by port basis. Referring to Figure 14, die port vector FIFO 70 examines the programmed switch mode of the receive port to determine when to place the frame pointer and associated information into the appropriate output queue 74 of the transmit port. For a first mode (low latency mode), the 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), me port vector FIFO 70 places the frame pointer into the output queue(s) 74 only after 64 bytes of the frame have been received. For a third mode (high latency mode), die port vector FIFO 70 places die 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 the management port 30 and 3) frames forwarding to the expansion bus port. In case 1), die 10 Mb/s port to 100 Mb/s port rate mismatch forces the forwarding mode to be me third, high latency mode. In case 2), all frames passed to the management port are third mode frames. In case 3), any frame forwarding to the 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 die entire port vector becomes that of the 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 the third 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 mat applies to the input (i.e., receive) port determines forwarding latency (how soon the switch 12 will forward a frame once it begins receiving the frame) and the 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 mree modes, frame data received at die receive FIFO 52 of an internal MAC port is forwarded to a buffer 140 in die 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, me 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 ie frame pointer and the forwarding port vector to the port vector FIFO 70. The port vector FIFO 70 places the frame pointer in the write side 76 of the output queues 74 for the output port(s) identified in the port vector. The receive port's switch mode defines die latency between when the port vector FIFO 70 receives die port vector (and me frame pointer) and places the frame pointer into the output queue(s) 74. This is described for die three modes below. Once the frame pointer passes to the read side 78 of die output queues 74, die buffer manager 72 reads the frame pointer and schedules transmission. The buffer manager 72 begins moving frame data from me address specified by die 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, diere is no network error protection because a frame is queued for transmission before it can be determined whedier 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 mat 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 the transmit MAC has not started a frame's transmission and the buffer header 142 indicates the frame terminated in a runt or an invalid CRC, the 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 the receive MAC are queued to the 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, the transmit MAC will complete the transmission with a bad CRC. If the transmit MAC has not started a frame transmission and me buffer header 142 indicates the frame (greater than or equal to sixty-four bytes) terminated in an invalid CRC, die buffer manager 72 returns the frame pointer to the reclaim queue 98 (for a unicopy forward) or the multicopy queue 96 (for a multicopy forward) widiout forwarding to die output port(s) 74. The third mode is a store-and-forward mode diat provides die 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 them to output ports. In this mode, the switch 12 screens out all fragments and CRC error frames before forwarding. In the third mode, once a valid frame completes successfully at the receiver (i.e., greater than or equal to sixty-four bytes with a valid CRC), the frame pointer is queued to the appropriate output queue(s) 74. Frames which terminate in a receive error (invalid CRC, runt (>64 bytes) etc.) are deleted and dieir 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 the selected mode of me receive port and the amount of data that has been received. In the embodiment described above, diere are three thresholds, aldiough there are different numbers of thresholds in other embodiments. In the exemplary embodiment, these mresholds 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 the mode in which the port is programmed. Aldiough the exemplary embodiment makes forwarding decisions based on the amount of received data, odier embodiments of the invention make forwarding decisions based on odier factors, such as die types of data received. In implementing the forwarding scheme of the 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 the receive port has finished receiving a frame, and if not, how much of the frame has already been received. The port vector FIFO 70 does not receive any information regarding the identity of the receive port from the rules checker 42 or 58. The only information the port vector receives that provides any identification of the port are the frame pointers. The port vector FIFO 70 queries the address table 160 with me frame pointer. Either the address table returns the receive port if die frame is still being received, or die address table 160 cannot find die frame pointer which means diat the frame has already been received. Once the frame is completely received, the frame pointer is moved out of the address table 160. This means that the third threshold (the frame complete) is met. Accordingly, the frame pointer may be dropped into the output queue 74 immediately. If the address table 160 returns the receive port, me 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 the port receives n bytes. At diat point, if that port is in the first mode, the port vector FIFO 70 starts processing the frame pointer by sending it to the appropriate output queue 74. If the receive port is not in die first mode, me 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 die 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, men 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 the mode for that receive port, and then monitors the flags from that receive port and releases the frame pointer according to the mode and the flags. The buffer manager 72 performs numerous functions in the switch 12 of the present invention. The buffer manager 72 comprises control logic and/or software to implement these functions, and is readily implemented by one of ordinary skill in die art given d e description of the functions of the buffer manager 72 provided above. Aldiough 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 the present invention being limited only by the terms of the appended claims.

Claims: CLAIMS WHAT IS CLAIMED IS: 1. An arrangement for reclaiming buffers that store frames for a network switch in a packet switched network, the arrangement comprising: a reclaim queue configured to queue frame pointers that point to a location in a buffer memory of a first buffer in which a frame is stored; a searchable first memory configured to store a count of the number of copies of a frame to be transmitted from the network switch; a free buffer pool configured to queue buffer pointers that point to locations of buffers in a buffer memory; a buffer manager configured to receive a frame pointer of a frame for transmission of a copy of that frame, search the first memory to determine if transmission of this copy would be the last copy of that frame to be transmitted and if so, cause die transmission of that last copy of diat frame from the network switch and place the buffer pointer of each buffer that stores the frame into the free buffer pool after the transmission of the contents of that buffer. 2. The arrangement of Claim 1, further comprising a multicopy queue configured to store entries, each entry comprising a frame pointer and an associated copy number, a positive copy number indicating the number of copies to be transmitted of die frame pointed to by die associated frame pointer, and a negative copy number representing a transmission of a copy of the frame pointed to by the associated frame pointer. 3. The arrangement of Claim 2, wherein the buffer manager is further configured to load the multicopy queue with one of said entries having a negative copy number when the search of the first memory determines that the transmission of the copy of the frame would not be the last copy of that frame transmitted by the network switch. 4. The arrangement of Claim 3, wherein the buffer manager is further configured to examine the copy number of entries exiting the multicopy queue, store in the first memory the entries having positive copy numbers, and search in the first memory for an entry with a frame pointer equal to the frame pointer of any entry having a negative copy number. 5. The arrangement of Claim 4, wherein the buffer manager is further configured to decrement the copy number of an entry in the first memory when the search in the first memory indicates diat the entry in the first memory has a frame pointer equal to the frame pointer of the entry having a negative copy number, if the copy number of the entry in the first memory is greater than one. 6. The arrangement of Claim 5, wherein the buffer manager is further configured to delete an entry in the first memory when the search in the first memory indicates that the entry in die first memory has a frame pointer equal to the frame pointer of the entry having a negative copy number, if the copy number of the entry in the first memory is equal to one, and to load die reclaim queue with the frame pointer of the entry that is deleted from the first memory. 7. The arrangement of Claim 6, wherein the buffer manager is further configured to obtain a frame pointer exiting from the reclaim buffer and place the buffer pointer of each buffer that stores the frame pointed to by the frame pointer into the free buffer pool. 8. The arrangement of Claim 7, wherein the buffers that store a single frame are linked together to form a linked list, me frame pointer of the frame being equal to the buffer pointer of a first one of die buffers that form the linked list, each buffer in the linked list containing the buffer pointer that points to e next buffer in the linked list for that frame, with die last buffer in the linked list containing an end of linked list indication instead of the buffer pointer to the next buffer . 9. The arrangement of Claim 8, wherein the buffer manager is further configured to examine the buffers in the linked list and retrieve the buffer pointers of die buffers in the linked list of die frame and place diem in the free buffer pool. 10. The arrangement of Claim 9, wherein each frame pointer of a frame for transmission received by the buffer manager has a bit diat indicates whether a single copy of the frame is to be transmitted or multiple copies of the frame are to be transmitted, and wherein the buffer manager is further configured to examine me bit and perform the search of the first memory only if the bit indicates that multiple copies of the frame are to be transmitted, and if the bit indicates that a single copy of me frame is to be transmitted, cause the transmission of the single copy of the frame and place the buffer pointer of each buffer that stores the frame into the free buffer pool after the transmission of the contents of that buffer. 11. A method of reclaiming buffers, which store a frame, following transmission of the frame from a network switch, die method comprising the steps of: attempting " to determine if a copy of the frame to be transmitted is the last copy of the frame to be transmitted; if it is determined that the copy of the frame to be transmitted is the last copy of the frame to be transmitted, reclaiming the buffers that store the frame after the contents of each buffer is transmitted from the network switch; if it is determined diat die copy of e frame to be transmitted is not the last copy of the frame to be transmitted, queuing an indication of the transmission of the copy of the frame after the frame is transmitted from the network switch; and if it is not determinable whether die copy of me frame to be transmitted is the last copy of the frame to be transmitted, queuing an indication of the transmission of the copy of the frame after the frame is transmitted from the network switch. 12. The method of Claim 11, further comprising the steps of obtaining one of the queued indications and determining if the last copy of me frame pointed to in me queued indication has been transmitted, and if so, loading into a reclaim queue a frame pointer mat points to a memory location in which the frame has been stored. 13. The method of Claim 12, further comprising the step of obtaining one of die frame pointers from the reclaim queue and reclaiming the buffers that store the frame. 14. The method of Claim 13, wherein the buffers that store a frame form a linked list of buffers tiiat are linked by buffer pointers that point to each buffer in the linked list, and wherein the step of reclaiming the buffers of a frame includes examining the linked list and loading the buffer pointer of each buffer storing a frame into a free buffer pool. 15. The method of Claim 14, further comprising maintaining a copy number representing the number of copies of a frame that remain to be transmitted, and wherein after obtaining one of the queued indications and determining that me last copy of the frame pointed to in the queued indication has not been transmitted, decrementing the copy number. 16. The method of Claim 15, wherein the step of determining whether the last copy of the frame has been transmitted includes examining die value of me copy number, with a copy number equal to one indicating that the last copy of die frame has already been transmitted. 17. The method of Claim 16, wherein the step of determining if transmission of a copy of a frame would be the last copy of diat frame to be transmitted includes examining the copy number of that frame, with a copy " number equal to one indicating mat the frame would be the last copy of e frame to be transmitted. 18. The method of Claim 17, further comprising the steps of queuing the copy number and the frame pointer of each frame for which multiple copies of the frame are to be transmitted in the same queue as the queued indications, examining the output of me queue, and if die output of me queue is the copy number and the frame pointer for a multiple copy frame, loading the copy number and the frame pointer for the multiple copy frame into a searchable memory, if the output of the queue is a queued indication, performing said step of attempting to determine if a copy of the frame to be transmitted is the last copy of the frame to be transmitted. 19. The method of Claim 18, wherein the step of attempting to determine includes searching the memory for a frame pointer that matches a frame pointer of the queued indication, and examining the copy number associated widi the frame pointer that matches the frame pointer of the queued indication. 20. An arrangement for making buffers available in a memory for storing data, comprising: a free buffer pool configured to store a plurality of buffer pointers, each buffer pointer pointing to a different location in memory at which an associated free buffer, which is a buffer currently available for storing data, is located ; reclaim circuitry configured to return buffer pointers of buffers that have become available for storing data to the free buffer pool; a counter that generates buffer pointers upon startup of the arrangement to provide free buffers available for storage of data; and a multiplexer with a first input coupled to the output of die free buffer pool and a second input coupled to the output of d e counter, and a control input that selects between the first and second inputs. 21. The arrangement of Claim 20, further comprising means for selecting the second input of the multiplexer as buffers are required for storage of data after startup of me arrangement, until the counter reaches a maximum count, and tiiereafter selecting the first input as further buffers are required for storage.

Inventors: EGBERT CHANDAN, ERIMLI BAHADIR, RUNALDUE THOMAS JEFFERSON, EGBERT, CHANDAN, ERIMLI, BAHADIR, RUNALDUE, THOMAS, JEFFERSON
Applicants: ADVANCED MICRO DEVICES INC, ADVANCED MICRO DEVICES, INC.
Application year: 1998
Main classification: H04L12
IPC codes: G06F5, H04L12, H04L29, H04Q3