TNETX3150A datasheet(72/113 Pages) TI1

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TNETX3150/TNETX3150A

ThunderSWITCH™ 15-PORT 10-/100-MBIT/S ETHERNET™ SWITCH

SPWS027F – FEBRUARY 1997 – REVISED SEPTEMBER 1997

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

IOB operation (continued)

When all multicast buffers have been linked onto the receive queue, the receive queue is linked to the transmit

queues on which the multicast data is to be transmitted. The ports to which the data is sent can be defined two

ways. If no external addressing logic is used, the multicast data is linked to all active ports. These are defined

in the port bit map held in the VLAN register for the port on which the data was received. Alternatively, the port

bit map presented on the EAM interface terminals is used. The data is linked to the active port subset of that

defined on the terminals.

Having determined the list of transmit queues onto which the IOB data is linked, the forward pointer for each

transmit queue is updated to point to the head of the receive queue (IOB data). In this way, the multicast data

buffers appear linked to multiple queues without the program of replicating the multicast data. The index buffer

is used to preserve the separate transmit queues as they form, following the IOB data frame. Each index buffer

contains a forward pointer referencing the continuation of the transmit queue for every port. As new transmit

queue data is queued, the forward pointers in the index buffer are updated to reflect the continuation of the

independent transmit queues (see Figure 30).

The IOB frame buffers can be returned to the free buffer queue only when all ports have transmitted the IOB

data. Since there can be a large discrepancy between the first port completing transmission and the last (due

to a long transmit queue before the IOB data), a tag field is used to record which ports have transmitted the IOB

data from the list of ports where the data was to be sent originally. The tag field also is stored in the index buffer.

When the last port tag is cleared, all the buffers can be returned to the free pool of buffers.

The buffers can be freed only after the last transmission, by which stage the forward pointer pointing to the head

of the IOB buffers is freed. The return address field of the index buffer is used to store the head address of the

IOB buffers. Thus, even after the last IOB transmission, the head of the IOB buffers remains known. Freeing

the buffers is done by writing the pointer at the top of the free queue to the last forward pointer of the IOB buffers

and moving the return address to the top of the free queue, adding the used IOB buffers to the top of the free

queue.

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