
MVTX2804
Data Sheet
23
Zarlink Semiconductor Inc.
occupancy at the destination. If the frame is not dropped, then the TxQ manager links the frame's FCB to the
correct per-port-per-class TxQ. Unicast TxQ's are linked lists of transmission jobs, represented by their
associated frames' FCBs. There is one linked list for each transmission class for each port. There are 8 classes
for each of the 8 Gigabit ports - a total of 32 unicast queues.
The TxQ manager is responsible for scheduling transmission among the queues representing different classes
for a port. When the port control module determines that there is room in the MAC Transmission FIFO (TxFIFO)
for another frame, it requests the handle of a new frame from the TxQ manager. The TxQ manager chooses
among the head-of-line (HOL) frames from the per-class queues for that port, using a Zarlink Semiconductor
scheduling algorithm.
As at the transmit end, each of the 8 ports has time slots devoted solely to reading data from memory at the
address calculated by port control. The Transmission DMA (TxDMA) is responsible for multiplexing the data and
the address. On a port's turn, the TxDMA will move 8 bytes (or up to the EOF) from memory into the port's
associated TxFIFO. After reading the EOF, the port control requests a FCB release for that frame. The TxDMA
The frame is transmitted from the TxFIFO to the line.
3.2 Multicast Data Frame Forwarding
After receiving the switch response, the TxQ manager has to make the dropping decision. A global decision to
drop can be made, based on global FDB utilization and reservations. If so, then the FCB is released and the
frame is dropped. In addition, a selective decision to drop can be made, based on the TxQ occupancy at some
subset of the multicast packet's destinations. If so, then the frame is dropped at some destinations but not
others, and the FCB is not released.
If the frame is not dropped at a particular destination port, then the TxQ manager formats an entry in the
multicast queue for that port and class. Multicast queues are physical queues (unlike the linked lists for unicast
frames). There are 4 multicast queues for each of the 8 Gigabit ports. There is one multicast queue for every
two unicast classes.
During scheduling, the TxQ manager treats the unicast queue and the multicast queue of the same class as one
logical queue.
The port control requests a FCB release only after the EOF for the multicast frame has been read by all ports to
which the frame is destined.
3.3 Frame Forwarding To and From CPU
Frame forwarding from the CPU port to a regular transmission port is nearly the same as forwarding between
transmission ports. The only difference is that the physical destination port must be indicated in addition to the
destination MAC address. If an invalid port is indicated the frame is forwarded accordingly to the destination
MAC address.
Frame forwarding to the CPU port is nearly the same as forwarding to a regular transmission port. The only
difference is in frame scheduling. Instead of using the patent-pending scheduling algorithms, scheduling for
the CPU port is simply based on strict priority. That is, a frame in a high priority queue will always be transmitted
before a frame in a lower priority queue. There are four output queues to the CPU and one receive queue.