Technical information

2-2

AIC-6915 Ethernet LAN Controller Programmer’s Manual

■

VLAN support:

– Address filtering based on VLAN

– Ability to delete VLAN tag and number from frame returned to the host

■

Optional second buffer list for allocating two different buffer sizes

Host Data Structures

Figure 2-1 illustrates the AIC-6915 receive data structures.

Producer and Consumer Indices

The transmit, receive, and completion descriptors are stored in circular queues. With the

descriptor queue, the host writes entries into the queue. The AIC-6915 reads from the

descriptor queue and writes to the completion queue, which is in turn read by the host.

The AIC-6915 maintains onchip Producer and Consumer indexes to each queue. The first

element of any queue has an index of 0.

The Producer Index indicates the next entry of the queue to be written, while the

Consumer Index indicates the next entry in the queue to be read.

If Producer and Consumer indices are equal, the queue is empty. If (Producer+1) mod

QueueSize is equal to Consumer then the queue is full. The maximum number of entries

placed in a queue at one time is the queue size minus one.

Receive DMA Descriptor Queues

The AIC-6915 contains two Producer/Consumer type DMA Receive Descriptor Queues

that contain a maximum of either 256 or 2048 entries. A variable option permits the use of

a smaller queue. Host Buffer addresses must be aligned to a word (4-byte) boundary. For

best performance, addresses should also be aligned to cacheline boundary.

Receive Completion Queue

(2KByte entries)

Adrs

Write

Read

ListLen

Host Memory

On-chip

Start End Len Stat

Adrs

Write Read

Receive Buffer Descriptor Queue

(256/2KByte

)

1 per Frame

Receive Buffers

BufferLength

(2 Queues are available for 2 different sizes of buffers)

Figure 2-1. The AIC-6915 Receive Data Structures

(2 completion Queues allow for 2 priorities)