DM9102AT datasheet(49/77 Pages) ETC

49 / 77 page

DM9102A

Single Chip Fast Ethernet NIC controller

Final

Version: DM9102A-DS-F03

August 28, 2000

Functional Description

System Buffer Management

1.Overview

The data buffers for reception and transmission of data

resides in the host memory. They are directed by the

descriptor list that is located in another region of the host

memory. All actions for the buffer management are operated

by the DM9102A in conjunction with the driver. The data

structures and processing algorithms are described in the

following text.

2. Data Structure and Descriptor List

There are two types of buffers that reside in the host

memory, the transmit buffer and the receive buffer. The

buffers are composed of many distributed regions in the

host memory. They are linked together and controlled by the

descriptor lists that reside in another region of the host

memory. The content of each descriptor includes pointer to

the buffer, count of the buffer, command and status for the

packet to be transmitted or received. Each descriptor list

starts from the address setting of CR3 (receive descriptor

base address) and CR4 (transmit descriptor base address).

The descriptor list is Chain structure.

3. Buffer Management -- Chain Structure Method

As the Chain structure depicted below, each descriptor

contains two pointers, one point to a single buffer and the

other to the next descriptor chained. The first descriptor is

chained to the last descriptor under host driver’s control.

With this structure, a descriptor can be allocated anywhere

in host memory and is chained to the next descriptor.

Buffer 1

Descriptor 1

Descriptor N

Packet N

control

buffer address 1

status

own

not valid

next descriptor address

buffer 1 length

4. Descriptor List: Buffer Descriptor Format

(a). Receive Descriptor Format

Each receive descriptor has four double-word entries and

may be read or written by the host or the DM9102A. The

descriptor format is shown below with a detailed functional

description.

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