DM9008 datasheet(40/68 Pages) ETC

40 / 68 page

DM9008

ISA/Plug & Play Super Ethernet Contoller

Final

Version :DM9008-DS-F02

June 14, 2000

Packet Reception

The local DMA receive channel uses a Buffer Ring Structure

comprised of a series of contiguous fixed length 256-byte

(128-word) buffers for storage of received packets. The

location of the Receive Buffer Ring is programmed in two

registers: Page Start and Page Stop. An ethernet packet

consists of a distribution of shorter link control packets and

longer data packets. The 256 byte buffer length provides a

good compromise between short packets and longer packets

for using memory most efficiently. In addition, these buffers

provide memory resources for storage of back-to-back packets

in loaded networks. The assignment of buffers for storing

packets is controlled by DM9008's Buffer Management Logic,

which provides three basic functions: linking of receive buffers

for long packets, recovery of buffers when a packet is rejected,

and recirculation of buffer pages that have been read by the

host. At initialization, a portion of the 64K byte (or 32K word)

address space is reserved for the receive buffer ring. For

applications, DM9008 should be programmed to 16K byte (or

8K word) address space (4000H-7FFFH) in NE2000 16-bit

mode, and 8K byte address space (4000-H-5FFFH) in NE2000

8-bit mode. Two eight-bit registers, the Page Start Address

(PSTOP), define the physical boundaries where the buffers

reside. DM9008 treats the list of buffers as a logical ring.

Whenever the DMA address reaches the Page Stop Address,

the DMA is reset to the Page Start Address.

Initialization of the Buffer Ring

Two static registers and two working registers control the

operation of the Buffer Ring. These are the Page Start

the Current Page Register (CURR) and the Boundary Pointer

buffer used to store a packet, and is used to restore the DMA to

Buffer Ring writing status. It also restores the DMA address in

the event of a Runt Packet, a CRC or Frame Alignment error.

The Boundary Register points to the first packet in the Ring not

yet read by the host If the local DMA address reaches the

boundary, reception is aborted. The Boundary Pointer is also

used to initialize the Remote DMA for removing a packet, and

is incremented when a packet is removed. A simple analogy

for remembering the function of these registers is that the

Current Page Register acts as a Write Pointer, whereas the

Boundary Pointer acts as a Read Pointer.

Beginning of Reception

When the first packet arrives, DM9008 begins storing the

packet at the location pointed to by CURR. An offset of 4 bytes

is saved in this first buffer to store the packet's corresponding

receive status.

Linking Receive Buffer Pages

If the length of the packet exhausts the first 256-byte buffer,

the DMA performs a forward link to the next buffer to store the

remainder of the packet. For a maximum length packet, the

buffer logic will link six buffers to store the entire packet.

Buffers cannot be skipped when linking; a packet will always

be stored in contiguous buffers. Before the next buffer can be

linked,

the

Buffer

Management

Logic

performs

two

comparisons. The first comparison tests for equalit between

the DMA address of the next buffer and the contents of

PSTOP. If the buffer address equals PSTOP, the buffer

management logic will restore the DMA to the first buffer in the

Receive Buffer Ring value programmed in PSTART. The

second comparison tests for equality between the DMA

address of the next buffer address and the contents of BNRY.

If the two values are equal, the reception is aborted. BNRY can

be used to protect against overwriting any area in the receive

buffer ring that has not yet been read. When linking buffers,

buffer management will never cross this pointer, effectively

avoiding any overwrites. If the buffer address does not match

either BNRY or PSTOP, the link to the next buffer is performed.

Linking Buffers

Before the DMA can enter the next contiguous 256-byte buffer,

the address is checked for equality to PSTOP and to BNRY. If

neither is reached, the DMA is allowed to use the next buffer.

DM9008 Datasheet(PDF) 40 Page - List of Unclassifed Manufacturers

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