DP8390D datasheet(8/56 Pages) NSC

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70 Packet Reception (Continued)

for storing packets is controlled by Buffer Management Log-

ic in the NIC The Buffer Management Logic provides three

basic functions linking receive buffers for long packets re-

covery of buffers when a packet is rejected and recircula-

tion of buffer pages that have been read by the host

At initialization a portion of the 64k byte (or 32k word) ad-

dress space is reserved for the receive buffer ring Two

eight bit registers

the Page Start Address Register

(PSTART) and the Page Stop Address Register (PSTOP)

define the physical boundaries of where the buffers reside

The NIC 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 Reg-

ister Page Stop Register (both described previously) the

Current Page Register and the Boundary Pointer Register

The Current Page Register points to the first buffer used to

store a packet and is used to restore the DMA for writing

status to the Buffer Ring or for restoring 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 ever

reaches the Boundary reception is aborted The Boundary

Pointer is also used to initialize the Remote DMA for remov-

ing a packet and is advanced when a packet is removed A

simple analogy to remember the function of these registers

is that the Current Page Register acts as a Write Pointer and

the Boundary Pointer acts as a Read Pointer

Note 1

At initialization the Page Start Register value should be loaded into

both the Current Page Register and the Boundary Pointer Register

Note 2

The Page Start Register must not be initialized to 00H

Receive Buffer Ring At Initialization

TLF8582 – 30

BEGINNING OF RECEPTION

When the first packet begins arriving the NIC begins storing

the packet at the location pointed to by the Current Page

allow room for storing receive status corresponding to this

packet

Received Packet Enters Buffer Pages

TLF8582 – 31

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 maximal 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 com-

parisons The first comparison tests for equality between

the DMA address of the next buffer and the contents of the

Page Stop Register If the buffer address equals the Page

Stop Register the buffer management logic will restore the

DMA to the first buffer in the Receive Buffer Ring value

programmed in the Page Start Address Register The sec-

ond comparison tests for equality between the DMA ad-

dress of the next buffer address and the contents of the

Boundary Pointer Register If the two values are equal the

reception is aborted The Boundary Pointer Register 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 the Boundary Pointer or Page Stop Address

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

the Boundary Pointer If neither are reached the DMA is

allowed to use the next buffer

Linking Receive Buffer Pages

1) Check for e to PSTOP

2) Check for e to Boundary

TLF8582 – 32

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