AX88796 datasheet(23/71 Pages) ASIX

23 / 71 page

ASIX ELECTRONICS CORPORATION

AX88796 L

3-in-1 Local Bus Fast Ethernet Controller

INITIALIZATION OF THE BUFFER RING

Two static registers and two working registers control the operation of the Buffer Ring. These are the Page Start

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 removing 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.

Buffer #1

Buffer #2

Buffer #3

…

Buffer #n

Physical Memory Map

Logic Receive Buffer Ring

Fig - 9 Receive Buffer Ring At Initialization

BEGINNING OF RECEPTION

When the first packet begins arriving the AX88796 and begins storing the packet at the location pointed to by the

Current Page Register. An offset of 4 bytes is reserved in this first buffer to allow room for storing receive status

corresponding to this packet.

LINKING RECEIVE BUFFER PAGES

If the length of the packet exhausts the first 256 bytes 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 comparisons. The first

comparison tests for equality between the DMA address of the next buffer and the contents of the Page Stop

to the first buffer in the Receive Buffer Ring value programmed in the Page Start Address Register. The second

comparison test for equality between the DMA address 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

4000h

8000h

Page Start

Page Stop

…

n-2

n-1

Boundary Page

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