82596DX Datasheet, PDF (55/77 Pages) Intel Corporation – HIGH-PERFORMANCE 32-BIT LOCAL AREA NETWORK COPROCESSOR

82596DX SX
Buffers on the receive side can be different lengths The 82596 will not place more bytes into a buffer than
indicated in the associated RBD The 82596 will fetch the next RBD before it is needed The 82596 will
attempt to receive frames as long as the FBL is not exhausted If there are no more buffers the 82596
Receive Unit will enter the No Resources state Before starting the RU the CPU must place the FBL pointer in
the RBD pointer field of the first RFD All remaining RBD pointer fields for subsequent RFDs should be ‘‘1s ’’ If
the Receive Frame Descriptor and the associated Receive Buffers are not reused (e g the frame is properly
received or the 82596 is configured to Save Bad Frames) the 82596 writes the address of the next free RBD
to the RBD pointer field of the next RFD
RECEIVE BUFFER DESCRIPTOR (RBD)
The RBDs are used to store received data in a flexible set of linked buffers The portion of the frame’s data
field that is outside the RFD is placed in a set of buffers chained by a sequence of RBDs The RFD points to
the first RBD and the last RBD is flagged with an EOF bit set to 1 Each buffer in the linked list of buffers
related to a particular frame can be any size up to 2¹⁴ bytes but must be word aligned (begin on an even
numbered byte) This ensures optimum use of the memory resources while maintaining low overhead All
buffers in a frame are filled with the received data except for the last in which the actual count can be smaller
than the allocated buffer space
Figure 39 RFA Flexible Memory Structure
290219 – 17
55