LAN9118MT Standard Microsystems (SMSC), LAN9118MT Datasheet - Page 58

• Manufacturer Part Number: LAN9118MT
• Manufacturer: Standard Microsystems (SMSC)

High-Performance Single-Chip 10/100 Non-PCI Ethernet Controller
Datasheet
Additionally, The LAN9118 has specific rules regarding the use of transmit buffers when in Store-and-
Forward mode (i.e., HW_CFG[SF] = 1). When this mode is enabled, the total space consumed in the
TX FIFO (MIL) must be limited to no more than 2KB - 3 DWORDs (2,036 bytes total). Any transmit
packet that is so highly fragmented that it takes more space than this must be un-fragmented (by
copying to a Driver-supplied buffer) before the transmit packet can be sent to the LAN9118.
One approach to determine whether a packet is too fragmented is to calculate the actual amount of
space that it will consume, and check it against 2,036 bytes. Another approach is to check the number
of buffers against a worst-case limit of 86 (see explanation below).
4.12.3.2
Calculating Worst-Case TX FIFO (MIL) Usage
The actual space consumed by a buffer consists only of any partial DWORD offsets in the first/last
DWORD of the buffer, plus all of the whole DWORDs in between. Any whole DWORD offsets and/or
alignments are stripped off before the buffer even gets into the TX data FIFO, and TX command words
are stripped off before the buffer is written to the TX FIFO, so none of those DWORDs count as space
consumed. The worst-case overhead for a TX buffer is 6 bytes, which assumes that it started on the
high byte of a DWORD and ended on the low byte of a DWORD. A TX packet consisting of 86 such
fragments would have an overhead of 516 bytes (6 * 86) which, when added to a 1514-byte max-size
transmit packet (1516 bytes, rounded up to the next whole DWORD), would give a total space
consumption of 2,032 bytes, leaving 4 bytes to spare; this is the basis for the "86 fragment" rule
mentioned above.
Revision 1.0 (03-17-05)
58
SMSC LAN9118
DATASHEET