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66 Intel 8255x 10/100 Mbps Ethernet Controller Family Open Source Software Developer Manual
Host Software Interface
Bits 5:0 - Byte Count. The byte count indicates the number of Command Block bytes to be
configured (and is always included in the count). It allows changing some of the parameters by
specifying a byte count less than the maximum number of configuration bytes (22 bytes). The
first eight bytes are used by the CU, and the remaining bytes are passed to the CSMA/CD unit
through the transmit DMA. The value permitted is 8 bytes.
Default - none.
Recommended -16h.
Note: If a runtime algorithm for Adaptive IFS is implemented, it is recommended that software issue an 8
byte configure command. If any of the first 8 bytes needs to be re-configured and the last 14 bytes
do not need to be changed, then it is more appropriate to use an 8 byte configure command. This is
a more efficient way of re-configuring the device.
• BYTE 1.
— Bits 6:4 - Transmit FIFO Limit. The transmit FIFO limit specifies the number of bytes
located in the 64 byte dual-ported transmit FIFO at which the device requests the bus in
order to transfer data from system memory to its internal transmit FIFO. The transmit
FIFO is organized in 32-bit wide Dwords. The FIFO limit programming is showed in the
table below.
Default - 0.
Recommended - 0.
— Bits 3:0 - Receive FIFO Limit. The receive FIFO limit specifies the number of bytes
located in the dual-ported receive FIFO at which the device requests the bus in order to
transfer data from its internal receive FIFO to system memory. The dual-ported receive
FIFO is organized into 32-bit wide Dwords. For the 82557, the FIFO size is 64 bytes. For
the 82558 and 82559 the FIFO is 128 bytes. The FIFO limit programming is showed in
the table below.
Default - 8.
Recommended - The default value is fine. However, lower values will result in better PCI
efficiency, whereas higher values will result in lower latencies.
NOTE: The configuration values are from 0 through Fh. The table shows the values in binary (4 bits wide).
Table 41. 82557 Dual-Port FIFO Settings - Transmit
Configuration Value Transmit FIFO Limit
Binary (Transmit Bits 6:4) Dwords Bytes
0 0 0 0 0
a
a. This line represents the default values.
0 0 1 1 4
0 1 0 2 8
0 1 1 3 12
1 0 0 4 16
1 0 1 5 20
1 1 0 6 24
1 1 1 7 28