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22 Intel 8255x 10/100 Mbps Ethernet Controller Family Open Source Software Developer Manual
PCI Interface
Table 7. Generated PCI Commands
The controllers do not generate I/O commands, Interrupt Acknowledge cycles, or Configuration
cycles. The controllers also do not support Dual Address Cycle (DAC). Targets (typically the
system bridge) must respond to all of the commands that the Ethernet controller generates.
4.2.1 Memory Write and Invalidate
The 82558, 82559, 82550, and 82551 have 4 internal DMA channels. Of these 4, the Receive DMA
channel is used to deposit packet data received from the link into system memory. The Receive
DMA channel uses both the Memory Write (MW) and the Memory Write and Invalidate (MWI)
commands. In order to use MWI the device must guarantee:
• A minimum transfer of one cache line.
• All byte-enable bits are active during each MWI access.
• The device may cross a cache line boundary only if it intends to transfer the entire next cache
line too.
In order to ensure the above conditions, the device may use the MWI command only if the
following conditions hold:
1. The cache line size written in the CLS register during PCI configuration is 8 or 16 Dwords.
2. The accessed address is cache line aligned.
3. The 82558 or 82559 has at least a cache line size of data byte in its Receive FIFO.
4. There is at least a cache line size of space left in the system memory buffer. In addition, the
device will use two configuration bits to enable and disable the use of MWI:
a. MWI Enable bit in the PCI Configuration Command register (Section 4.1, “PCI
Configuration Space”).
b. MWI Enable bit in the device Configure command (Section 6.4.2.3, “Configure (010b)”).
If any one of these conditions does not hold, the device uses the MW command. If a MWI cycle is
started and one of the conditions does not hold any more (for example, the data space in the
memory buffer is less than the CLS), then the device terminates the MWI cycle at the end of the
cache line. The next cycle is a MWI or MW cycle according to the conditions listed above.
If a MWI cycle is terminated by a Retry from the target, the device attempts to retry the access
using the MWI command. If a MWI cycle is terminated in the middle of a cache line by a
disconnect from the target (including Disconnect-C), the device issues a new cycle from the
disconnected point using the MW command. If the disconnect occurs on a cache line boundary, the
PCI Command Name Circumstance Used
0x6 MR TxCB “S” bit read.
0x7 MW
CB and RFD. Writing statistics counters or dump data
buffer to memory. Writing received packet data into
receive buffers.
0xC MRM Reading transmit data buffers.
0xE MRL CB, TBD, and RFD.
0xF
MWI (82558 &
82559)
Writing received packet data into receive buffers.