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110 Intel 8255x 10/100 Mbps Ethernet Controller Family Open Source Software Developer Manual
Host Software Interface
6.5.4.2 RU Resume Command
The RU Resume (RU_RESUME) command resumes frame reception. The RU performs the
following tasks:
1. The RU goes to the ready state and configures a new RFD if the RU is in the suspended state
and not actively discarding a frame.
2. The RU sets the RU resume request flag if the RU is in the suspended state and the device is
discarding a frame.
3. The RU ignores the command if it is not in the suspended state.
6.5.4.3 RU Abort Command
The RU abort (RU_ABORT) command immediately stops frame reception and enters the idle state.
1. The RU requests an RNR interrupt if the RU is in the ready state.
2. The RU stops all DMA activity and starts discarding incoming data.
3. The device transitions the RU to the idle state.
6.5.5 Updating SCB Status
The device updates the SCB status word when any of the following events occur.
1. When a control command is accepted. (The 8255x updates the SCB status and clears the SCB
command word to indicate that acceptance has completed.)
2. When the CU or RU changes state.
3. After receiving a frame.
4. When a command with its I bit set is completed.
Note: The device may first clear the SCB command byte and return at a later time to update new status in
the SCB status byte.
6.6 Flow Control
The 82557 does not have any support for flow control. However, the 82558 and later generation
controllers support frame based (IEEE) flow control (FC). In addition, the 82558 also supports a
proprietary PHY based mechanism for flow control (known as Bay flow control).
Flow control is implemented as a means of reducing the possibility of receive buffer overflows,
which can result dropped received packets, and allows local control of network congestion levels.
This can be accomplished by sending an indication to a transmitting station of an almost full
receive buffer condition at a receiving station.