Intel 82559 用户手册

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The following sequence is performed by the CU at the completion of execution of an action 
command:

1. The devices writes command specific status to the status word of the current CB (usually the C 

and OK bits are written to). If the command is a transmit command, the C and OK bits are 
updated when the last transmit buffer DMA has completed, not after the actual transfer of the 

frame on the serial link. This allows the transmit buffer resources to be returned as soon as the 

data is copied into the device internal transmit FIFO instead of waiting for actual transmission 
on the wire. Transmit status is kept in the transmit statistical counters of the device.

2. If the I bit is set, the device sets a request for the CX interrupt.

3. If the EL bit is set, after completion of the command the CU becomes idle. If the S bit is set, 

the CU becomes suspended. Otherwise, the CU requests the beginning of the next action 

command. A transition from an active to suspended state also generates a CNA interrupt if the 

device is configured to do so.

4. The device updates the status word in the SCB. (In step 1, the transmit command status is 

actually set at the end of transmit DMA, not at the completion of the actual transmit 

command.)

This command results in no action by the device except for those performed in the normal 

command processing. It is used to manipulate the CBL. The NOP command format is shown 
below.

00h

EL

S

I

0000000000

000

C

X

OK XXXXXXXXXXXXX

04h

Link Address (A31:A0)

This is the 32-bit address of the next command block. It is added to the CU base to 
obtain the actual address.

If this bit is set to one, it indicates that this command block is the last one on the CBL. 
The CU will go from the active to the idle state after the execution of the CB is finished. 
This transition will always cause an interrupt with the CNA/CI bit set in the SCB.

If this bit is set to one, the CU will be suspended after the completion of this CB. A CNA 
interrupt will be generated if the device is configured for this. The CU transitions from the 
active to the suspended state after the execution of the CB.

If the I bit is set to one, the device generates an interrupt after the execution of the CB is 
finished. If I is not set to one, the CX interrupt will not be generated.

These bits are reserved and should all be set to 0.