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36 Intel 8255x 10/100 Mbps Ethernet Controller Family Open Source Software Developer Manual
Host Software Interface
Note: The SCB Status word is not updated immediately in response to SCB commands. For example, the
CU status will remain in the idle state for a period of time after the CU start command is issued.
Software should not rely exclusively on the state of the SCB Status word to determine when it is
appropriate to issue commands requiring the device to be in a specific state. Software may be
required to keep an internal state engine on the commands recently issued to the device to insure
that data read from the register is valid.
6.3.2.2 SCB Command Word
The SCB Command word is also addressable as two bytes. The upper byte is called the Interrupt
Control byte. The least significant byte is called the Command byte.
The Interrupt Control byte allows software to either force the generation of an interrupt or mask
device interrupts from occurring. The 82558 and later devices also allow individual interrupt
sources from within the device to be masked (this feature is not available in the 82557).
Bits 7:6 CUS
This field contains the CU status (2 bits). Valid values are for this field are:
00 Idle
01 Suspended
10 LPQ Active
11 HQP Active
Bits 5:2 RUS
This field contains the RU status (4 bits). Valid values are:
0000 Idle
0001 Suspended
0010 No resources
0011 Reserved
0100 Ready
0101 Reserved
0110 Reserved
0111 Reserved
1000 Reserved
1001 Reserved
1010 Reserved
1011 Reserved
1100 Reserved
1101 Reserved
1110 Reserved
1111 Reserved
Bits 1:0 Reserved These bits are reserved and should not be used.
Figure 10. SCB Command Word
31 26 25 24 23 20 19 18 16
Specific Interrupt Mask Bits SI M CU Command 0 RU Command
Table 13. SCB Status Word Bits Descriptions
Bit Symbol Description