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58 Intel 8255x 10/100 Mbps Ethernet Controller Family Open Source Software Developer Manual
Host Software Interface
6.4.1.1 General Action Command Format
The format common to all action commands and the algorithms for beginning and completing the
execution (also common to all action commands) is described below.
The general format of the Command Block (CB) includes the following fields:
6.4.1.1.1 Beginning Execution
An action command can be started by either the CU start or CU resume control command.
Otherwise, it may follow a previous action command. However, the actual command start may be
delayed by RU activity.
The following sequence is performed by the CU at the beginning of execution of each action
command:
1. The device reads 4 Dwords of the CB in one continuous PCI burst (if possible).
2. The device analyzes the contents of the command word to determine the necessary action.
3. The device reads and analyzes the link offset of the next CB and saves it.
4. The device performs specific actions according to the action command specified in the current
command field.
If the commands are chained, the CU prefetches the next command block by accessing the address
specified in the link offset field of the current CB. The device reads, analyzes, and saves the
command word of the next CB (the following fields are saved for later use: EL, S, I and CMD).
6.4.1.1.2 Completing Execution
Command completion time is asynchronous to the beginning of the command. It is determined by
the command type, RU activity, CU control commands, bus latency, etc. The CU is always in the
active state at this time. The EL, S, and I bits determine the next actions.
100 Transmit
One transmit command is used to send a single frame. If more than one frame
needs to be sent, the host CPU can link multiple transmit commands together.
101 Load Microcode
This command downloads microcode to the device.
Note: Documentation for microcode is beyond the scope of this manual.
110 Dump
This command causes the device to dump its internal registers into memory.
The registers included are those loaded by the configure and address setup
commands, plus status and other internal working registers.
111 Diagnose
The diagnose command puts the device CSMA/CD subsystem through a self-
test procedure and reports the result of the internal test.
Table 37. Operation Codes
Opcode Name Description
Figure 14. General Action Command Format
Offset Command Word Bits 31:16 Status Word Bits 15:0
00h EL S I 0000000000 CMD C X OK XXXXXXXXXXXXX
04h Link Offset
08h Optional Address and Data Fields