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Diagnostic POST code LEDs
During the system boot process, BIOS executes a number of platform configuration processes, each of which is assigned a specific hex code number. As each configuration routine is started, BIOS will display the given POST code to the diagnostic POST code LEDs found on the rear of the mainboard. To assist in troubleshooting a system hang during the POST process, the diagnostic LEDs can be used to identify the last POST process to be executed.
Each POST code will be represented by a combination of colors from the four LEDs. The LEDs are capable of displaying three colors: green, red, and amber. Each bit in the upper nibble is represented by a red LED and each bit in the lower nibble is represented by a green LED. If both bits are set in the upper and lower nibbles then both red and green LEDs are lit, resulting in an amber color. If both bits are clear, then the LED is off.
Diagnostic LED placement diagram
USB Port | USB Port | Diagnostic LEDs | |
Back edge of server board |
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| MSB | LSB |
The following table lists the possible POST codes that may be displayed on the Diagnostic POST code LEDs during system initialization.
| Diagnostic LED Decoder |
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Checkpoint | G=Green, R=Red, A=Amber | Description | |||
code |
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| MSB |
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| LSB |
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Host Processor |
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0x10h | OFF | OFF | OFF | R | |
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| processor (bootstrap processor) |
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0x11h | OFF | OFF | OFF | A | Host processor cache initialization |
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| (including AP) |
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0x12h | OFF | OFF | G | R | Starting application processor |
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| initialization |
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