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Chapter 4
Power-On Self-Test (POST)
Each time you turn on the system, the Power-on Self Test (POST) is initiated. Several items are tested during
POST, but is for the most part transparent to the user.
The Power-On Self Test (POST) is a BIOS procedure that boots the system, initializes and diagnoses the
system components, and controls the operation of the power-on password option. If POST discovers errors in
system operations at power-on, it displays error messages on screen, generates a check point code at port
80h or even halts the system if the error is fatal.
NOTE:
When Post executes a task, it uses a series of preset numbers called check points to belatched atport
80h, indicating the stages it is currently running. This latch can be read and shown on a debug
board.The following table describes the BIOS common tasks carried out by POST. Each task is denoted
by an unique check point number. For other unique check point numbers that are not listed in the table,
refer to the corresponding product service guide.
Post Checkpoints List: The list may vary accordingly depending on your BIOS
Bootblock Initialization Code Checkpoints
Checkpoint
Description
Before D0
If boot block debugger is enabled, CPU cache-as-RAM functionality is enabled at this
point. Stack will be enabled from this point.
D0
Early Boot Strap Processor (BSP) initialization like microcode update, frequency and other
CPU critical initialization. Early chipset initialization is done.
D1
Early super I/O initialization is done including RTC and keyboard controller. Serial port is
enabled at this point if needed for debugging. NMI is disabled. Perform keyboard controller
BAT test. Save power-on CPUID value in scratch CMOS. Go to flat mode with 4GB limit
and GA20 enabled.
D2
Verify the boot block checksum. System will hang here if checksum is bad.
D3
Disable CACHE before memory detection. Execute full memory sizing module. If memory
sizing module not executed, start memory refresh and do memory sizing in Boot block
code. Do additional chipset initialization. Re-enable CACHE. Verify that flat mode is
enabled.
D4
Test base 512KB memory. Adjust policies and cache first 8MB. Set stack.
D5
Bootblock code is copied from ROM to lower system memory and control is given to it.
BIOS now executes out of RAM. Copies compressed boot block code to memory in right
segments. Copies BIOS from ROM to RAM for faster access. Performs main BIOS
checksum and updates recovery status accordingly.
D6
Both key sequence and OEM specific method is checked to determine if BIOS recovery is
forced. If BIOS recovery is necessary, control flows to checkpoint E0. See Bootblock
Recovery Code Checkpoints section of document for more information.
D7
Restore CPUID value back into register. The Bootblock-Runtime interface module is
moved to system memory and control is given to it. Determine whether to execute serial
flash.
D8
The Runtime module is uncompressed into memory. CPUID information is stored in
memory.
D9
Store the Uncompressed pointer for future use in PMM. Copying Main BIOS into memory.
Leaves all RAM below 1MB Read-Write including E000 and F000 shadow areas but
closing SMRAM.
