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Chapter 4
Checkpoints
A checkpoint is either a byte or word value output to I/O port 80h.The BIOS outputs checkpoints throughout
bootblock and Power-On Self Test (POST) to indicate the task the system is currently executing. Checkpoint
sare very useful in aiding software developers or technicians in debugging problems that occur during the pre-
boot process.
Viewing BIOS checkpoints
Viewing all checkpoints generated by the BIOS requires acheckpoint card, also referred to as a POST card or
POST diagnostic card. These are ISA or PCI add-in cards that show the value of I/O port 80h on a LED
display. Checkpoints may appear on the bottom right corner of the screen during POST. This display method
islimited, since it only displays checkpoints thatoccur after the video card has been activated.
Bootblock Initialization Code Checkpoints
The Bootblock initialization code sets up the chipset,memory, and other components before system memory is
available. The following table describes the type of checkpoints that may occur during the bootblock
initialization portion of the BIOS.
NOTE:
Please note that checkpoints may differ between different platforms based on system
configuration.Checkpoints may change due to vendor requirements,system chipset or option ROMs
from add-in PCI devices.
Checkpoint
Description
Before D1
Early chipset initialization is done. Early super I/O initialization is done includingRTC and
keyboard controller. NMI is disabled.
D0
Go to flat mode with 4GB limit and GA20 enabled. Verify the bootblock checksum.
D1
Perform keyboard controller BAT test. Check if waking up from power managementsuspend
state. Save power-onCPUID value in scratch CMOS.
D2
Disable CACHE before memory detection. Execute full memory sizing module.Verify that flat
mode is enabled.
D3
If memory sizing module not executed, start memory refresh and do memory sizingin
Bootblock code. Do additional chipsetinitialization. Re-enable CACHE. Verifythat 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 toit. BIOS
now executes out of RAM.
D6
Both key sequence and OEM specific method is checked to determine if BIOSrecovery is
forced. Main BIOS checksum is tested. If BIOS recovery is necessary,control flows to
checkpoint E0. See Bootblock Recovery Code Checkpoints sectionfor 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.
DA
Restore CPUID value back into register. Give control to BIOS POST (ExecutePOSTKernel).
See POST Code Checkpoints section of document for more information.
