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Chapter 4 55
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. Checkpoints
are 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 a checkpoint 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 is
limited, since it only displays checkpoints that occur 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 including
RTC 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 management
suspend state. Save power-on CPUID 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 sizing
in Bootblock 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.
D6 Both key sequence and OEM specific method is checked to determine if BIOS
recovery is forced. Main BIOS checksum is tested. If BIOS recovery is necessary,
control flows to checkpoint E0. See Bootblock Recovery Code Checkpoints section
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.
DA Restore CPUID value back into register. Give control to BIOS POST
(ExecutePOSTKernel). See POST Code Checkpoints section for more information.