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4 BIOS Overview
Order in Which the POST Tests are Performed
Order in Which the POST Tests are Performed
Each time the system is powered on, or a reset is performed, the POST is
executed. The POST process verifies the basic functionality of the system
components and initializes certain system parameters.
The POST starts by displaying a graphic screen of the Hewlett-Packard logo
when the PC is restarted.
Devices, such as memory and newly installed hard disks, are configured
automatically. The user is not requested to confirm the change. Newly
removed hard disks are detected, and the user is prompted to confirm the
new configuration by pressing . Note, though, that the POST does not
detect when a hard disk drive has been otherwise changed.
During the POST, the BIOS and other ROM data is copied into high-speed
shadow RAM. The shadow RAM is addressed at the same physical location as
the original ROM in a manner which is completely transparent to applications.
It therefore appears to behave as very fast ROM. This technique provides
faster access to the system BIOS firmware.
The following table lists the POST checkpoint codes written at the start of
each test.
Checkpoint
Code
POST Routine Description
02h Verify Real Mode
03h Disable Non-Maskable Interrupt (NMI)
04h Get CPU type
06h Initialize system hardware
08h Initialize chipset with initial POST values
09h Set IN POST flag
0Ah Initialize CPU registers
0Bh Enable CPU cache
0Ch Initialize caches to initial POST values
0Eh Initialize I/O component
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