BIOS Overview
product description 33
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 started.
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 F4.
NOTE 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
0Fh Initialize the local bus IDE
10h Initialize Power Management
11h Load alternate registers with initial POST values
12h Restore CPU control word during warm boot
13h Initialize PCI Bus Mastering devices
14h Initialize keyboard controller
17h Initialize cache before memory autosize
18h 8254 timer initialization
1Ah 8237 DMA controller initialization
1Ch Reset Programmable Interrupt Controller
24h Set ES segment register to 4GB
26h Enable A20 line
28h Autosize DRAM
29h Initialize POST Memory Manager
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