MSI MS-9282 Dokumentacja Pobierz pdf (Strona 3)

mid-1990s, it became typical for the BIOS ROM to include a "BIOS configuration utility" or "BIOS
setup utility", accessed at system power-up by a particular key sequence. This program allowed the
user to set system configuration options, of the type formerly set using DIP switches, through an
interactive menu system controlled through the keyboard. In the interim period, IBM-compatible PCs
—including the IBM AT—held configuration settings in battery-backed RAM and used a bootable
configuration program on disk, not in the ROM, to set the configuration options contained in this
memory. The disk was supplied with the computer, and if it was lost the system settings could not be
changed.
A modern Wintel-compatible computer provides a setup routine essentially unchanged in nature from
the ROM-resident BIOS setup utilities of the late 1990s; the user can configure hardware options using
the keyboard and video display. Also, when errors occur at boot time, a modern BIOS usually displays
user-friendly error messages, often presented as pop-up boxes in a TUI style, and offers to enter the
BIOS setup utility or to ignore the error and proceed if possible. Instead of battery-backed RAM, the
modern Wintel machine may store the BIOS configuration settings in flash ROM, perhaps the same
flash ROM that holds the BIOS itself.
Because it is the only visible feature of the BIOS to the average user, who is not familiar with
hardware programming techniques and device abstraction layers, he often misidentifies the BIOS
configuration utility as the BIOS, as in typical statements such as, "To add a second internal hard disk,
you have to go into your BIOS and enable it," or, "To change the boot password you have to use the
BIOS." This misuse of terms has become so common that it may now be considered that "BIOS
configuration utility" or "BIOS setup menu" is a new second definition of the word "BIOS".
Operation
CPU reset
When an x86 microprocessor is reset, it loads its program counter with a fixed address near the top of
the one-megabyte address space assignable in real mode. (Depending on the microprocessor model, it
may also assert all of the address lines above the first 20, causing the real-mode address space to be
mapped into the last megabyte of the physical address space, until the first FAR jump or call.) The
address of the BIOS's memory is located such that it will be executed upon processor reset. There are
only a few bytes in memory after the reset startup address, so a jump instruction near the end of the
BIOS ROM directs the processor to start executing the BIOS startup code at an earlier address.
If the system has just been powered up or the reset button was pressed ("cold boot"), the full power-on
self-test (POST) is run. If Ctrl+Alt+Delete was pressed ("warm boot"), a special flag value is stored in
nonvolatile BIOS memory before the processor is reset, and after the reset the BIOS startup code
detects this flag and does not run the POST. This saves the time otherwise used to detect and test all
memory. The non-volatile RAM (NVRAM) is in the real-time clock (RTC).
POST
The POST checks, identifies, and initializes system devices such as the CPU, RAM, interrupt and
DMA controllers and other parts of the chipset, video display card, keyboard, hard disk drive, optical
disc drive and other basic hardware.
Early IBM PCs had a little-known routine in the POST that would attempt to download a program
from into RAM through the keyboard port. (Note that no serial or parallel ports were standard on early
IBM PCs, but a keyboard port of either the XT or AT / PS/2 type has been standard on practically
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