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Nonvolatile BIOS memory refers to a small memory on PC motherboards that is used to store BIOS settings. It is traditionally called CMOS RAM because it uses a volatile, low-power complementary metal–oxide–semiconductor (CMOS) SRAM (such as the Motorola MC146818 [1] or similar) powered by a small battery when system and standby power is off. [2]
The BIOS also updates the ESCD each time the hardware configuration changes, after deciding how to re-allocate resources like IRQ and memory mapping ranges. After the ESCD has been updated, the decision need not be made again, which thereafter results in faster startup without conflicts until the next hardware configuration change.
This implies that after the hardware reset, the CPU will start execution at the physical address 0xFFFF0. In IBM PC compatible computers, This address maps to BIOS ROM. The memory word at 0xFFFF0 usually contains a JMP instruction that redirects the CPU to execute the initialization code of BIOS. This JMP instruction is absolutely the first ...
In later computer systems, the BIOS contents are stored on flash memory so it can be rewritten without removing the chip from the motherboard. This allows easy, end-user updates to the BIOS firmware so new features can be added or bugs can be fixed, but it also creates a possibility for the computer to become infected with BIOS rootkits.
The BIOS begins its POST when the CPU is reset. The first memory location the CPU tries to execute is known as the reset vector . In the case of a hard reboot , the northbridge will direct a code fetch request to the BIOS located on the system flash memory .
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Later the decreased cost of the CMOS technology allowed the same devices to be fabricated using it, adding the letter "C" to the device numbers (27xx(x) are n-MOS and 27Cxx(x) are CMOS). While parts of the same size from different manufacturers are compatible in read mode, different manufacturers added different and sometimes multiple ...
Examples of computer firmware include: The BIOS firmware used on PCs; The (U)EFI-compliant firmware used on Itanium systems, Intel-based Macs, and many newer PCs; Hard disk drive, solid-state drive, optical disc drive and optical disc recorder firmware [5] Video BIOS of a graphics card