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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 MRC is part of reference BIOS code, which relates to memory initialization in the BIOS. It includes information about memory settings, frequency, timing, driving and detailed operations of the memory controller. The MRC is written in a C-language code, which can be edited and compiled by board makers. It provides a space to develop advanced ...
Later computers, including most IBM-compatibles with 80286 CPUs, had a battery-backed nonvolatile BIOS memory (CMOS RAM chip) that held BIOS settings. [48] These settings, such as video-adapter type, memory size, and hard-disk parameters, could only be configured by running a configuration program from a disk, not built into the ROM.
In most cases, there is a special optional procedure for accessing BIOS parameters, to view and potentially make changes in settings. It may be possible to control how the computer uses the memory SPD data—to choose settings, selectively modify memory timings, or possibly to completely override the SPD data (see overclocking).
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.
At this point, the BIOS or operating system will program the memory-mapped addresses and I/O port addresses into the device's BAR configuration registers. These addresses stay valid as long as the system remains turned on. Upon power-off, these settings are lost and the procedure is repeated next time the system is powered back on.
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Furthermore, on a modern x86 system, BIOS calls can only be performed in Real mode, or Virtual 8086 mode. v8086 is not an option in Long mode. This means that a modern operating system, which operates in Protected mode (32 bit), or Long mode (64 bit), would need to switch into real mode and back to call the BIOS - a hugely expensive operation.