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What you need to know about BIOS and CMOS.


BIOS and CMOS are two computing terms that cause a lot of confusion. This is largely because they are used interchangeably, while at other times they are used colloquially to refer to other things. The easy way to understand what these terms mean is to have a hardware crash course.



Just as throwing a bunch of ingredients into a pot cannot make a meal, likewise throwing computer components in a box does not make a PC. The CPU needs a way to communicate with all the components that has been put together. Traditionally this communication was achieved through a chip called the Northbridge (the bridge part is because the chip acted as a bridge between the CPU and the rest of the computer. Still trying to figure out where they got the north from). But since this would have been too much for just one chip, the wise guys of the PC industry decided to shift some responsibility to another chip called the Southbridge. Together both North and South bridges are called “the chipset.” This is no longer the case with modern PCs. In modern PCs the Northbridge has been merged into the CPU while the Southbridge is called the Input/Output Controller Hub in Intel systems and Fusion Controller Hubin AMD systems. But for easy understanding let’s just go with chipset.

  • BIOS

The chipset connects to every device on the PC (including the CPU) via two channels, the data bus and address bus. The CPU uses these channels through the chipset to pass data and commands to every device in the PC. But this begs the question of how does the CPU know how to talk to devices using the buses. It does this by special programming loaded into memory that teaches it about a device, this programming is called basic input/output services (BIOS). To reiterate, the programs that the CPU uses to talk to devices are called services. These programs are stored on a special memory chip on the motherboard called read-only memory (ROM). Traditionally, read-only memory meant whatever was stored couldn’t be changed. But modern motherboards use a type of ROM that allows data on it to be changed through a process called “flashing the ROM”, hence they’re called flash ROM. Every motherboard has a flash ROM chip that enables the CPU talk to the basic hardware of the PC, this chip is also known as the system ROM. To talk to all the basic hardware requires hundreds of services (little blocks of code), these services stored on the ROM chip are collectively known as system BIOS. Note that any program stored on a ROM chip is also referred to as firmware.


  • CMOS

So what’s the relation between BIOS and CMOS? The system BIOS has two types of hardware to support, hardware that doesn’t change (like the keyboard controller) and hardware that changes from time to time (like RAM or HDD). For the latter, the system needs a place to store the specific details of the devices, this enables it to differentiate say between a HDD and a SSD. A separate memory chip called the complementary metal-oxide semiconductor (CMOS), stores the information that describes specific device parameters. Note that CMOS does not store programs, it only stores data that is read by the BIOS to complete the programs needed to talk to changeable hardware. Another thing to note is that CMOS acts like a clock keeping the current date and time.

  • System Setup Utility

Every PC comes with a program built into the system ROM called the System Setup Utility which enables access and modification of the CMOS data. This is where all the confusion is as this setup utility is often referred to as CMOS or even BIOS. You might have heard someone say “enter BIOS and make the necessary changes.” Technically this is incorrect but colloquially acceptable. Other terms you might hear are CMOS setup program, CMOS, BIOS setup utility, UEFI/BIOS setup, or UEFI firmware settings. Just know they are all referring to the same thing.


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