What is BIOS /Hard Drive

 Basic hard drive/BIOS

As soon as you flip that switch, your CPU starts executing shit located at F000:FFF0. This area contains the BIOS, Basic Input/Output System. This code is written in assembly and is stored in chips called EPROMs in your computer. This code will perform something known as POST, Power On Self Test. This checks for installed devices and checks if they all work. In particular it checks for the video card and runs the video
Breaking Through the BIOS Barrier: The Definitive BIOS Optimization Guide for PCs
What is Bios

BIOS usually located at C000h. Next it checks for other ROMs to see if they have installed BIOSes. Usually it then finds and executes the hard drive BIOS located at C8000h. Then it starts something like a "system inventory" where it checks for other installed devices and tests them. It does some more stuff that's all basicly useless for us right now, until it finally transfers control over to the operating system. That's the part that we're interested in. Back in the old days, only one OS was installed on a computer. If you bought a certain computer, you could only run the OS that was made for it. Nothing else. Obviosly that wasn't such a good thing as you would have to buy a new computer if you wanted a different OS, so BIOS makers came up with the Boot Sector. In case you didn't know System BIOS for IBM PCs, Compatibles, and EISA Computers (2nd Edition)yet, a Sector is the smallest area your hard drive can access. According to the ATA standards each sector is exactly 512 bytes. However ATA standards only apply to hard drives, things like floopies can use whatever they want. Knowing this we can move on to the boot sector.
What is BIOS /Hard Drive


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