Standard IBM PC BIOS
This section refers to how option ROMs are used with standard IBM PC compatible BIOSes (Most desktop Computers.)
A common example of an option ROM is the Video BIOS found on all standard PC video cards. This is a special case of an option ROM, as it is loaded very early on in the boot process (otherwise nothing would appear on the screen during the POST.) It is usually easy to tell when this particular ROM is initialized, as it is the first time something appears on the monitor since the computer was switched on. Unless working with an embedded system, the video BIOS is almost always located in the C0000 memory segment (i.e. the very start of the memory area reserved for option ROMs.) Other ROMs can be located from segments
C8000 all the way up to
E0000. The BIOS Boot Specification (BBS) requires that option ROMs be aligned to 2kB boundaries (e.g. segments
C8000, C8800, C9000, C9800, etc.).
After the basic POST checks are complete, the option ROMs are normally initialized. This gives them the chance to hook (intercept) system interrupts, in order to provide increased functionality to the system.
Read more about this topic: Option ROM
... Since these two methods don't offer a large amount of control over the boot process, the BIOS Boot Specification (BBS) was developed ... at the start of the boot process, it could inform the BIOS about its functionality, and then it could be called later on in the boot process depending on the order the user had selected ...
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