This is purely academic, out of date, out of curiosity. Let's back to 1990s, before the Windows. The BIOS remapped the INT 08H+ for their own interrupt handler, for instance, int 10H for VGA MODE switch, int 13H for hard driver.
But, as we can see in Intel® 64 and IA-32 Architectures Software Developer Manuals, we can know that, there are also many real-address exception that cans cause interrupt. Though, these instructions of their triggers are rare, but it still exists and useable which means these exceptions are always possible.
So now, MS-DOS will not cover the BIOS's interrpt vectors.
And if a programmer intentionally or unintentionally causes CPU 's default interrupt, for instance, PRINT SCREEN and BOUND Range Exceeded both caused INT 5, which means, you can only handle one interrupt. what's more, IRQ makes if you want to use keyboard you can't trigger those interrupts.
As so many intel's default interrupt is remapped by the bios, things became unreasonable.
So, what's inside it in those years?
BTW I also asked in stackoverflow, I don't know how to close that.