(Though, technically, that last one is an OS problem that got patched in the BIOS instead.) However, that's also not the only workaround that was done in practice, because many machines had onboard IDE controllers with the BIOS only capable of addressing up to 504MiB (528 MB), and then 2GiB, and then 4GiB. Firmware neglecting to whitelist which PCIe lanes will have their option ROMs initialized.) That's where some of the Thunderbolt/USB4 vulnerabilities came from. (And option ROMs are still a thing, even with UEFI. That's how you get things like the IBM EGA card retrofitting new int 10h services that wouldn't be built into the BIOS until the IBM AT and the 1986 revision of the IBM XT BIOS, network cards adding netboot support, or hard drive controller cards adding support for things like booting off SCSI.
The IBM PC design makes a provision for option ROMs, which allow expansion cards to patch extensions into the BIOS.
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