When in "long mode", meaning a 64-bit OS, you can execute everything other than 16-bit protected mode code. You can still run 16-bit code that was designed for

When in "long mode", meaning a 64-bit OS, you can execute everything other than 16-bit protected mode code. You can still run 16-bit code that was designed for real mode. This means you could run some really old DOS programs, but not ones with a 16-bit DOS extender and not 16-bit Windows programs. Of course, a 64-bit OS could pop in and out of 64-bit mode as needed.