What actually is real mode?

Real mode is the default operating mode of x86 CPUs immediately after boot.

It exists mainly for backward compatibility with extremely old software written for the original x86 processors. In this mode, the CPU behaves similarly to the old Intel 8086.

That means:

• Only 16-bit registers are directly accessible
• Memory access is heavily restricted
• There is no memory protection
• Advanced CPU features are unavailable
• Only around 1 MB of RAM can be addressed

Why Do Modern CPUs Still Use It?

Mostly because of compatibility.

A huge amount of old software depended on the original x86 boot behavior. Completely removing real mode would break decades of assumptions about how PCs start up.

So modern x86 CPUs still begin in real mode before the operating system switches into something more modern like protected mode