Why is protected mode needed in addition to compatibility mode in Intel x86 64 bit CPUs?

Question

I'm reading intel software developer manual (section 3.1). Here is said that

The IA-32 architecture supports three basic operating modes: protected mode, real-address mode, and system management mode.

and

Intel 64 architecture adds IA-32e mode. IA-32e mode has two sub-modes.

This two sub-modes is Compatibility mode and 64-bit mode. Thus Intel 64 architecture contains 4 operating modes (protected mode, real-address mode, system management mode, IA-32e mode). But I don't understand for what protected mode is needed in Intel 64? Compatibility sub-mode of IA-32e permit to run legacy 16- and 32-bits application without re-compilation to 64-bit OS and 64-bit sub-mode permits OS to run application which access to 64-bit linear address space. Where protected mode is used when I'm using 64-bit OS? Application running ensured by IA-32e mode.

Minaret

It was a drop-in replacement for machines that boot an operating system that never heard of IA-32e mode. Backwards compatibility is very important to get a new chip design to start selling quickly. – Lated

In fact, you lose one, the V8086 mode which is not supported in IA-32e mode. I suggest you review the x86 & Windows history, and then you'll get the answer. – Tomsk

Normally a 64-bit OS doesn't use protected-mode at all, after booting. Pure protected mode is for 32-bit OSes. – Loughlin

Answer 1

I believe the organization is like this. At any given time, the entire CPU must be in precisely one of the following modes (excluding the maintenance mode):

IA-32 Real Mode: This is how the processor powers on.

IA-32 Protected Mode: 32-bit execution environment. The usual protection mechanisms are in play; the CPU can selectively move into virtual-8086 mode.

IA-32e Mode: This is only available on 64-bit CPUs. The CPU can selectively move between compatibility and long mode.

That is, if you want to execute any 64-bit code, you need to go into IA-32e mode. You can now decide on a per-segment basis whether to execute 32-bit code in compatibility mode or 64-bit code in long mode.

Gober