The long mode is activated through a control bit called LMA (Long Mode Active). When the LMA is inactivated, the processor will operate in the standard x86 mode and will be compatible to the operating systems and applications of 16 and 32 bits, that is, it will be compatible with everything that exists nowadays. When the LMA is activated (long mode), the 64-bit extension will be enabled, offering a new 64-bit CPU. The long mode is also divided into two sub-modes: the 64-bit mode and the compatible mode. These two sub-modes are controlled by the D and L bits, present at the descriptor pointed by the CS (Code Segment) register. The compatible mode is interesting because it allows, as a program, to run 16- or 32-bit applications inside the 64-bit mode. It is something similar to the virtual mode 86 of the 386 processors. Figure 1 shows an explanation of those modes.

	CS.L=0 CS.D=0	CS.L=0 CS.D=1	CS.L=1 CS.D=0	CS.L=1 CS.D=1
LMA=0 Legal Mode	Standard 16-bit Mode	Standard 32-bit Mode	Standard 16-bit Mode	Standard 32-bit Mode
LMA=1 Long Mode Active	16-bit Compatibility Mode	32-bit Compatibility Mode	64-bit Mode	Reserved

Figure 1: Operation modes of the x86-64 family.

The old x86 mode (32 or 16 bits) is called Legal Mode (LMA=0). When in this mode, the x86-64 CPUs can work with data of 16 or 32 bits. Notice that, when in this mode, the state of the CS.L bit has no meaning, that is, it is a don't care.

When put in 64-bit Long Mode (LMA=1, CS.L=1 and CS.D=0), the standard size of the operand is of 32 bits and the standard size for addressing is of 64 bits. With the use of instruction prefixes, the size of the operand can be altered to 64 or 16 bits and the size of the address to 32 bits.

If set in the Long Mode, Compatibility Mode (LMA=1 and CS.L=0), we have the binary compatibility with the applications written in x86 16 and 32 bits. That is really interesting, because an operating system in long mode can run the present 16- and 32-bit programs only by setting to zero the CS.L bit of the descriptor pointed by the code segment of those applications. In the compatible sub-mode, the CS.D bit continues to select between the 16- and 32-bit modes. It should also be noticed that, when the processor is in legal mode, the state of the CS.L bit has no meaning, that is, it is a don't care. Figure 2 shows the programming details of this new architecture.

 

Mode	Operating System	Recompilation Necessity	Address Lenght	Operand Size	Extended Registers	Registers Size
Long Mode - 64-bit Mode	New 64-bit O.S.	Yes	64 bits	32 bits	Yes	64 bits
Long Mode - Compatibility Mode	New 64-bit O.S.	No	32 or 16 bits	32 bits	No	32 bits
Legal Mode	32-bit O.S.	No	32 bits	32 bits	No	32 bits
Legal Mode	16-bit O.S.	No	16 bits	16 bits	No	32 bits

Figure 2: Programming characteristics of the AMD x86-64 architecture.