This page tries to clear ideas about x86-64 CPUs (AMD64 and Intel's equivalent EM64T implementation). IA-64 (Itanium) are really a different beast and not addressed here.
Features
What does Long Mode offer ?
- Long mode extends general registers to 64 bits (RAX, RBX, RIP, RSP, RFLAGS, etc), and adds an additional 8 integer registers (R8, R9, ..., R15) plus 8 more SSE registers (XMM8 to XMM15) to the CPU. Linear addresses are extended to 64 bit (however, a given CPU may implement less than this) and the physical address space is extended to 52 bits (a given CPU may implement less than this). In essence long mode adds another mode to the CPU
- Real mode
- Legacy mode (32 bit protected mode)
- Long mode (64 bit protected mode)
- System Management mode
Long mode does not support hardware task switching or virtual 8086 tasks, and most of the segment register details are ignored (a flat memory model is required). In long mode the current CS determines if the code currently running is 64 bit code (true long mode) or 32 bit code (compatability mode), or even 16-bit protected mode code (still in compatability mode).
The first 64 bit CPUs from both Intel and AMD will support 40 bit physical addresses and 48 bit linear addresses.
Setting up ...
How do I detect if the CPU is 64 bits ?
You can find that out by checking CPUID. All AMD64 compliant processors have the longmode-capable-bit turned on in the extended feature flags (bit 29) in EDX, after calling CPUID with EAX=0x80000001. There are also other bits required by long mode, but you can see those yourself in CPUID at 
AMD general purpose instruction reference
How do i enable Long Mode ?
- The steps for enabling long mode are
- Disable paging
- Set the PAE enable bit in CR4
- Load CR3 with the physical address of the PML4
- Enable long mode by setting the EFER.LME flag in MSR 0xC00000080
- Enable paging
Now the CPU will be in compatability mode, and instructions are still 32-bit. To enter long mode, the D/B bit (bit 22, 2nd dword) of the GDT code segment must be clear (as it would be for a 16-bit code segment), and the L bit (bit 21, 2nd dword) of the GDT code segment must be set. Once that is done, the CPU is in 64-bit long mode.
Are there restrictions on 32 code running in Legacy Mode ?
x86-64 processors can operate in a legacy mode, they still start in real mode and protected mode is still available (along with the associated v8086 mode). This means an x86 operating system, even DOS, will still run just fine. The only difference is that physical addresses can be up to 52 bits (or as many bits as implemented by the CPU) when PAE is used.
However, there is nothing like Virtual8086 Mode (16 bits support) once in long/compatibility mode.
Can i enable Long Mode directly ?
Protected mode must be entered before activating long mode. A minimal protected-mode environment must be established to allow long-mode initialization to take place. This environment must include the following:
- A protected-mode IDT for vectoring interrupts and exceptions to the appropriate handlers while in protected mode.
- The protected-mode interrupt and exception handlers referenced by the IDT.
Gate descriptors for each handler must be loaded in the IDT.
--AMD64 docs, volume 2, section 14.4 (Enabling Protected Mode), 24593 Rev. 3.10 February 2005
That being said, 
we have a thread where Brendan shows how you can enable 64-bit long mode with no 32-bit IDT and no 32-bit segments ... Be assured, however, that any paging-related exception that occurs in long mode before you enable 64-bit IDT will cause the processor to reset due to a triple fault ...
64bit Environment Models
There are three 64bit programming models you need to consider; LP64, ILP64, LLP64, each mode has its own pitfalls. The I/L/P stand for Int, Long, Pointer, and the 64 means thats how many bits in each.
This LP64 means Longs and Pointers are 64bits wide. LL is a special case and means long-long...
DataTypes
This table lists the breakdown of sizes in the various programming models.
| Datatype | LP64 | ILP64 | LLP64 | ILP32 | LP32 |
| char | 8 | 8 | 8 | 8 | 8 |
| short | 16 | 16 | 16 | 16 | 16 |
| _int | 32 | -- | 32 | -- | -- |
| int | 32 | 64 | 32 | 32 | 16 |
| long | 64 | 64 | 32 | 32 | 32 |
| long long | -- | -- | 64 | -- | -- |
| pointer | 64 | 64 | 64 | 32 | 32 |
64bit OS Modes
The following table lists what some current 64bit OS have as a programming model.
| OS | Mode |
| Windows XP-64 / IA64 | LLP64 |
| Linux | LP64 |
| Solaris | LP64 |
| DEC OSF/1 Alpha | LP64 |
| SGI Irix | LP64 |
| HP UX 11 | LP64 |
Categories: CollectedKnowledge, HardWareCpu