Long mode
Encyclopedia
In the x86-64
computer architecture
, long mode is the mode where a 64-bit
application (or operating system
) can access 64-bit instructions and registers
. 32-bit programs and 16-bit protected mode
programs are executed in a compatibility sub-mode; real mode
or virtual 8086 mode
programs cannot run in this mode.
processor when running in real mode or protected mode, which are supported sub-modes when the processor is not in long mode.
A bit in the CPUID
extended attributes field informs programs in real or protected modes if the processor can go to long mode, which allows a program to detect an x86-64 processor. This is similar to the CPUID attributes bit that Intel IA-64 processors use for allowing programs to detect they are running under IA-32 emulation.
, memory addressing has not yet been increased to the full 64 bits. For the time being, it is impractical to equip computers with sufficient memory to require a full 64 bits. As long as that remains the case, load/store unit(s), cache
tags, MMUs
and TLBs
can be simplified without any loss of usable memory. Despite this limitation, software is programmed using full 64-bit pointers, and will therefore be able to use progressively larger address spaces as they become supported by future processors and operating systems.
/ Opteron
(K8) CPUs, had 48-bit virtual
and 40-bit physical address
ing.
The virtual address space of these processors is divided into two 47-bit regions, one starting at the lowest possible address, the other extending down from the largest. Attempting to use addresses falling outside this range will cause a general protection fault
.
The physical address limit constrains how much RAM
can be installed in the machine. On a ccNUMA multiprocessor
system (Opteron) this includes the memory which is installed in the remote nodes, because the CPUs can directly address (and cache) all memory regardless if it is on the home node or remote. The 1 TB
limit (40-bit) for physical memory for the K8 is huge by typical personal computer standards, but might have been a limitation for use in supercomputers. Consequently, the K10
(or "10h") microarchitecture
implements 48-bit physical addresses and so can address up to 256 TB
of RAM.
When there is need, the microarchitecture can be expanded step by step without side-effects from software and simultaneously save cost with its implementation. For future expansion, the architecture supports expanding virtual address space to 64 bits, and physical memory addressing to 52 bits (limited by the page table
entry format). These would allow the processor to address 264 bytes (16 exabyte
s) of virtual address space and 252 bytes (4 petabyte
s) of physical address space.
X86-64
x86-64 is an extension of the x86 instruction set. It supports vastly larger virtual and physical address spaces than are possible on x86, thereby allowing programmers to conveniently work with much larger data sets. x86-64 also provides 64-bit general purpose registers and numerous other...
computer architecture
Computer architecture
In computer science and engineering, computer architecture is the practical art of selecting and interconnecting hardware components to create computers that meet functional, performance and cost goals and the formal modelling of those systems....
, long mode is the mode where a 64-bit
64-bit
64-bit is a word size that defines certain classes of computer architecture, buses, memory and CPUs, and by extension the software that runs on them. 64-bit CPUs have existed in supercomputers since the 1970s and in RISC-based workstations and servers since the early 1990s...
application (or operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
) can access 64-bit instructions and registers
Processor register
In computer architecture, a processor register is a small amount of storage available as part of a CPU or other digital processor. Such registers are addressed by mechanisms other than main memory and can be accessed more quickly...
. 32-bit programs and 16-bit protected mode
Protected mode
In computing, protected mode, also called protected virtual address mode, is an operational mode of x86-compatible central processing units...
programs are executed in a compatibility sub-mode; real mode
Real mode
Real mode, also called real address mode, is an operating mode of 80286 and later x86-compatible CPUs. Real mode is characterized by a 20 bit segmented memory address space and unlimited direct software access to all memory, I/O addresses and peripheral hardware...
or virtual 8086 mode
Virtual 8086 mode
In the 80386 microprocessor and later, virtual 8086 mode allows the execution of real mode applications that are incapable of running directly in protected mode while the processor is running a protected mode operating system.VM86 mode uses a segmentation scheme identical to that of real mode In...
programs cannot run in this mode.
Overview
An x86-64 processor acts identically as an IA-32IA-32
IA-32 , also known as x86-32, i386 or x86, is the CISC instruction-set architecture of Intel's most commercially successful microprocessors, and was first implemented in the Intel 80386 as a 32-bit extension of x86 architecture...
processor when running in real mode or protected mode, which are supported sub-modes when the processor is not in long mode.
A bit in the CPUID
CPUID
The CPUID opcode is a processor supplementary instruction for the x86 architecture. It was introduced by Intel in 1993 when it introduced the Pentium and SL-Enhanced 486 processors....
extended attributes field informs programs in real or protected modes if the processor can go to long mode, which allows a program to detect an x86-64 processor. This is similar to the CPUID attributes bit that Intel IA-64 processors use for allowing programs to detect they are running under IA-32 emulation.
Memory limitations
While register sizes have increased to 64 bits from the previous x86 architectureX86 architecture
The term x86 refers to a family of instruction set architectures based on the Intel 8086 CPU. The 8086 was launched in 1978 as a fully 16-bit extension of Intel's 8-bit based 8080 microprocessor and also introduced segmentation to overcome the 16-bit addressing barrier of such designs...
, memory addressing has not yet been increased to the full 64 bits. For the time being, it is impractical to equip computers with sufficient memory to require a full 64 bits. As long as that remains the case, load/store unit(s), cache
CPU cache
A CPU cache is a cache used by the central processing unit of a computer to reduce the average time to access memory. The cache is a smaller, faster memory which stores copies of the data from the most frequently used main memory locations...
tags, MMUs
Memory management unit
A memory management unit , sometimes called paged memory management unit , is a computer hardware component responsible for handling accesses to memory requested by the CPU...
and TLBs
Translation Lookaside Buffer
A translation lookaside buffer is a CPU cache that memory management hardware uses to improve virtual address translation speed. All current desktop and server processors use a TLB to map virtual and physical address spaces, and it is ubiquitous in any hardware which utilizes virtual memory.The...
can be simplified without any loss of usable memory. Despite this limitation, software is programmed using full 64-bit pointers, and will therefore be able to use progressively larger address spaces as they become supported by future processors and operating systems.
Current limits
The first CPUs implementing the x86-64 architecture, namely the AMD Athlon 64Athlon 64
The Athlon 64 is an eighth-generation, AMD64-architecture microprocessor produced by AMD, released on September 23, 2003. It is the third processor to bear the name Athlon, and the immediate successor to the Athlon XP...
/ Opteron
Opteron
Opteron is AMD's x86 server and workstation processor line, and was the first processor which supported the AMD64 instruction set architecture . It was released on April 22, 2003 with the SledgeHammer core and was intended to compete in the server and workstation markets, particularly in the same...
(K8) CPUs, had 48-bit virtual
Virtual address
In computer technology, a virtual address is an address identifying a virtual, i.e. non-physical, entity.-Description:The term virtual address is most commonly used for an address pointing to virtual memory or, in networking, when referring to a virtual network address...
and 40-bit physical address
Physical address
In computing, a physical address, also real address, or binary address, is the memory address that is represented in the form of a binary number on the address bus circuitry in order to enable the data bus to access a particular storage cell of main memory.In a computer with virtual memory, the...
ing.
The virtual address space of these processors is divided into two 47-bit regions, one starting at the lowest possible address, the other extending down from the largest. Attempting to use addresses falling outside this range will cause a general protection fault
General protection fault
A general protection fault in the Intel x86 and AMD x86-64 architectures, and other unrelated architectures, is a fault that can encompass several cases in which protection mechanisms within the processor architecture are violated by any of the programs that are running, either the kernel or a...
.
The physical address limit constrains how much RAM
Random-access memory
Random access memory is a form of computer data storage. Today, it takes the form of integrated circuits that allow stored data to be accessed in any order with a worst case performance of constant time. Strictly speaking, modern types of DRAM are therefore not random access, as data is read in...
can be installed in the machine. On a ccNUMA multiprocessor
Multiprocessor
Computer system having two or more processing units each sharing main memory and peripherals, in order to simultaneously process programs.Sometimes the term Multiprocessor is confused with the term Multiprocessing....
system (Opteron) this includes the memory which is installed in the remote nodes, because the CPUs can directly address (and cache) all memory regardless if it is on the home node or remote. The 1 TB
Terabyte
The terabyte is a multiple of the unit byte for digital information. The prefix tera means 1012 in the International System of Units , and therefore 1 terabyte is , or 1 trillion bytes, or 1000 gigabytes. 1 terabyte in binary prefixes is 0.9095 tebibytes, or 931.32 gibibytes...
limit (40-bit) for physical memory for the K8 is huge by typical personal computer standards, but might have been a limitation for use in supercomputers. Consequently, the K10
AMD K10
The AMD Family 10h is a microprocessor microarchitecture by AMD. Though there were once reports that the K10 had been canceled, the first third-generation Opteron products for servers were launched on September 10, 2007, with the Phenom processors for desktops following and launching on November...
(or "10h") microarchitecture
Microarchitecture
In computer engineering, microarchitecture , also called computer organization, is the way a given instruction set architecture is implemented on a processor. A given ISA may be implemented with different microarchitectures. Implementations might vary due to different goals of a given design or...
implements 48-bit physical addresses and so can address up to 256 TB
Terabyte
The terabyte is a multiple of the unit byte for digital information. The prefix tera means 1012 in the International System of Units , and therefore 1 terabyte is , or 1 trillion bytes, or 1000 gigabytes. 1 terabyte in binary prefixes is 0.9095 tebibytes, or 931.32 gibibytes...
of RAM.
When there is need, the microarchitecture can be expanded step by step without side-effects from software and simultaneously save cost with its implementation. For future expansion, the architecture supports expanding virtual address space to 64 bits, and physical memory addressing to 52 bits (limited by the page table
Page table
A page table is the data structure used by a virtual memory system in a computer operating system to store the mapping between virtual addresses and physical addresses. Virtual addresses are those unique to the accessing process...
entry format). These would allow the processor to address 264 bytes (16 exabyte
Exabyte
The exabyte is a unit of information or computer storage equal to one quintillion bytes . The unit symbol for the exabyte is EB...
s) of virtual address space and 252 bytes (4 petabyte
Petabyte
A petabyte is a unit of information equal to one quadrillion bytes, or 1000 terabytes. The unit symbol for the petabyte is PB...
s) of physical address space.