Local Descriptor Table
Encyclopedia
The Local Descriptor Table (LDT) is a memory table used in the x86 architecture
in protected mode
and containing memory segment descriptor
s: start in linear memory, size, executability, writability, access privilege, actual presence in memory, etc.
The LDT is the sibling of the Global Descriptor Table
(GDT) and similarly defines up to 8191 memory segments accessible to programs.
, the LDT is essential to implementing separate address space
s for multiple processes. There will be generally one LDT per user process, describing privately held memory, while shared memory
and kernel memory will be described by the GDT. The operating system
will switch the current LDT when scheduling a new process, using the LLDT machine instruction. On the contrary, the GDT is generally not switched (although this may happen if virtual machine monitors like VMware
are running on the computer).
The lack of symmetry between both tables is underlined by the fact that the current LDT can be automatically switched on certain events, notably if TSS
-based multitasking is used, while this is not possible for the GDT. The LDT also cannot store certain privileged types of memory segments (eg. TSSes). Finally, the LDT is actually defined by a descriptor inside the GDT, while the GDT is directly defined by a linear address.
Creating shared memory through the GDT has some drawbacks. Notably such memory is visible to every process and with equal rights. In order to restrict visibility and to differentiate the protection of shared memory, for example to only allow read-only access for some processes, one can use separate LDT entries, pointed at the same physical memory areas and only created in the LDTs of processes which have requested access to a given shared memory area.
LDT (and GDT) entries which point to identical memory areas are called aliases. Aliases are also typically created in order to get write access to code segments: an executable selector cannot be used for writing. (Protected mode programs constructed in the so-called tiny memory model
, where everything is located in the same memory segment, must use separate selectors for code and data/stack, making both selectors technically "aliases" as well.) In the case of the GDT, aliases are also created in order to get access to system segments like the TSSes.
Segments have a "Present" flag in their descriptors, allowing them to be removed from memory if the need arises. For example, code segments or unmodified data segments can be thrown away, and modified data segments can be swapped out to disk. However, because entire segments need to be operated on as a unit, it is necessary to limit their size in order to ensure that swapping can happen in a timely fashion. However, using smaller, more easily swappable segments means that segment registers must be reloaded more frequently which is itself a time-consuming operation.
microprocessor introduced paging - allocating separate physical memory pages (themselves very small units of memory) at the same virtual addresses, with the advantage that disk paging is far faster and more efficient than segment swapping. Therefore, modern 32-bit
x86 operating systems use the LDT very little, primarily to run legacy 16-bit
code.
Should 16-bit code need to run in a 32-bit environment while sharing memory (this happens e.g. when running OS/2
1.x programs on OS/2 2.0 and later), the LDT must be written in such a way that every flat (paged) address has also a selector in the LDT (typically this results in the LDT being filled with 64 KiB entries). This technique is sometimes called LDT tiling. The limited size of the LDT means the virtual flat address space has to be limited to 512 megabytes (8191 times 64 KiB) - this is what happens on OS/2, although this limitation was fixed in version 4.5. It is also necessary to make sure that objects allocated in the 32-bit environment do not cross 64 KiB boundaries; this generates some address space waste.
If 32-bit code does not have to pass arbitrary memory objects to 16-bit code, e.g. presumably in the OS/2 1.x emulation present in Windows NT
or in the Windows 3.1 emulation layer
, it is not necessary to artificially limit the size of the 32-bit address space.
X86 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...
in protected mode
Protected mode
In computing, protected mode, also called protected virtual address mode, is an operational mode of x86-compatible central processing units...
and containing memory segment descriptor
Segment descriptor
In memory addressing for Intel x86 computer architectures, segment descriptors are a part of the segmentation unit, used for translating a logical address to a linear address...
s: start in linear memory, size, executability, writability, access privilege, actual presence in memory, etc.
The LDT is the sibling of the Global Descriptor Table
Global Descriptor Table
The Global Descriptor Table or GDT is a data structure used by Intel x86-family processors starting with the 80286 in order to define the characteristics of the various memory areas used during program execution, including the base address, the size and access privileges like executability and...
(GDT) and similarly defines up to 8191 memory segments accessible to programs.
History
On x86 processors not having paging features, like the Intel 80286Intel 80286
The Intel 80286 , introduced on 1 February 1982, was a 16-bit x86 microprocessor with 134,000 transistors. Like its contemporary simpler cousin, the 80186, it could correctly execute most software written for the earlier Intel 8086 and 8088...
, the LDT is essential to implementing separate address space
Address space
In computing, an address space defines a range of discrete addresses, each of which may correspond to a network host, peripheral device, disk sector, a memory cell or other logical or physical entity.- Overview :...
s for multiple processes. There will be generally one LDT per user process, describing privately held memory, while shared memory
Shared memory
In computing, shared memory is memory that may be simultaneously accessed by multiple programs with an intent to provide communication among them or avoid redundant copies. Depending on context, programs may run on a single processor or on multiple separate processors...
and kernel memory will be described by the GDT. The 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...
will switch the current LDT when scheduling a new process, using the LLDT machine instruction. On the contrary, the GDT is generally not switched (although this may happen if virtual machine monitors like VMware
VMware
VMware, Inc. is a company providing virtualization software founded in 1998 and based in Palo Alto, California, USA. The company was acquired by EMC Corporation in 2004, and operates as a separate software subsidiary ....
are running on the computer).
The lack of symmetry between both tables is underlined by the fact that the current LDT can be automatically switched on certain events, notably if TSS
Task State Segment
The task state segment is a special structure on x86-based computers which holds information about a task. It is used by the operating system kernel for task management...
-based multitasking is used, while this is not possible for the GDT. The LDT also cannot store certain privileged types of memory segments (eg. TSSes). Finally, the LDT is actually defined by a descriptor inside the GDT, while the GDT is directly defined by a linear address.
Creating shared memory through the GDT has some drawbacks. Notably such memory is visible to every process and with equal rights. In order to restrict visibility and to differentiate the protection of shared memory, for example to only allow read-only access for some processes, one can use separate LDT entries, pointed at the same physical memory areas and only created in the LDTs of processes which have requested access to a given shared memory area.
LDT (and GDT) entries which point to identical memory areas are called aliases. Aliases are also typically created in order to get write access to code segments: an executable selector cannot be used for writing. (Protected mode programs constructed in the so-called tiny memory model
Memory model (computing)
In computing, a memory model describes the interactions of threads through memory and specifies the assumptions the compiler is allowed to make when generating code for segmented memory or paged memory platforms.-History and significance:...
, where everything is located in the same memory segment, must use separate selectors for code and data/stack, making both selectors technically "aliases" as well.) In the case of the GDT, aliases are also created in order to get access to system segments like the TSSes.
Segments have a "Present" flag in their descriptors, allowing them to be removed from memory if the need arises. For example, code segments or unmodified data segments can be thrown away, and modified data segments can be swapped out to disk. However, because entire segments need to be operated on as a unit, it is necessary to limit their size in order to ensure that swapping can happen in a timely fashion. However, using smaller, more easily swappable segments means that segment registers must be reloaded more frequently which is itself a time-consuming operation.
Modern usage
The Intel 80386Intel 80386
The Intel 80386, also known as the i386, or just 386, was a 32-bit microprocessor introduced by Intel in 1985. The first versions had 275,000 transistors and were used as the central processing unit of many workstations and high-end personal computers of the time...
microprocessor introduced paging - allocating separate physical memory pages (themselves very small units of memory) at the same virtual addresses, with the advantage that disk paging is far faster and more efficient than segment swapping. Therefore, modern 32-bit
32-bit
The range of integer values that can be stored in 32 bits is 0 through 4,294,967,295. Hence, a processor with 32-bit memory addresses can directly access 4 GB of byte-addressable memory....
x86 operating systems use the LDT very little, primarily to run legacy 16-bit
16-bit
-16-bit architecture:The HP BPC, introduced in 1975, was the world's first 16-bit microprocessor. Prominent 16-bit processors include the PDP-11, Intel 8086, Intel 80286 and the WDC 65C816. The Intel 8088 was program-compatible with the Intel 8086, and was 16-bit in that its registers were 16...
code.
Should 16-bit code need to run in a 32-bit environment while sharing memory (this happens e.g. when running OS/2
OS/2
OS/2 is a computer operating system, initially created by Microsoft and IBM, then later developed by IBM exclusively. The name stands for "Operating System/2," because it was introduced as part of the same generation change release as IBM's "Personal System/2 " line of second-generation personal...
1.x programs on OS/2 2.0 and later), the LDT must be written in such a way that every flat (paged) address has also a selector in the LDT (typically this results in the LDT being filled with 64 KiB entries). This technique is sometimes called LDT tiling. The limited size of the LDT means the virtual flat address space has to be limited to 512 megabytes (8191 times 64 KiB) - this is what happens on OS/2, although this limitation was fixed in version 4.5. It is also necessary to make sure that objects allocated in the 32-bit environment do not cross 64 KiB boundaries; this generates some address space waste.
If 32-bit code does not have to pass arbitrary memory objects to 16-bit code, e.g. presumably in the OS/2 1.x emulation present in Windows NT
Windows NT
Windows NT is a family of operating systems produced by Microsoft, the first version of which was released in July 1993. It was a powerful high-level-language-based, processor-independent, multiprocessing, multiuser operating system with features comparable to Unix. It was intended to complement...
or in the Windows 3.1 emulation layer
Windows on Windows
In computing, Windows on Windows - commonly referred to by its acronym WOW or WoW - is a software component of 32-bit versions of the Microsoft Windows NT family of operating systems that provides limited support for running legacy Win16 applications - applications written for Windows 3.x...
, it is not necessary to artificially limit the size of the 32-bit address space.