In
IBM PC compatible
IBM PC compatible computers are similar to the original IBM PC, XT, and AT, all from computer giant IBM, that are able to use the same software and expansion cards. Such computers were referred to as PC clones, IBM clones or IBM PC clones. ...
computing, DOS memory management refers to software and techniques employed to give applications access to more than 640
kibibytes
The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable uni ...
(640*1024 bytes) (KiB) of "conventional memory". The 640 KiB limit was specific to the IBM PC and close compatibles; other machines running
MS-DOS
MS-DOS ( ; acronym for Microsoft Disk Operating System, also known as Microsoft DOS) is an operating system for x86-based personal computers mostly developed by Microsoft. Collectively, MS-DOS, its rebranding as IBM PC DOS, and a few ope ...
had different limits, for example the
Apricot PC
The Apricot PC (originally called the ''ACT Apricot'') is a personal computer produced by Apricot Computers, then still known as Applied Computer Techniques or ACT. Released in late 1983, it was ACT's first independently developed microcomputer, ...
could have up to 768 KiB and the
Sirius
Sirius is the list of brightest stars, brightest star in the night sky. Its name is derived from the Ancient Greek language, Greek word , or , meaning 'glowing' or 'scorching'. The star is designated α Canis Majoris, Latinisation ...
Victor 9000, 896 KiB. Memory management on the IBM family was made complex by the need to maintain backward compatibility to the original PC design
and
real-mode
Real mode, also called real address mode, is an operating mode of all x86-compatible CPUs. The mode gets its name from the fact that addresses in real mode always correspond to real locations in memory. Real mode is characterized by a 20-bit seg ...
DOS, while allowing computer users to take advantage of large amounts of low-cost memory and new generations of processors. Since DOS has given way to
Microsoft Windows
Windows is a group of several proprietary graphical operating system families developed and marketed by Microsoft. Each family caters to a certain sector of the computing industry. For example, Windows NT for consumers, Windows Server for serv ...
and other 32-bit operating systems not restricted by the original arbitrary 640 KiB limit of the IBM PC, managing the memory of a personal computer no longer requires the user to manually manipulate internal settings and parameters of the system.
The 640 KiB limit imposed great complexity on hardware and software intended to circumvent it; the physical memory in a machine could be organised as a combination of base or conventional memory (including lower memory), upper memory, high memory (not the same as upper memory), extended memory, and expanded memory, all handled in different ways.
Conventional memory
The
Intel 8088
The Intel 8088 ("''eighty-eighty-eight''", also called iAPX 88) microprocessor is a variant of the Intel 8086. Introduced on June 1, 1979, the 8088 has an eight-bit external data bus instead of the 16-bit bus of the 8086. The 16-bit registers an ...
processor used in the original IBM PC had 20 address lines and so could directly address 1 MiB (2
20 bytes) of memory. Different areas of this address space were allocated to different kinds of memory used for different purposes. Starting at the lowest end of the address space, the PC had read/write
random access memory
Random-access memory (RAM; ) is a form of computer memory that can be read and changed in any order, typically used to store working Data (computing), data and machine code. A Random access, random-access memory device allows data items to b ...
(RAM) installed, which was used by DOS and application programs. The first part of this memory was installed on the motherboard of the system (in very early machines, 64 KiB, later revised to 256 KiB). Additional memory could be added with cards plugged into the expansion slots; each card contained straps or switches to control what part of the address space accesses memory and devices on that card.
On the IBM PC, all the address space up to 640 KiB was available for RAM. This part of the address space is called "conventional memory" since it is accessible to all versions of DOS automatically on startup. Segment 0, the first 64 KiB of conventional memory, is also called
low memory area. Normally expansion memory is set to be contiguous in the address space with the memory on the motherboard. If there was an unallocated gap between motherboard memory and the expansion memory, the memory would not be automatically detected as usable by DOS.
Upper memory area
The upper memory area (UMA) refers to the address space between 640 KiB and 1024 KiB (0xA0000–0xFFFFF). The 128 KiB region between 0xA0000 and 0xBFFFF was reserved for
VGA
Video Graphics Array (VGA) is a video display controller and accompanying de facto graphics standard, first introduced with the IBM PS/2 line of computers in 1987, which became ubiquitous in the PC industry within three years. The term can no ...
screen memory and legacy SMM. The 128 KiB region between 0xC0000 and 0xDFFFF was reserved for device
Option ROM
An Option ROM for the PC platform (i.e. the IBM PC and derived successor computer systems) is a piece of firmware that resides in ROM on an expansion card (or stored along with the main system BIOS), which gets executed to initialize the device and ...
s, including
Video BIOS
Video BIOS is the BIOS of a graphics card in a (usually IBM PC-derived) computer. It initializes the graphics card at the computer's boot time. It also implements INT 10h interrupt and VESA BIOS Extensions (VBE) for basic text and videomode output ...
. The 64 KiB of the address space from 0xE0000 to 0xEFFFF was reserved for the
BIOS
In computing, BIOS (, ; Basic Input/Output System, also known as the System BIOS, ROM BIOS, BIOS ROM or PC BIOS) is firmware used to provide runtime services for operating systems and programs and to perform hardware initialization during the ...
or
Option ROM
An Option ROM for the PC platform (i.e. the IBM PC and derived successor computer systems) is a piece of firmware that resides in ROM on an expansion card (or stored along with the main system BIOS), which gets executed to initialize the device and ...
s. The IBM PC reserved the uppermost 64 KiB of the address space from 0xF0000 to 0xFFFFF for the BIOS and
Cassette BASIC
The IBM Personal Computer Basic, commonly shortened to IBM BASIC, is a programming language first released by IBM with the IBM Personal Computer, Model 5150 (IBM PC) in 1981. IBM released four different versions of the Microsoft BASIC interpre ...
read-only memory (ROM).
For example, the
monochrome video adapter memory area ran from 704 to 736 KiB (0xB0000–0xB7FFF). If only a monochrome display adapter was used, the address space between 0xA0000 and 0xAFFFF could be used for RAM, which would be contiguous with the conventional memory.
The system BIOS ROMs must be at the upper end of the address space because the CPU starting address is fixed by the design of the processor. The starting address is loaded into the
program counter
The program counter (PC), commonly called the instruction pointer (IP) in Intel x86 and Itanium microprocessors, and sometimes called the instruction address register (IAR), the instruction counter, or just part of the instruction sequencer, is ...
of the CPU after a hardware reset and must have a defined value that endures after power is interrupted to the system. On reset or power up, the CPU loads the address from the system ROM and then jumps to a defined ROM location to begin executing the system
power-on self-test
A power-on self-test (POST) is a process performed by firmware or software routines immediately after a computer or other digital electronic device is powered on.
This article mainly deals with POSTs on personal computers, but many other embed ...
, and eventually load an operating system.
Since an
expansion card
In computing, an expansion card (also called an expansion board, adapter card, peripheral card or accessory card) is a printed circuit board that can be inserted into an electrical connector, or expansion slot (also referred to as a bus slo ...
such as a video adapter,
hard drive controller, or network adapter could use allocations of memory in many of the upper memory areas, configuration of some combinations of cards required careful reading of documentation, or experimentation, to find card settings and
memory mappings that worked. Mapping two devices to use the same physical memory addresses could result in a stalled or unstable system.
Not all addresses in the upper memory area were used in a typical system; unused physical addresses would return undefined and system-dependent data if accessed by the processor.
Expanded memory
As memory prices declined, application programs such as
spreadsheet
A spreadsheet is a computer application for computation, organization, analysis and storage of data in tabular form. Spreadsheets were developed as computerized analogs of paper accounting worksheets. The program operates on data entered in cel ...
s and
computer-aided drafting
Computer-aided design (CAD) is the use of computers (or ) to aid in the creation, modification, analysis, or optimization of a design. This software is used to increase the productivity of the designer, improve the quality of design, improve co ...
were changed to take advantage of more and more physical memory in the system.
Virtual memory
In computing, virtual memory, or virtual storage is a memory management technique that provides an "idealized abstraction of the storage resources that are actually available on a given machine" which "creates the illusion to users of a very l ...
in the 8088 and
8086
The 8086 (also called iAPX 86) is a 16-bit microprocessor chip designed by Intel between early 1976 and June 8, 1978, when it was released. The Intel 8088, released July 1, 1979, is a slightly modified chip with an external 8-bit data bus (allowi ...
was not supported by the processor hardware, and disk technology of the time would make it too slow and cumbersome to be practical. Expanded memory was a system that allowed application programs to access more RAM than directly visible to the processor's address space. The process was a form of
bank switching
Bank switching is a technique used in computer design to increase the amount of usable memory beyond the amount directly addressable by the processor instructions. It can be used to configure a system differently at different times; for example ...
. When extra RAM was needed,
driver software would temporarily make a piece of expanded memory accessible to the processor; when the data in that piece of memory was updated, another part could be swapped into the processor's address space. For the IBM PC and
IBM PC/XT
The IBM Personal Computer XT (model 5160, often shortened to PC/XT) is the second computer in the IBM Personal Computer line, released on March 8, 1983. Except for the addition of a built-in Hard disk drive, hard drive and extra expansion slots, ...
, with only 20 address lines, special-purpose expanded memory cards were made containing perhaps a megabyte, or more, of expanded memory, with logic on the board to make that memory accessible to the processor in defined parts of the 8088 address space.
Allocation and use of expanded memory was not transparent to application programs. The application had to keep track of which bank of expanded memory contained a particular piece of data, and when access to that data was required, the application had to request (through a driver program) the expanded memory board to map that part of memory into the processor's address space. Although applications could use expanded memory with relative freedom, many other software components such as drivers and
terminate-and-stay-resident programs (TSRs) were still normally constrained to reside within the 640K "conventional memory" area, which soon became a critically scarce resource.
The 80286 and the high memory area
When the
IBM PC/AT
The IBM Personal Computer/AT (model 5170, abbreviated as IBM AT or PC/AT) was released in 1984 as the fourth model in the IBM Personal Computer line, following the IBM PC/XT and its IBM Portable PC variant. It was designed around the Intel 80 ...
was introduced, the
segmented memory
Memory segmentation is an operating system memory management technique of division of a computer's primary memory into segments or sections. In a computer system using segmentation, a reference to a memory location includes a value that identifi ...
architecture of the Intel family processors had the byproduct of allowing slightly more than 1 MiB of memory to be addressed in the "real" mode. Since the 80286 had more than 20 address lines, certain combinations of segment and offset could point into memory above the 0x0100000 (2
20) location. The 80286 could address up to 16 MiB of system memory, thus removing the behavior of memory addresses "wrapping around". Since the required address line now existed, the combination F800:8000 would no longer point to the physical address 0x0000000 but the correct address 0x00100000.
As a result, some DOS programs would no longer work. To maintain compatibility with the PC and XT behavior, the AT included an ''A20 line'' gate (Gate A20) that made memory addresses on the AT wrap around to low memory as they would have on an 8088 processor. This gate could be controlled, initially through the
keyboard controller, to allow running programs which were designed for this to access an additional 65,520 bytes (64 KiB of memory in
real mode
Real mode, also called real address mode, is an operating mode of all x86-compatible CPUs. The mode gets its name from the fact that addresses in real mode always correspond to real locations in memory. Real mode is characterized by a 20-bit seg ...
. At
boot
A boot is a type of footwear. Most boots mainly cover the foot and the ankle, while some also cover some part of the lower calf. Some boots extend up the leg, sometimes as far as the knee or even the hip. Most boots have a heel that is cle ...
time, the
BIOS
In computing, BIOS (, ; Basic Input/Output System, also known as the System BIOS, ROM BIOS, BIOS ROM or PC BIOS) is firmware used to provide runtime services for operating systems and programs and to perform hardware initialization during the ...
first enables A20 when counting and testing all of the system's memory, and disables it before transferring control to the operating system. Enabling the A20 line is one of the first steps a
protected mode
In computing, protected mode, also called protected virtual address mode, is an operational mode of x86-compatible central processing units (CPUs). It allows system software to use features such as virtual memory, paging and safe multi-tasking d ...
x86
x86 (also known as 80x86 or the 8086 family) is a family of complex instruction set computer (CISC) instruction set architectures initially developed by Intel based on the Intel 8086 microprocessor and its 8088 variant. The 8086 was introd ...
operating system
An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs.
Time-sharing operating systems schedule tasks for efficient use of the system and may also in ...
does in the bootup process, often before control has been passed onto the kernel from the bootstrap (in the case of Linux, for example).
The ''high memory area'' (''HMA'') is the
RAM
Ram, ram, or RAM may refer to:
Animals
* A male sheep
* Ram cichlid, a freshwater tropical fish
People
* Ram (given name)
* Ram (surname)
* Ram (director) (Ramsubramaniam), an Indian Tamil film director
* RAM (musician) (born 1974), Dutch
* ...
area consisting of the first 64 KiB, minus 16
bytes
The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable uni ...
, of the
extended memory
In DOS memory management, extended memory refers to memory above the first megabyte (220 bytes) of address space in an IBM PC or compatible with an 80286 or later processor. The term is mainly used under the DOS and Windows operating systems. DOS ...
on an IBM PC/AT or compatible microcomputer.
Originally, the logic gate was a gate connected to the
Intel 8042
The MCS-48 microcontroller series, Intel's first microcontroller, was originally released in 1976. Its first members were 8048, 8035 and 8748. The 8048 is probably the most prominent member of the family. Initially, this family was produced u ...
keyboard controller. Controlling it was a relatively slow process. Other methods have since been added to allow for more efficient multitasking of programs which require this wrap-around with programs that access all of the system's memory. There was at first a variety of methods, but eventually the industry settled on the PS/2 method of using a bit in
port
A port is a maritime facility comprising one or more wharves or loading areas, where ships load and discharge cargo and passengers. Although usually situated on a sea coast or estuary, ports can also be found far inland, such as Ham ...
92h to control the A20 line.
Disconnecting A20 would not wrap all memory accesses above 1 MiB, just those in the 1 MiB, 3 MiB, 5 MiB, etc. ranges.
Real mode
Real mode, also called real address mode, is an operating mode of all x86-compatible CPUs. The mode gets its name from the fact that addresses in real mode always correspond to real locations in memory. Real mode is characterized by a 20-bit seg ...
software only cared about the area slightly above 1 MiB, so Gate A20 was enough.
Virtual 8086 mode
In the 80386 microprocessor and later, virtual 8086 mode (also called virtual real mode, V86-mode, or VM86) allows the execution of real mode applications that are incapable of running directly in protected mode while the processor is runnin ...
, introduced with the
Intel 80386
The Intel 386, originally released as 80386 and later renamed i386, is a 32-bit microprocessor introduced in 1985. The first versions had 275,000 transistors[virtual memory
In computing, virtual memory, or virtual storage is a memory management technique that provides an "idealized abstraction of the storage resources that are actually available on a given machine" which "creates the illusion to users of a very l ...]
facilities of the processor: physical memory may be mapped to multiple virtual addresses, thus allowing that the memory mapped at the first megabyte of virtual memory may be mapped again in the second megabyte of virtual memory. The operating system may intercept changes to Gate A20 and make corresponding changes to the virtual memory address space, which also makes irrelevant the efficiency of Gate-A20 toggling.
The first user of the HMA among Microsoft products was
Windows 2.0 in 1987, which introduced the
HIMEM.SYS
HIMEM.SYS is a DOS device driver which allows DOS programs to store data in extended memory according to the Extended Memory Specification (XMS). The memory beyond the first 1 MB of address space is required by Windows 9x/ Me in order to load ...
device driver. Starting with versions 5.0 of
DR-DOS
DR-DOS (written as DR DOS, without a hyphen, in versions up to and including 6.0) is a disk operating system for IBM PC compatibles. Upon its introduction in 1988, it was the first DOS attempting to be compatible with IBM PC DOS and MS-D ...
(1990) and of
MS-DOS
MS-DOS ( ; acronym for Microsoft Disk Operating System, also known as Microsoft DOS) is an operating system for x86-based personal computers mostly developed by Microsoft. Collectively, MS-DOS, its rebranding as IBM PC DOS, and a few ope ...
(1991), parts of the operating system could be loaded into HMA as well, freeing up to 46 KiB of
conventional memory
In DOS memory management, conventional memory, also called base memory, is the first 640 kilobytes of the memory on IBM PC or compatible systems. It is the read-write memory directly addressable by the processor for use by the operating system ...
. Other components, such as device drivers and TSRs, could be loaded into the
upper memory area
In DOS memory management, the upper memory area (UMA) refers to memory between the addresses of 640 KB and 1024 KB ( 0xA0000–0xFFFFF) in an IBM PC or compatible. IBM reserved the uppermost 384 KB of the 8088 CPU's 1024 KB ...
(UMA).
A20 handler
The ''A20 handler'' is software controlling access to the high memory area.
Extended memory
In DOS memory management, extended memory refers to memory above the first megabyte (220 bytes) of address space in an IBM PC or compatible with an 80286 or later processor. The term is mainly used under the DOS and Windows operating systems. DOS ...
managers usually provide this functionality. In DOS, high memory area managers, such as
HIMEM.SYS
HIMEM.SYS is a DOS device driver which allows DOS programs to store data in extended memory according to the Extended Memory Specification (XMS). The memory beyond the first 1 MB of address space is required by Windows 9x/ Me in order to load ...
had the extra task of managing A20 and provided an
API
An application programming interface (API) is a way for two or more computer programs to communicate with each other. It is a type of software interface, offering a service to other pieces of software. A document or standard that describes how ...
for opening/closing A20. DOS itself could utilize the area for some of its storage needs, thereby freeing up more conventional memory for programs. This functionality was enabled by the "
DOS
DOS is shorthand for the MS-DOS and IBM PC DOS family of operating systems.
DOS may also refer to:
Computing
* Data over signalling (DoS), multiplexing data onto a signalling channel
* Denial-of-service attack (DoS), an attack on a communicat ...
=HIGH" directive in the
CONFIG.SYS
CONFIG.SYS is the primary configuration file for the DOS and OS/2 operating systems. It is a special ASCII text file that contains user-accessible setup or configuration directives evaluated by the operating system's DOS BIOS (typically residing ...
configuration file.
A20 gate on later processors
The
Intel 80486
The Intel 486, officially named i486 and also known as 80486, is a microprocessor. It is a higher-performance follow-up to the Intel 386. The i486 was introduced in 1989. It represents the fourth generation of binary compatible CPUs following the ...
and
Pentium
Pentium is a brand used for a series of x86 architecture-compatible microprocessors produced by Intel. The original Pentium processor from which the brand took its name was first released on March 22, 1993. After that, the Pentium II and Pe ...
added a special pin named ''A20M#'', which when asserted low forces bit 20 of the physical address to be zero for all on-chip cache or external memory accesses. This was necessary since the 80486 introduced an on-chip cache, and therefore masking this bit in external logic was no longer possible. Software still needs to manipulate the gate and must still deal with external peripherals (the
chipset
In a computer system, a chipset is a set of electronic components
An electronic component is any basic discrete device or physical entity in an electronic system used to affect electrons or their associated fields. Electronic components are ...
) for that.
80386 and subsequent processors
Intel processors from the
386 onward allowed a
virtual 8086 mode
In the 80386 microprocessor and later, virtual 8086 mode (also called virtual real mode, V86-mode, or VM86) allows the execution of real mode applications that are incapable of running directly in protected mode while the processor is runnin ...
, which simplified the hardware required to implement expanded memory for DOS applications. Expanded memory managers such as
Quarterdeck
The quarterdeck is a raised deck behind the main mast of a sailing ship. Traditionally it was where the captain commanded his vessel and where the ship's colours were kept. This led to its use as the main ceremonial and reception area on bo ...
's
QEMM
Quarterdeck Expanded Memory Manager (QEMM) is a memory manager produced by Quarterdeck Office Systems in the late 1980s through the late 1990s. It was the most popular third-party memory manager for the MS-DOS and other DOS operating systems.
...
product and Microsoft's
EMM386
EMM386 is the expanded memory manager of Microsoft's MS-DOS, IBM's PC DOS, Digital Research's DR-DOS, and Datalight's ROM-DOS which is used to create expanded memory using extended memory on Intel 80386 CPUs. There also is an EMM386.EXE available ...
supported the expanded memory standard without requirement for special memory boards.
On 386 and subsequent processors, memory managers like QEMM might move the bulk of the code for a driver or TSR into extended memory and replace it with a small fingerhold that was capable of accessing the extended-memory-resident code. They might analyze memory usage to detect drivers that required more RAM during startup than they did subsequently, and recover and reuse the memory that was no longer needed after startup. They might even remap areas of memory normally used for memory-mapped I/O. Many of these tricks involved assumptions about the functioning of drivers and other components. In effect, memory managers might reverse-engineer and modify other vendors' code on the fly. As might be expected, such tricks did not always work. Therefore, memory managers also incorporated very elaborate systems of configurable options, and provisions for recovery should a selected option render the PC unbootable (a frequent occurrence).
Installing and configuring a memory manager might involve hours of experimentation with options, repeatedly rebooting the machine, and testing the results. But conventional memory was so valuable that PC owners felt that such time was well-spent if the result was to free up 30 KiB or 40 KiB of conventional memory space.
Extended memory
In the context of IBM PC compatible computers, ''extended memory'' refers to memory in the address space of the 80286 and subsequent processors, beyond the 1 megabyte limit imposed by the 20 address lines of the 8088 and 8086. Such memory is not directly available to DOS applications running in the so-called "real mode" of the 80286 and subsequent processors. This memory is only accessible in the protected or virtual modes of 80286 and higher processors.
See also
*
Global EMM Import Specification
EMM386 is the expanded memory manager of Microsoft's MS-DOS, IBM's PC DOS, Digital Research's DR-DOS, and Datalight's ROM-DOS which is used to create expanded memory using extended memory on Intel 80386 CPUs. There also is an EMM386.EXE availabl ...
(GEMMIS)
*
Virtual DMA Services (VDS)
*
Virtual Control Program Interface
In computing, the Virtual Control Program Interface (VCPI) is a specification published in 1989 by Phar Lap Software that allows a DOS program to run in protected mode, granting access to many features of the processor not available in real mode ...
(VCPI)
*
Extended Virtual Control Program Interface
In computing, the Virtual Control Program Interface (VCPI) is a specification published in 1989 by Phar Lap Software that allows a DOS program to run in protected mode, granting access to many features of the processor not available in real mode ...
(XVCPI)
*
DOS Protected Mode Interface
In computing, the DOS Protected Mode Interface (DPMI) is a specification introduced in 1989 which allows a DOS program to run in protected mode, giving access to many features of the new PC processors of the time not available in real mode. It w ...
(DPMI)
*
DOS Protected Mode Services
DOS Protected Mode Services (DPMS) is a set of extended DOS memory management services to allow DPMS-enabled DOS drivers to load and execute in extended memory and protected mode.
Not being a DOS extender by itself, DPMS is a minimal set of ex ...
(DPMS)
*
Helix Cloaking
DOS Protected Mode Services (DPMS) is a set of extended DOS memory management services to allow DPMS-enabled DOS drivers to load and execute in extended memory and protected mode.
Not being a DOS extender by itself, DPMS is a minimal set of ex ...
References
{{reflist, refs=
[{{cite book , author-first=Tom , author-last=Shanley , title=Protected mode software architecture , publisher=]Taylor & Francis
Taylor & Francis Group is an international company originating in England that publishes books and academic journals. Its parts include Taylor & Francis, Routledge, F1000 (publisher), F1000 Research or Dovepress. It is a division of Informa ...
, date=1996 , isbn=0-201-55447-X , page=60
[{{cite book , author-first=Scott , author-last=Mueller , author-link=Scott Mueller , title=Upgrading and Repairing PCs , edition=4th , publisher= Que , date=1994 , isbn=1-56529-932-9 , chapter=7 , url=https://archive.org/details/upgradingrepairi00muel_7 ]
[{{cite book , author-first=Scott , author-last=Mueller , author-link=Scott Mueller , title=Upgrading and Repairing PCs , edition=11th , publisher= Que Corporation , date=1999 , isbn=0-7897-1903-7 , chapter=6: Memory , url=https://archive.org/details/upgradingrepairi00muel_1 ]
External links
* Microsoft support
Overview of Memory-Management Functionality in MS-DOS*
Computer Chronicles
''(The) Computer Chronicles'' is an American half-hour television series, which was broadcast from 1983 to 2002 on Public Broadcasting Service (PBS) public television and which documented various issues from the rise of the personal computer from ...
(1990).
High Memory Management. From the
Internet Archive
The Internet Archive is an American digital library with the stated mission of "universal access to all knowledge". It provides free public access to collections of digitized materials, including websites, software applications/games, music, ...
.
Memory management