Shared Memory Architecture
Encyclopedia
In computer architecture
, Shared Memory Architecture (SMA) refers to a design where the graphics chip does not have its own dedicated memory, and instead shares the main system RAM with the CPU
and other components.
This design is used with many integrated graphics solutions to reduce the cost and complexity of the motherboard
design, as no additional memory chips
are required on the board. There is usually some mechanism (via the BIOS
or a jumper
setting) to select the amount of system memory to use for graphics, which means that the graphics system can be tailored to only use as much RAM as is actually required, leaving the rest free for applications. A side effect
of this is that when some RAM is allocated for graphics, it becomes effectively unavailable for anything else, so an example computer with 512 MiB
RAM set up with 64MiB graphics RAM will appear to the operating system
and user to only have 448 MiB RAM installed.
The disadvantage of this design is lower performance because system RAM usually runs slower than dedicated graphics RAM, and there is more contention as the memory bus
has to be shared with the rest of the system. It may also cause performance issues with the rest of the system if it is not designed with the fact in mind that some RAM will be 'taken away' by graphics.
Other approach that gave similar final state is boost up of graphics, that is used in some SGi
computers, most notably O2/O2+
. The memory in these machines is simply one fast pool (2,1 GB/s per second in 1996) shared between system and graphics is done on demand including pointer redirection communication between main system and graphics subsystem. This is called Unified Memory Architecture (UMA).
, released in 1984. Video memory
was shared with the first 128KiB of RAM. The exact size of the video memory could be reconfigured by software to meet the needs of the current program.
Another early design was the Commodore
Amiga
, introduced in 1985. It initially featured 256 KiB
of "chip RAM
" (later up to 2048 KiB, depending on the model). This RAM was used by both the CPU (as main memory) and the Amiga's custom chipset
(for sound/graphics/IO). By default, most Amiga computers were shipped with chip RAM only, but could be expanded with RAM that only the CPU could access (called "Fast RAM"), through expansion boards.
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....
, Shared Memory Architecture (SMA) refers to a design where the graphics chip does not have its own dedicated memory, and instead shares the main system RAM with the CPU
Central processing unit
The central processing unit is the portion of a computer system that carries out the instructions of a computer program, to perform the basic arithmetical, logical, and input/output operations of the system. The CPU plays a role somewhat analogous to the brain in the computer. The term has been in...
and other components.
This design is used with many integrated graphics solutions to reduce the cost and complexity of the motherboard
Motherboard
In personal computers, a motherboard is the central printed circuit board in many modern computers and holds many of the crucial components of the system, providing connectors for other peripherals. The motherboard is sometimes alternatively known as the mainboard, system board, or, on Apple...
design, as no additional memory chips
Silicon Chip
Silicon Chip is an Australian electronics magazine. It was started in November, 1987 by Leo Simpson. Following the demise of Electronics Australia, it is the only hobbyist-related electronics magazine remaining in Australia.- Magazine :...
are required on the board. There is usually some mechanism (via the BIOS
BIOS
In IBM PC compatible computers, the basic input/output system , also known as the System BIOS or ROM BIOS , is a de facto standard defining a firmware interface....
or a jumper
Jumper (computing)
In electronics and particularly computing, a jumper is a short length of conductor used to close a break in or bypass part of an electrical circuit...
setting) to select the amount of system memory to use for graphics, which means that the graphics system can be tailored to only use as much RAM as is actually required, leaving the rest free for applications. A side effect
Unintended consequence
In the social sciences, unintended consequences are outcomes that are not the outcomes intended by a purposeful action. The concept has long existed but was named and popularised in the 20th century by American sociologist Robert K. Merton...
of this is that when some RAM is allocated for graphics, it becomes effectively unavailable for anything else, so an example computer with 512 MiB
Mebibyte
The mebibyte is a multiple of the unit byte for digital information. The binary prefix mebi means 220, therefore 1 mebibyte is . The unit symbol for the mebibyte is MiB. The unit was established by the International Electrotechnical Commission in 2000 and has been accepted for use by all major...
RAM set up with 64MiB graphics RAM will appear to 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...
and user to only have 448 MiB RAM installed.
The disadvantage of this design is lower performance because system RAM usually runs slower than dedicated graphics RAM, and there is more contention as the memory bus
Memory bus
The memory bus is the computer bus which connects the main memory to the memory controller in computer systems. Originally, general-purpose buses like VMEbus and the S-100 bus were used, but to reduce latency, modern memory buses are designed to connect directly to DRAM chips, and thus are...
has to be shared with the rest of the system. It may also cause performance issues with the rest of the system if it is not designed with the fact in mind that some RAM will be 'taken away' by graphics.
Other approach that gave similar final state is boost up of graphics, that is used in some SGi
Silicon Graphics
Silicon Graphics, Inc. was a manufacturer of high-performance computing solutions, including computer hardware and software, founded in 1981 by Jim Clark...
computers, most notably O2/O2+
SGI O2
The O2 is an entry-level Unix workstation introduced in 1996 by Silicon Graphics, Inc. to replace their earlier Indy series. Like the Indy, the O2 used a single MIPS microprocessor and was intended to be used mainly for multimedia. Its larger counterpart was the SGI Octane...
. The memory in these machines is simply one fast pool (2,1 GB/s per second in 1996) shared between system and graphics is done on demand including pointer redirection communication between main system and graphics subsystem. This is called Unified Memory Architecture (UMA).
History
The first PC to use the SMA was the IBM PCjrIBM PCjr
The IBM PCjr was IBM's first attempt to enter the home computer market. The PCjr, IBM model number 4860, retained the IBM PC's 8088 CPU and BIOS interface for compatibility, but various design and implementation decisions led the PCjr to be a commercial failure.- Features :Announced November 1,...
, released in 1984. Video memory
Video memory
Video memory is a term generally used in computers to describe some form of writable memory, usually RAM, dedicated to the purpose of holding the information necessary for a graphics card to drive a display device...
was shared with the first 128KiB of RAM. The exact size of the video memory could be reconfigured by software to meet the needs of the current program.
Another early design was the Commodore
Commodore International
Commodore is the commonly used name for Commodore Business Machines , the U.S.-based home computer manufacturer and electronics manufacturer headquartered in West Chester, Pennsylvania, which also housed Commodore's corporate parent company, Commodore International Limited...
Amiga
Amiga
The Amiga is a family of personal computers that was sold by Commodore in the 1980s and 1990s. The first model was launched in 1985 as a high-end home computer and became popular for its graphical, audio and multi-tasking abilities...
, introduced in 1985. It initially featured 256 KiB
Kibibyte
The kibibyte is a multiple of the unit byte for quantities of digital information. The binary prefix kibi means 1024; therefore, 1 kibibyte is . The unit symbol for the kibibyte is KiB. The unit was established by the International Electrotechnical Commission in 1999 and has been accepted for use...
of "chip RAM
Chip RAM
Chip RAM is a commonly used term for the integrated RAM used in Commodore's line of Amiga computers. Chip RAM is shared between the central processing unit and the Amiga's dedicated chipset...
" (later up to 2048 KiB, depending on the model). This RAM was used by both the CPU (as main memory) and the Amiga's custom chipset
Original Amiga chipset
The Original Chip Set was a chipset used in the earliest Commodore Amiga computers and defined the Amiga's graphics and sound capabilities...
(for sound/graphics/IO). By default, most Amiga computers were shipped with chip RAM only, but could be expanded with RAM that only the CPU could access (called "Fast RAM"), through expansion boards.
See also
- IBM PCjrIBM PCjrThe IBM PCjr was IBM's first attempt to enter the home computer market. The PCjr, IBM model number 4860, retained the IBM PC's 8088 CPU and BIOS interface for compatibility, but various design and implementation decisions led the PCjr to be a commercial failure.- Features :Announced November 1,...
- Video memoryVideo memoryVideo memory is a term generally used in computers to describe some form of writable memory, usually RAM, dedicated to the purpose of holding the information necessary for a graphics card to drive a display device...
- Shared memoryShared memoryIn 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...
, in general, other than graphics