In
computer architecture
In computer engineering, computer architecture is a description of the structure of a computer system made from component parts. It can sometimes be a high-level description that ignores details of the implementation. At a more detailed level, the ...
, shared graphics memory refers to a design where the
graphics chip
A video display controller or VDC (also called a display engine or display interface) is an integrated circuit which is the main component in a video-signal generator, a device responsible for the production of a TV video signal in a computin ...
does not have its own dedicated memory, and instead shares the main system
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
* ...
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
''CHiPs'' is an American crime drama television series created by Rick Rosner and originally aired on NBC from September 15, 1977, to May 1, 1983. It follows the lives of two motorcycle officers of the California Highway Patrol (CHP). The seri ...
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
In medicine, a side effect is an effect, whether therapeutic or adverse, that is secondary to the one intended; although the term is predominantly employed to describe adverse effects, it can also apply to beneficial, but unintended, consequence ...
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 64 MiB graphics RAM will appear to the
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 i ...
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
In computer architecture, a bus (shortened form of the Latin '' omnibus'', and historically also called data highway or databus) is a communication system that transfers data between components inside a computer, or between computers. This e ...
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.
A similar approach that gave similar results is the boost up of graphics used in some
SGi computers, most notably the
O2/O2+. The memory in these machines is simply one fast pool (2.1 GB per second in 1996) shared between system and graphics. Sharing is performed on demand, including pointer redirection communication between main system and graphics subsystem. This is called Unified Memory Architecture (UMA).
History
Most early personal computers used a shared memory design with graphics hardware sharing memory with the CPU. Such designs saved money as a single bank of DRAM could be used for both display and program. Examples of this include the Apple II computer, the Commodore 64, the Radio Shack Color Computer, the Atari ST, and the Apple Macintosh.
A notable exception was the IBM PC. Graphics display was facilitated by the use of an expansion card with its own memory plugged into an ISA slot.
The first IBM PC to use the SMA was the
IBM PCjr, released in 1984.
Video memory
Dynamic random-access memory (dynamic RAM or DRAM) is a type of random-access semiconductor memory that stores each bit of data in a memory cell, usually consisting of a tiny capacitor and a transistor, both typically based on metal-oxid ...
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.
An early hybrid system was the Commodore Amiga which could run as a shared memory system, but would load executable code preferentially into non-shared "
fast 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 (CPU) and the Amiga's dedicated chipset (hence the name). It was also, rather misleadingly, ...
" if it was available.
See also
*
IBM PCjr
*
Video memory
Dynamic random-access memory (dynamic RAM or DRAM) is a type of random-access semiconductor memory that stores each bit of data in a memory cell, usually consisting of a tiny capacitor and a transistor, both typically based on metal-oxid ...
*
Shared memory
In computer science, shared memory is memory that may be simultaneously accessed by multiple programs with an intent to provide communication among them or avoid redundant copies. Shared memory is an efficient means of passing data between progr ...
, in general, other than graphics
External links
PC Magazine Definition for SMAIBM PCjr information
{{Graphics Processing Unit
Memory management
ro:Arhitectură cu memorie partajată
ru:SMA