In computer architecture, shared graphics memory 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 512MiB 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.
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 (2.1GB 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.[citation needed]
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 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" if it was available.
Amiga is a family of personal computers produced by Commodore from 1985 until the company's bankruptcy in 1994, with production by others afterward. The original model is one of a number of mid-1980s computers with 16-bit or 16/32-bit processors, 256 KB or more of RAM, mouse-based GUIs, and significantly improved graphics and audio compared to previous 8-bit systems. These include the Atari ST—released earlier the same year—as well as the Macintosh and Acorn Archimedes. The Amiga differs from its contemporaries through custom hardware to accelerate graphics and sound, including sprites, a blitter, and four channels of sample-based audio. It runs a pre-emptive multitasking operating system called AmigaOS.
The Amiga 500, also known as the A500, was the first popular version of the Amiga home computer, "redefining the home computer market and making so-called luxury features such as multitasking and colour a standard long before Microsoft or Apple sold these to the masses." It contains the same Motorola 68000 as the Amiga 1000, as well as the same graphics and sound coprocessors, but is in a smaller case similar to that of the Commodore 128.
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The Amiga 2000 (A2000) is a personal computer released by Commodore in March 1987. It was introduced as a "big box" expandable variant of the Amiga 1000 but quickly redesigned to share most of its electronic components with the contemporary Amiga 500 for cost reduction. Expansion capabilities include two 3.5" drive bays and one 5.25" bay that could be used by a 5.25" floppy drive, a hard drive, or CD-ROM once they became available.
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. It was also, rather misleadingly, known as "graphics RAM".
The Apple IIc is a personal computer introduced by Apple Inc. shortly after the launch of the original Macintosh in 1984. It is essentially a compact and portable version of the Apple IIe. The IIc has a built-in floppy disk drive and a keyboard, and was often sold with its matching monitor. The c in the name stands for compact, referring to the fact it is a complete Apple II setup in a smaller notebook-sized housing. It is compatible with a wide range of Apple II software and peripherals.
The IBM PCjr was a home computer produced and marketed by IBM from March 1984 to May 1985, intended as a lower-cost variant of the IBM PC with hardware capabilities better suited for video games, in order to compete more directly with other home computers such as the Apple II and Commodore 64.
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The Adventure Game Interpreter (AGI) is a game engine developed by Sierra On-Line. The company originally developed the engine for King's Quest (1984), an adventure game that Sierra and IBM wished to market in order to attract consumers to IBM's lower-cost home computer, the IBM PCjr.
A video display controller (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 computing or game system. Some VDCs also generate an audio signal, but that is not their main function. VDCs were used in the home computers of the 1980s and also in some early video picture systems.
The Amiga is a family of home computers that were designed and sold by the Amiga Corporation from 1985 to 1994.
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Home computers were a class of microcomputers that entered the market in 1977 and became common during the 1980s. They were marketed to consumers as affordable and accessible computers that, for the first time, were intended for the use of a single, non-technical user. These computers were a distinct market segment that typically cost much less than business, scientific, or engineering-oriented computers of the time, such as those running CP/M or the IBM PC, and were generally less powerful in terms of memory and expandability. However, a home computer often had better graphics and sound than contemporary business computers. Their most common uses were word processing, playing video games, and programming.
Tandy Graphics Adapter is a computer display standard for the Tandy 1000 series of IBM PC compatibles, which has compatibility with the video subsystem of the IBM PCjr but became a standard in its own right.
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