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.
In computer engineering, computer architecture is a set of rules and methods that describe the functionality, organization, and implementation of computer systems. Some definitions of architecture define it as describing the capabilities and programming model of a computer but not a particular implementation. In other definitions computer architecture involves instruction set architecture design, microarchitecture design, logic design, and implementation.
A central processing unit (CPU), also called a central processor or main processor, is the electronic circuitry within a computer that carries out the instructions of a computer program by performing the basic arithmetic, logic, controlling, and input/output (I/O) operations specified by the instructions. The computer industry has used the term "central processing unit" at least since the early 1960s. Traditionally, the term "CPU" refers to a processor, more specifically to its processing unit and control unit (CU), distinguishing these core elements of a computer from external components such as main memory and I/O circuitry.
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.
A motherboard is the main printed circuit board (PCB) found in general purpose microcomputers and other expandable systems. It holds and allows communication between many of the crucial electronic components of a system, such as the central processing unit (CPU) and memory, and provides connectors for other peripherals. Unlike a backplane, a motherboard usually contains significant sub-systems such as the central processor, the chipset's input/output and memory controllers, interface connectors, and other components integrated for general purpose use and applications.
BIOS is non-volatile firmware used to perform hardware initialization during the booting process, and to provide runtime services for operating systems and programs. The BIOS firmware comes pre-installed on a personal computer's system board, and it is the first software to run when powered on. The name originates from the Basic Input/Output System used in the CP/M operating system in 1975. Originally proprietary to the IBM PC, the BIOS has been reverse engineered by companies looking to create compatible systems. The interface of that original system serves as a de facto standard.
In electronics and particularly computing, a jumper is a short length of conductor used to close, open or bypass part of an electronic circuit. They are typically used to set up or configure printed circuit boards, such as the motherboards of computers.
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.
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 designed by chip standards bodies such as JEDEC. Examples are the various generations of SDRAM, and serial point-to-point buses like SLDRAM and RDRAM. An exception is the Fully Buffered DIMM which, despite being carefully designed to minimize the effect, has been criticized for its higher latency.
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).
Silicon Graphics, Inc. was an American high-performance computing manufacturer, producing computer hardware and software. Founded in Mountain View, California in November 1981 by Jim Clark, its initial market was 3D graphics computer workstations, but its products, strategies and market positions developed significantly over time.
The O2 is an entry-level Unix workstation introduced in 1996 by Silicon Graphics, Inc. (SGI) 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 is the SGI Octane. The O2 was SGI's last attempt at a low-end workstation.
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 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.
The IBM PCjr is a home computer that was produced and marketed by IBM from March 1984 to May 1985. The PCjr was positioned as a complement to the very successful IBM Personal Computer (PC), competing with other home computers such as the Apple II series and the Commodore 64. It retains the IBM PC's 8088 CPU and BIOS interface, but provides enhanced graphics and sound capabilities over the original IBM PC, ROM cartridge slots, joystick ports, and an infrared wireless keyboard. The PCjr supports expansion via "sidecar" modules, which are attached to the side of the unit. New software such as King's Quest I showcased its PC gaming capabilities.
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.
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".
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Related Research Articles
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Home computers were a class of microcomputers that entered the market in 1977, that started with what Byte Magazine called the "trinity of 1977", and which 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 nontechnical 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 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 playing video games, but they were also regularly used for word processing, doing homework, and programming.
Tandy Graphics Adapter (TGA) is a computer display standard for an IBM PC compatible video subsystem that improved on IBM's Color Graphics Adapter (CGA) technology. Whereas CGA could display only four colors at a time at a screen resolution of 320×200 pixels, a TGA system could display up to 16 colors. While not strictly an adapter—the TGA hardware was available only integrated onto computer motherboards, not on a separate card—TGA is so called to parallel CGA, to which TGA is related and with which it competed. TGA is also known as Tandy graphics.
