The CPU central processing unit and the GPU graphic processing unit

The CPU and the GPU

In a simple system, there may be only one processor, the Central Processing Unit (CPU) of the system, which must do both the normal processing and the graphical processing. The main graphical function of the processor is to take specifications of graphical primitives (such as lines, circles, and polygons) generated by application programs and to assign values to the pixels in the frame buffer that best represent these entities. For example, a triangle is specified by its three vertices, but to display its outline by the three line segments connecting the vertices, the graphics system must generate a set of pixels that appear as line segments to the viewer. The conversion of geometric entities to pixel colours and locations in the frame buffer is known as rasterization, or scan conversion.

In early graphics systems, the frame buffer was part of the standard memory that could be directly addressed by the CPU. Today, virtually all graphics systems are characterized by special-purpose Graphics Processing Units (GPUs), custom-tailored to carry out specific graphics functions. The GPU can be either on the mother board of the system or on a graphics card. The frame buffer is accessed through the graphics processing unit and usually is on the same circuit board as the GPU. GPUs have evolved to where they are as complex as or even more complex than CPUs. They are characterized by both special-purpose modules geared toward graphical operations and a high degree of parallelism—recent GPUs contain over 100 processing units, each of which is user programmable. GPUs are so powerful that they can often be used as mini supercomputers for general purpose computing.

Graphics Processing Unit

A Graphics Processing Unit or GPU (also occasionally called visual processing unit or VPU) is a specialized circuit designed to rapidly manipulate and alter memory in such a way so as to accelerate the building of images in a frame buffer intended for output to a display. GPUs are used in embedded systems, mobile phones, personal computers, workstations, and game consoles. Modern GPUs are very efficient at manipulating computer graphics, and their highly parallel structure makes them more effective than general-purpose CPUs for algorithms where processing of large blocks of data is done in parallel. In a personal computer, a GPU can be on a video card, or it can be on the motherboard, or in certain CPUs, on the CPU die. An example is the GeForce 6600GT GPU shown in Figure. More than 90% of new desktop and notebook computers have integrated GPUs, which are usually far less powerful than those on a dedicated video card.

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Figure  GeForce 6600GT GPU 8

GPU forms

There are various GPU forms characterized by their interfaces with the main board. The common ones are mentioned below.

Dedicated graphics cards

The GPUs of the most powerful class typically interface with the motherboard by means of an expansion slot such as PCI Express (PCIe) or Accelerated Graphics Port (AGP) and can usually be replaced or upgraded with relative ease, assuming the motherboard is capable of supporting the upgrade. A few graphics cards still use Peripheral Component Interconnect (PCI) slots, but their bandwidth is so limited that they are generally used only when a PCIe or AGP slot is not available.

A dedicated GPU is not necessarily removable, nor does it necessarily interface with the motherboard in a standard fashion. The term “dedicated” refers to the fact that dedicated graphics cards have RAM that is dedicated to the card’s use, not to the fact that most dedicated GPUs are removable. Dedicated GPUs for portable computers are most commonly interfaced through a non-standard and often proprietary slot due to size and weight constraints. Such ports may still be considered PCIe or AGP in terms of their logical host interface, even if they are not physically interchangeable with their counterparts.

Integrated graphics solutions

Integrated graphics solutions, shared graphics solutions, or Integrated Graphics Processors (IGP) utilize a portion of a computer’s system RAM rather than dedicated graphics memory. They are integrated into the motherboard. Exceptions are AMD’s IGPs that use dedicated side-port memory on certain motherboards, and APUs, where they are integrated with the CPU die. Computers with integrated graphics account for 90% of all PC shipments. These solutions are less costly to implement than dedicated graphics solutions, but are less capable. Historically, integrated solutions were often considered unfit to play 3D games or run graphically intensive programs but could run less intensive programs such as Adobe Flash. Modern desktop motherboards often include an integrated graphics solution and have expansion slots available to add a dedicated graphics card later.

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As a GPU is extremely memory intensive, an integrated solution may find itself competing for the already relatively slow system RAM with the CPU, as it has minimal or no dedicated video memory. System RAM may be 2 GB/s to 16 GB/s, yet dedicated GPUs enjoy between 10 GB/s to over 300 GB/s of bandwidth depending on the model (for instance the GeForce GTX 590 and Radeon HD 6990 provide approximately 320 GB/s between dual memory controllers). Older integrated graphics chipsets lacked hardware transform and lighting, but newer ones include it 9

Hybrid solutions

This newer class of GPUs competes with integrated graphics in the low-end desktop and notebook markets. The most common implementations of this are ATI’s HyperMemory and NVIDIA’s TurboCache. Hybrid graphics cards are somewhat more expensive than integrated graphics, but much less expensive than dedicated graphics cards. These share memory with the system and have a small dedicated memory cache, to make up for the high latency of the system RAM. Technologies within PCI Express can make this possible. While these solutions are sometimes advertised as having as much as 768MB of RAM, this refers to how much can be shared with the system memory.

The Graphics pipeline

In 3D computer graphics, the terms graphics pipeline or rendering pipeline most commonly refers to the current state of the art method of rasterization-based rendering as supported by commodity graphics hardware. The graphics pipeline typically accepts some representation of a three-dimensional primitive as an input and results in a 2D raster image as output. OpenGL and Direct3D are two notable 3D graphic standards, both describing very similar graphic pipeline.

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The rendering pipeline is mapped onto current graphics acceleration hardware such that the input to the graphics card (GPU) is in the form of vertices. These vertices then undergo transformation and per-vertex lighting. At this point in modern GPU pipelines a custom vertex shader program can be used to manipulate the 3D vertices prior to rasterization. Once transformed and lit, the vertices undergo clipping and rasterization resulting in fragments as shown in figure. A second custom shader program can then be run on each fragment before the final pixel values are output to the frame buffer for display.

FIGURE 1.1(c) Arithmetic pipeline (Ed Angel (1991)


FIGURE 1.1(d) Geometric pipeline. (Ed Angel, 1991)

The graphics pipeline is well suited to the rendering process because it allows the GPU to function as a stream processor since all vertices and fragments can be thought of as independent. This allows 10 all stages of the pipeline to be used simultaneously for different vertices or fragments as they work their way through the pipe. In addition to pipelining vertices and fragments, their independence allows graphics processors to use parallel processing units to process multiple vertices or fragments in a single stage of the pipeline at the same time.

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