PC Gaming Week: The anatomy of a graphics card - from overclocking to memory

Core, Fabrication Process, Memory

Ever wondered just what goes on in the graphics cards that power your favourite games? If you're a PC gamer, the answer is problably yes. The following article is from The Ultimate PC Building Handbook, published by Future, which contains 180 pages of PC upgrading advice from the experts at PC Format and Maximum PC.

The core

The processing core functions in a similar manner to a PC's processor - it uses clock cycles and cores, and speed is measured in hertz. The specialised demands of this hardware means a graphics processing unit, or GPU, is built in a different way.

The mathematical demands of a GPU mean that high-end graphics cards have more transistors than high-end processors; an Nvidia GeForce GTX 780, for instance, has seven billion, which is more than four times the number included in Intel's Core i7-4770K.

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Transistors form stream processors, which function as tiny processing cores. These cores can be used to produce specific parts of images, and graphics cards have hundreds - or at the high end, thousands. The huge numbers help with the intensely parallelised workloads handled by GPUs and, as ever, higher clock speeds mean faster results.

Nvidia and AMD organise hundreds of stream processors into large clusters that aid workload delegation and organisation. Like a desktop CPU, they share caches and memory interfaces.

The fabrication process

This refers to the method that wafers - and, therefore, the dies that create graphics cores - are produced. AMD and Nvidia contract external firms such as TSMC and GlobalFoundries to build these, and they're always pushing to build wafers with smaller transistors.

Transistor size is measured in nanometres, and reducing this number has several advantages. If transistors are smaller, more can be packed into a graphics card's die - which means more raw processing horsepower.

Smaller transistors have improved thermal efficiency, which means higher clock speeds - and, again, more power.

It's a constant challenge to produce smaller transistors consistently. Current GPUs use 28nm dies, and there's no firm date for a jump to 20nm.

Memory

Graphics cards are powerful enough that they need dedicated memory - and, as usual, the more and the faster the better. It's used to temporarily store the data thats been generated by the graphics card before it's used to render frames on-screen.

Current graphics cards use GDDR5 memory, which is based on the DDR3 memory used in desktop PCs and laptops. Graphics memory is configured differently to DDR3, with higher bandwidth at the expense of latency in order to handle consistent, large amounts of data - a challenge encountered by graphics cards.

Even the cheapest GPU will be accompanied by 2GB of GDDR5, but that's the bare minimum for playing top-tier games. Mid-range cards are now often sold with 3GB or 4GB, and top-end cards sometimes have more.

Make sure a new graphics card has enough memory and at a high enough speed to last the course, because it can't be upgraded, unlike RAM inside a desktop PC.

Overclocking, Slots, Physical Challenges, AMD/Nvidia

Overclocking

Graphics cards use the same basic components as processors, and they're even easier to overclock than their socketed stablemates.

For starters, GPU overclocking doesn't require fiddling in the BIOS. It can be done in software, with some options available in AMD and Nvidia's drivers, and other third-party tools that can improve speeds by clicking a button.

Graphics overclocking involves tweaking the core and memory clocks, and sometimes the boost limits - so cards can dynamically overclock to even higher speeds.

The GPU equivalent of upping the voltage involves raising the card's power limit, which is usually as simple as raising a slider.

It's easy to overclock a graphics card but, as with desktop processors, it is a risky business. If damage is caused to the card because of overclocking, it can void its warranty - as well as break the card.

Slots and display connections

A graphics card interfaces with the rest of a PC via a PCI Express slot, which can cope with the kind of demanding data transfers required by graphics cards. Motherboards have PCI Express sockets in a variety of sizes, and most graphics cards use x16 slots - the largest. These fall into two standards: PCI Express 2.0 and 3.0. The latter is newer and has ample bandwidth for even the most powerful cards, where 2.0 might struggle.

Also check whether a PCI Express slot is able to use its full bandwidth. Most motherboards have one PCI Express x16 slot that runs in x16 mode, but secondary slots are often restricted to just x8 or x4 modes - so they're only able to use a half or a quarter of their potential bandwidth - due to chipset restrictions.

Physical challenges

Graphics card installation isn't as simple as pushing the card into a slot - the card needs to have the right connections, and the case needs to be large enough.

Powerful cards generate more heat, which means larger and more sophisticated cooling systems are required to keep the core chilled.

Larger heatsinks are sometimes too long to fit inside smaller cases, and double-width coolers can also block motherboard slots below the card.

That's not the only physical consideration. More powerful cards require more electricity, with some top chips needing two eight-pin power plugs to function. Most power supplies come with these connections, but it's worth checking in advance.

Every graphics card has a variety of outputs that connect to monitors. The most common connections found on modern cards are HDMI, DVI and DisplayPort - the latter is the newest, and is useful for connecting extremely high-resolution displays.

It's worth checking which outputs a card has before purchase to make sure they match your monitor - or so you can order an adaptor.

AMD and Nvidia decoded

AMD and Nvidia are the big players in graphics, with both firms battling for GPU supremacy. Both companies use different naming schemes. Nvidia's cards are all prefixed with 'GeForce', and top-end cards also use 'GTX'; low-end products only use 'GT'. Beyond that, the bigger the number the better - cards such as the GTX 750 and 760 are mid-range products, with the GTX 770 and 780 at the high end. Cards with the 'Ti' suffix are slightly more powerful versions of their origin card.

AMD's Radeon naming scheme has recently changed. Now its cards are divided into three groups: R5 cards are entry-level products; R7-branded parts sit in the mid-range; and high-end chips are denoted by the R9 prefix. Beyond that, it's a similar story to Nvidia - the bigger the number, the better the card, and the higher the price. Cards with an X attached to their names are more powerful.

The companies tend to follow each other closely when it comes to price and performance - it's handy to look for overclocked cards and game bundles to help differentiate between the two when buying a new card.

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