The CPUs included in a laptop are vastly different from those used in a desktop in terms of a multitude of factors such as their respective power consumption, form factor, socket type, etc. In this article, we have discussed all you need to know about each of the respective differences one at a time. For the more curious among you, before we start with the differences between laptop and desktop processor, we have also included a brief description of what processors are and how they operate.
What are processors?
Processors are highly complex electronic devices which might be thought of as being the brains of the computer. It takes input from one or more input devices, computes the result and communicates the same to the user through one or more output devices. The basic functions of a modern processor might be subdivided into three parts- Arithmetic operations, Logical operations and I/O operations. The processor performs these operations by using predefined instructions to operate on bits of data, where a bit is simply a binary state containing 0 or 1.
The aforementioned set of instructions which tells the processor how to operate on a given chunk of data is called an instruction set. Most modern laptop/desktop processors use two major instruction sets, namely, x86_64 and ARM. Devices which operate using the mechanism outlined in the previous paragraph are traditionally divided into two parts- Microprocessors and Microcontrollers. If you would like to find out more, we have a separate article discussing Microprocessors vs. Microcontrollers in detail.
In modern processors, the complexity of the underlying circuitry can be more closely imagined as a sprawling city on a nano scale. For a sense of said scale, the AMD Ryzen 5 5600X, which is a mid-range processor, has a typical area of only a few square millimeters. However, on those few square millimeters, exists 19.2 billion transistors executing billions of instructions per second. Naturally, harmony in the various sub processes needs to be maintained without which such a complex electronic device cannot perform properly.
At a very high level, a processor synchronizes all of its various operations using an electronic “clock” inside its circuitry. Governed by the clock, the processor completes processing cycles where various instructions are executed during each cycle. The number of such cycles completed per second constitutes the clock speed of the processor and is measured in hertz (Hz). Modern processors are generally capable of billions of cycles a second and thus their clock speed is measured in gigahertz or GHz where giga stands for an order of magnitude equal to 10 raised to the power of 9.
Key Differences between a laptop and a desktop processor
As a completely separate market segment, laptops have vastly different requirements as compared to desktops. Naturally, for a processor to be successful in either market, it has to respond very favourably to the unique challenges and requirements inherent to the respective markets. For example, laptops typically derive their power from a battery due to their mobile nature. Consequently, a laptop offering increased screen-on time would be a more competitive product in the market. Thus, a processor running in such a laptop would also have to be highly power and thermally efficient. Going forward, we have discussed each of such differences one by one.
A laptop processor is generally a cut-down variant of desktop processors, although typically both said laptop and desktop chips tend to be based on the same underlying microarchitecture. The desktop chips can afford to draw much higher power since there is no battery optimization involved, while a laptop on the other hand is not capable of supporting, say, a 125W CPU. Why is that? Well, the battery technology that would be required for supporting such kind of hardware does not exist at the present moment. Such a battery would have to be capable of quick discharge, high capacity, be lightweight and take up a minimum amount of space at the same time, due to chassis limitations.
For instance, a top-of-the-line Dell XPS 15 with an 86 Wh battery would be capable of powering an Intel Core i7 11700k, running at a constant power draw of 125W, for a little over 41 minutes. Said number is for the processor alone. It does not consider the energy budget allocated for powering the screen, the chipset, storage devices, the RAM, etc.
Thus, laptop processors are generally limited to a TDP of 15 W. In high-performance laptops, such as the ones designed for gaming, the processors are generally limited to about 45 Watts.
Laptop processors are also designed to be more thermally efficient than their desktop counterparts. Due to a laptop processor’s power limitations, it does not generate a ton of heat to begin with. However, it still generates enough heat to seriously harm the system if said heat is not dealt with properly. This is where a cooling system comes into play.
Due to a typical laptop chassis’s volumetric limitations, there isn’t much room to fit a beefy cooling solution. Flattened heat pipes and fans are generally used while vapor-chamber cooling can also be found in more expensive systems. Due to the processor sitting directly underneath the keyboard, improper management of the waste heat can lead to major discomfort when typing. At the same time, the processor might automatically limit its clock speed (thermal throttling) in an attempt to manage the output heat, or outright shut down if the temperature reaches a critical value. If the processor repeatedly turns off due to high temperatures, it is quite harmful for the system’s longevity.
Desktops are typically more modular when compared to their laptop counterparts. Keeping parts non-modular can result in tighter power, temperature and quality control, reduce costs and can even provide a more integrated experience. However, modularity in the system allows the user experience to be upgraded anytime on a per-part basis instead of a per-system basis. Hence, desktop processors are generally designed for two types of sockets- LGA (Land Grid Array) and PGA (Pin Grid Array). Both LGA and PGA have their respective advantages and disadvantages but they are equally modular.
In laptops, however, processors are generally soldered into the motherboard in a socket type known as BGA (Ball Grid Array). This eliminates any possibility of a user-upgradeable processor. Instead, a customer purchasing a laptop can only use the included processor for the lifetime of the product and replace the whole system with a new model when it becomes too slow or cumbersome for his/her use case.
As mentioned previously, laptop processors tend to be severely power constrained. This limitation directly affects their performance as well. Thus a laptop Intel Core i7 11850H would not perform as fast as a desktop variant Intel Core i7 11700k. Due to the increased thermal budget, an 11700k is also allowed to boost higher for longer as compared to the 11850H.
Desktop CPUs can be anywhere from 25 – 100% faster than the equivalent laptop part and are also generally cheaper.
Laptops have very limited space available for cramming a beefy cooling solution due to its extreme form factor geared towards maximum mobility. Thus, in a laptop, cooling is generally handled by a couple heat pipes and two to three fans. The heat pipes move the heat away from the processor, while the fans blow cool air, transferring the heat from the pipes to the air, heating it up and exhausting it out. On more expensive systems, one might find the addition of a vapor chamber cooling solution while on a couple systems like the new MacBook Air with the coveted M1 chip, all the cooling is managed passively, i.e. without using any fans or other active components.
Desktop cooling solutions, on the other hand, can range from simple air coolers such as the Noctua NH-U14S, the CoolerMaster Hyper 212, etc., all the way up to crazy liquid cooled setups with multiple 360 or 280 mm radiators and custom loops, paired with enough RGB LEDs to make the system look like something out of a neon-lit city from a dystopian future.
RAM Capacity, Clock Speed and Core Count
This difference isn’t so much of a difference as it is a general trend. In other words, it is not applicable to all case comparisons between a laptop and a desktop system. Instead, it is a general trend of how each type of system is configured.
In terms of the total RAM capacity, laptops are generally limited to 16-32 GB RAM while desktops can feature far greater capacities, although limited by factors such as the total number of RAM slots in the motherboard, capacity per DIMM and the Operating System’s maximum RAM support.
Desktop CPU clock speeds also tend to be higher as compared to the equivalent laptop variant. This is also a direct result of the increased power and thermal budget found in desktops as compared to laptops.
The total number of cores in a desktop chip also tends to follow the previous trend of being higher than the laptop counterparts. However, this trend is quickly changing as engineers find new ways to build more power efficient chips which nearly negate the performance delta between a laptop and an equivalent desktop processor.