We can add a graphics card via PCIe, so why not a processor?

For any central processor, the biggest hurdle is communication with RAM, there is no more than that. It is about global limitations that will always exist for material reasons and at the same time make certain scenarios impossible. The issue at hand is having the processor as an expansion card, so we’re really going to talk about a scenario like that for graphics cards, where the main memory and CPU will be on the same PCI Express card.

Can another processor be added through an expansion card?

Imagine for a moment the following scenario: instead of using a socket to connect the CPU to the motherboard, Intel and AMD decided that the PC’s CPUs would come in the form of an expansion card that could be plugged into a PCI Express port.

At first glance, this would be ideal, as it would allow us to have universal motherboards for both Intel and AMD. Moreover, at the same time, the chipsets on the board will be universal and we will not have a large number of models of motherboards for different processors. It would be perfect, don’t you think? Well, this has never been implemented on a PC and the reasons, as many might think, are not economic, but technical. Although if it did, it wouldn’t be anything new either.

A little bit of historical perspective

Before the personal computer as we now know it was standardized, there was a standard based on MITS Altair called the S-100, where all components were connected through a common board that was a series of card slots. Exactly symmetrical expansion. Even the central processor was on a separate expansion card. Of course, they were systems for the minority at a time when the users themselves had to know both programming and soldering to create their own boards or sell them to third parties.

The above picture corresponds to an Intel 8086. Curiously, the three largest chips are not the CPU, but also the memory controller that has not been integrated at that time and the peripheral manager. the rest? ram. Note that even with a processor under 5MHz, the system RAM is not on another board. This is because if the distance between the processor and the memory is too high, it is harmful because the signal transmission slows down. Theoretically, the idea would only be to put the RAM and CPU on the board itself, but that brings us to the next question.

Can two cards of this kind communicate with each other?

The fact that we have two memory pools at a great distance from each other presents a series of problems, as we not only have to ensure that there are coherence mechanisms in a non-uniform system, which is a hardware nightmare. In addition to the fact that when data or instructions are found in another board’s memory, the latency will be very high.

What times will we talk about? The PCI Express interface can add, if we’re being generous, 200ns per access to what we’ll already get with a direct memory connection, so instructions will take longer to complete and overall performance will drop dramatically for the time it takes. It will take the CPU to receive the instructions. What has been done, instead, is to use a general purpose processor as a central accelerator, but this has not been done due to the fact that it has long been discovered that GPUs for certain tasks are more efficient than adding additional cores.

So no, you can’t add more cores to the CPU as an expansion card, would it be ideal to be able to? No doubt, but we hope you understand the reasons why it is completely impossible.