If you’ve ever installed a CPU (or watched it done) you’ve almost certainly seen a small square object slot into a flat socket on a motherboard. For a brief time in PC history, though, desktop processors looked like NES cartridges, sometimes with fans strapped to them!
The Rise of the Slot CPU: Pentium II’s Debut
In May of 1997 Intel launched the Pentium II. Rather than launching these processors in a package where a grid of pins were inserted into a grid of holes in a motherboard socket, the “Slot 1” design used an edge connector. This made it more like a graphics card, which to this day still used a processor on card design.
Even back then, the competition between Intel and AMD was red-hot in the CPU market, so unsurprisingly AMD soon followed with it’s imaginatively-named “Slot A.” Hey, “1” and “A” are both at the top of their respective stacks! Since AMD played follow-the-leader so swiftly, you’d be forgiven for thinking that there must have been something to this approach, and you’d be right.
Why a Slot CPU?
There are genuine benefits to moving your CPU onto a card format like this. How important those benefits in practice are debatable, but they are undeniable:
- Thermal and power benefits: If your CPU is on a card, you can build exactly the right cooling solution for it. You can cool it from both sides of the PCB, and ensure the right airflow gets to your parts. This is still true for graphics cards today, and the Pentium II was a hot and hungry chip by the standards of the time. I had no problem overclocking my Pentium II 400 to 500Mhz using just the stock cooling, so there was clearly a nice chunk of overhead.
- CPU installation for dummies: You don’t have to go far on the internet to find horror tales of people bending or breaking pins on their CPUs. You only need to mess up a single pin our of the hundreds or thousands for the entire CPU to be ruined. A slot design like this is just as robust as installing RAM or an expansion card. You’d have to be Mr. Bean to mess up a slot installation.
- Integrated Cache Memory: During the Pentium II era, cache memory was not on-die as it is with modern CPUs. By going with a card design, Intel and AMD could put cache memory chips close to the CPU, with plenty of bandwidth and little latency.
Even today, Slot 1 Pentium II CPUs seem somehow more futuristic than even the latest CPUs of today, so you can imagine how we thought this was going to be what CPUs would look like going ahead. What actually happened is that some Pentium III CPUs used Slot 1, but that was the end of it, with Socket 370 taking the PIII back to a traditional socket design. Since then desktop CPUs have generally used a socket design, increasing the number of pins. The major change was moving the pins from the CPU package to the motherboard, so if you bent any pins, at least the CPU would be OK.
Why Didn’t Card Slot CPUs Work Out?
Even in hindsight, it’s hard to pinpoint any single factor that killed off the idea of a CPU card. By the early 2000s, both Intel and AMD phased out the slot-style CPUs in favor of the more familiar socket approach. I think this shift was largely driven by the need for more compact, cost-effective, and efficient designs. Three key factors are responsible, in my opinion:
- Size and space: While the card design approach gives you more control over cooling design, you don’t have as much room to expand as with a vertical design. By the Pentium 4, CPU heat had already gone out of control, and even today, high-performance CPUs need tall air coolers with plenty of heatsink volume and even liquid cooling.
- Cost and complexity: The slot approach means making a traditional CPU, and then also making the slotted PCB that it goes into. This is more complex and costly, and the financial incentive for the slot design probably faded quickly as CPU technology progressed.
- On-die cache: While the Pentium Pro already had on-die cache (integrated into the CPU itself) in 1995, this was too expensive for consumer CPUs, so the Pentium II launched with its L2 cache as an external chip running at half the CPU’s clock speed. During this time, Intel experimented with on-die cache for the cheaper Celeron CPUs, which lead to a situation where cheaper Celeron’s with on-die cache could outperform more expensive Pentium IIs with off-die cache. By the time the “Coppermine” Pentium IIIs launched on-die L2 cache was already the norm and so one major reason for the slot design just fell away.
The socket design for CPUs still seems to be the best way to go about it, but we shouldn’t make the mistake of thinking we’ll never see another approach. CPUs are once again heading for a number of technological walls and new and radical CPU designs might need a different way to connect to the rest of the computer. Half the fun of being a computer geek is seeing what wild new approach engineers come up with!
