What was the advantage (if any) of slot 1/slot a CPUs compared to socketed CPUs? Why were slots abandoned so quickly? Is there any chance of slots being used again on motherboards?
I wonder why this OP dropped off with no replies?
Since I firmly believe that every sensible GQ deserves a response, I’ll give it a shot (even though I’m not in the CPU business). Feel free to chime in and correct me!
From the earliest member of Intel’s x86 series of CPUs (the 8086, introduced in 1978) through the Pentium MMX and Pentium Pro (1997), sockets were used (starting with 40-pin DIPs for the 8086/8088, and going through Socket types 1, 2, 3, 4, 5, 6, and 7). The CPUs from 8086 through 80486 had no on-chip cache, although external caches were often added as socketed DIP IC’s on the motherboard. The early Pentiums had two on-chip L1 8kB caches (for instructions and data), and some could support external Cache-On-A-Stick (COAST) modules as shown here. However, as CPU clock speeds increased, these external L2 caches could not keep pace due to the physical distance between CPU and cache, together with the resistance and inductance of the sockets.
The obvious solution for close-coupling of the CPU and L2 cache is to put them all on one die, but process technologies were not yet up to that task for large caches. So, Intel came up with two solutions: the Socket 8 for the Pentium Pro in 1995, and Slot 1 for the Pentium II in 1997 (timeline here). In both of these cases, an external cache was put on a daughterboard next to the CPU. Intel really pushed Slot 1 (and later Slot 2) as the wave of the future.
However, as new process technologies allowed more and more transistors on each chip with ever-improving yields, the Level 2 cache could be integrated on the same die as the CPU, and the whole raison d’être of the Slot vanished. Slots 1 & 2 take up more motherboard real estate than the later sockets, and smaller footprint = lower cost. In addition, the daughterboard used in the slot is an additional cost and manufacturing step, so sockets won out as soon as it became technically feasible to put the L2 cache on-chip.
In summary, the slot was a short-term solution to a problem that was solved quickly thereafter.
Will we ever see them again? One could imagine various future scenarios in which daughterboard-based CPUs could be useful (maybe with close-coupled multiple processors?), and a well-designed “slot” format (daughterboard at right angle to motherboard) can reduce real estate and increase airflow compared to a “piggyback” daughterboard parallel to the motherboard. So, if ever the advantage of a daughterboard-based CPU system outweigh the extra cost and complexity, the slot will reappear.
Hope this helps!
Great answer! Thanks 