Serial CPUs?
It occurred to me yesterday that the type of interconnects used to attach devices to computers has come full circle. This left me wondering how much further the current trend is going to go.
Many of the earliest devices were attached to computers using various serial cables. Printers, terminals, and drives mostly started life as having some kind of serial interconnect. RS-232 (used with modems and older printers), EIA-422, ST-506 (used with the first PC hard drives), and ESDI (a successor to the ST-506 interface) were all serial interconnects.
Slowly, as the demand for higher bandwidth grew, those interconnects started moving to wider and wider parallel buses. ATA (formerly known as IDE, now known as PATA) for hard drives, SCSI for drives and other bulk data devices, and the parallel port for printers were the main examples of this on PCs. For many years these interfaces were clocked incrementally faster, and in some cases the buses grew wider as well.
Internally, where device attachment largely started with parallel connections, the same trend has been happening, with the ISA bus starting at 8 bits, then moving to 16, and then to 32 bits with EISA, MCA, VESA, PCI, and AGP. The PCI bus later developed a 64-bit variant known as PCI-X, and the speeds clocked higher as well.
However, those parallel interconnects have hit a brick wall, and it appears that they will not be back any time soon. As data transfer speeds increase, it becomes increasingly difficult to ensure that all the data on a parallel bus is valid at the same time. This necessitates correspondingly more complex and expensive circuitry to drive such a bus. When combined with the ability for engineers to build much faster circuitry, it has become much more effective to built interfaces based on serial mechanisms instead of parallel ones.
So, all major PC devices have now moved to serial connections of one form or another. FireWire is used for many digital media devices, SATA and SAS are used for storage drives, and USB is used for just about everything. Even the internal PCI bus has moved to a serial mechanism with PCIe. Even though a PCIe bus may transfer multiple bits simultaneously, it is physically composed of multiple serial "lanes" which are independently self-clocked.
This leads me to my question. All of the replaceable components on PCs, both externally and internally, have moved to serial interconnects except for two: RAM and CPUs. Admittedly, these two devices have the highest bandwidth requirements of all of the devices, so the technological constraints are higher. However, I still wonder. Are serial-attached CPUs as inevitable as all the other serial interconnects?
Bruce on 07.04.07 @ 12:42 PM CST [link] [1 Comment]