There are also some some hybrid SOCs, like TI's Sitara chips that the Beagleboard is built around, that have a ARM core for linux, then a couple MCU cores for d

There are also some some hybrid SOCs, like TI's Sitara chips that the Beagleboard is built around, that have a ARM core for linux, then a couple MCU cores for doing fast-ish realtime stuff. (TI's FAQ says a four instruction busyloop that just toggles a pin can run at about 50MHz on a PRU)