Well, the market leaders by units in the field will be whatever is used in appliances like toaster ovens and microwaves - these are often
8-bit processors running on the metal. Followed by smart phones (iOS and Android, both are linux varients). Followed by automobiles (no idea).Those 77 GHz radars used in automobiles are components. Probably only FPGAs or ASICs inside, but invisiblefrom the outside.
Well, "realtime" is not a binary property - it comes in degrees and kinds of urgency, not just yes or no.
Here is a good example from phased-array radars:
All otherwise unused radar timeline is spent sending search waveforms out. This is a background, non-realtime operation. It only matters that one gets enough search waveforms out onmoving-window average that things cannot slip through the radar fences and escape detection.
When a search waveform gets a hit, a realtime algorithm is triggered. One must get a verify waveform out within say a few tenths of a second, to ensure that the search hit was not a random blip, and to get a more accurate target location in both alt-az angle and slant range. Velocity is not yet known.
If verified, a track initiate sequence commences, the purpose being to determine the direction and speed of target motion before the target gets too away for re-acquisition to be practical. So all this is realtime, but a microsecond or even a millisecond here and there makes no difference. This is dominated by the physics of target motion and of radar, and not usually by computer issues.
The resulting data is used to create a new track data structure, and subsequent track maintenance requires only roughly periodic track waveforms to refresh the track data. In the air traffic world, the refesh is done every ten seconds or so, unless there is special interest in a track. For precision approach, two seconds is common.
Again, embedded does not mean small and really fast, although that's a common combination. Nor does it depend on the number of processors or cores employed. There is an official definition:
Embedded Application Support), also known as POSIX.13:
"3.2.7 Embedded Computer System: A computer (and its software) is considered embedded if it is an integral component of a larger system and is used to control and/or directly monitor that system, using special hardware devices."
One item from the corresponding Rationale: "Is the internal microprocessor controlling a disk drive an example of an embedded system? Yes, regardless of what the disk drive is used for. The software (firmware, actually) within the disk drive controls the HDA (head disk assembly) hardware and is hard realtime as well."
Special hardware devices are such things as radar hardware and rotary cement kilns. Disks, displays, and printers are not special devices.
As discussed above, there is in fact a clear definition of embedded. I'll grant that people did struggle with this, largely because embedded systems come in a wide variety, so counter-examples abounded. The earlier definition is what remained.
All of these things are possible solutions. The main constraint was computer power, which was often constrained by the physical environment in which the computer must operate.
Anti-aircraft missile guidance systems cycled at 64 to 128 Hz thirty years ago (set by the physics of arial pursuit) but even back then quaternions were used despite the very limited computer power that could fit into a 9" diameter missile body, because such a missile can go in any direction in 3D as it chases an wildly maneuvering target, and the mathematical equivalent of gimbal lock was expensive to prevent, so on whole, quaternions were faster to compute.
Joe Gwinn