A practical example I once had to solve: I was writing the BIOS/flash-loader/debugger code for a Z80 clone[1] we made. The compiler send instruction packets ov

A practical example I once had to solve: I was writing the BIOS/flash-loader/debugger code for a Z80 clone[1] we made. The compiler send instruction packets over the RS-232 serial port at 115200 kBd (kbaud). Using 8N1, this is 12800 B/s which means we have a deadline to read the current byte out of the serial port register ever 78us or data will be lost as the new incoming byte on the serial port is written to the port's IO register. This sounded like a lot of time, until I was informed some of the boards we sold ran at (iirc) 128kHz. After accounting for memory access time and the runtime of relevant instructions, it worked out that we had at most a budget of 60-70 instructions/byte, which wasn't anywhere close to enough to process the byte (including a simple checksum). Even using a normal ISR wasn't possible: the usual register push/pop would have a major portion of our instruction budget. I ended up dedicating a set of registers to receiving the bytes in realtime into as buffer, while processing the data non-realtime using the remaining registers. Depending on how strict your deadlines are, "realtime" can mean accounting for every clock cycle the CPU is running. An OS (realtime or otherwise) is for people with softer deadlines and CPU time to spare. [1] https://en.wikipedia.org/wiki/Rabbit_2000 https://en.wikipedia.org/wiki/Rabbit_2000