Unfortunately there are no questions on Stack regarding ARM and assembler at all.

My concern -- is time critical devices. Let's take for an example one of my AVR-based device (program compiled with GCC) which should do something up to INT0 interrupt. It working with 8 MHz internal oscillator (125 ns one machine cycle) but it took up to 5 microseconds to react for the interrupt. After the code investigation I came to the conclusion that in the beginning of interrupt service routine processor make a lot of work to save it's state which is almost uncontrollable for high level programming languages (such as C is). If I'd use assembler I could for example throw a pin change in the very beginning and keep the rest of necessary calculations after that. Or I could have much more control over the registers' use and therefore much less time to save those registers.

If I'd go to ARM (which I'm planing to do soon) I will have much faster processor core with much more registers and memory space which looks promising. But will I ever be able to have any control over such time critical processes to obtain for example reaction time within let's say hundred nanosecs'?