Electronic – NVIC memory map ARM cortex M3 STM32f103

The ARMs implement an interrupt table to store the address for each interrupt handler (or callback, basically the same thing). Basically, all of the addresses of the interrupt handlers are stored in program memory at a predefined location. When an interrupt occurs, the processor knows where in the table that interrupt's entry is, grabs it and branches to the address stored there.

How do you populate this table? Usually all this information is contained in a startup file. Typically you just define a constant array of pointers, fill it with the addresses of the callbacks you want to use. Here is a website detailing how to create a startup file for a specific ARM microcontroller. The tricky thing about this is that almost all of the particulars of making the file depend heavily on the linker, compiler and chip you're using, so you'll either have to find an example out there or muddle through making your own.

It's very reasonable to program ARM in assembler. It's a straightforward RISC architecture with few surprises and plenty of registers. You can mix C and assembler provided you have a good understanding of the calling conventions.

There is a special low-latency ARM interrupt mode called FIQ, which swaps out some of the registers to a bank in hardware so they do not need to be saved in the ISR. 100ns latency to doing something useful is still going to be hard - at 100MHz that's 10 clock cycles, and FIQ takes up to 12 before it executes the first instruction.