ARM modes and why are there so many?

Question

I'm currently reading/learning about ARM architecture ... and I was wondering why there are so many modes (FIQ, User, System, Supervisor, IRQ, ...).

My question is why do we need so many modes? Wouldn't just User and System be enough?

Thanks in advance.

Answer 1

It's just an architectural decision. The big advantage of the multiple modes is that they have some banked registers. Those extra registers allow you to write much less complicated exception routines.

If you were to pick only two, just USR and SYS are probably as good a choice as any, but what would happen when you took an exception? The normal ARM model is to go to an exception mode, set the banked link register for that exception mode to point to the instruction you want to return to after you resolve the exception, save the processor state in the exception mode's SPSR register, and then jump to the exception vector. USR and SYS share all their registers - using this model, you'd blow away your function return address (in LR) every time you took an interrupt!

The FIQ mode in particular has even more banked registers than the other exception modes. Those extra registers are in keeping with the "F" part of FIQ - it stands for "Fast". Not having to save and restore more processor context in software will speed up your interrupt handler.

Answer 2

Not too much to add to Carl's answer. Not sure what family / architecture of ARM processors you're talking about, so I'll just assume based on your question (FIQ, IRQ, etc.) that you're talking about ARM7/9/11. I won't enumerate every difference between every mode in every ARM architecture variant.

In addition to what Carl said, a few other advantages of having different modes for different circumstances:

• for example, in the FIQ, you don't have to branch off right away, you can just keep on executing. With other exceptions you have to branch right away

• with different modes, you have natural support for separate stacks. If you're multitasking (e.g., RTOS) and you don't have a separate stack when you're in an interrupt mode, you have to build-in extra space onto each task stack for the worst-case interrupt situation

• with different modes, certain registers (e.g. CPSR, MMU regs, etc. - depends on architecture) are off-limits. Same thing with certain instructions. You don't want to let user code modify privileged registers, now do you?