Re: RFA/ARM: Switch mode when setting PC

[Andrew Cagney <cagney@gnu.org>]

> For example, if the user writes a 32-bit value into the PC, the CPSR state 
>> > probably shouldn't be changed (even if the bottom bit is altered) -- this 
>> > is how ARM's debuggers behave.  However, if the user 'calls' a function 
>> > that is in the 'other state', then the CPSR should be updated (and 
>> > presumably restored afterwards).
>> > 
>> > I'm not sure if GDB has a way of separating these two cases.  It's an 
>> > interesting problem.
> 
>> 
>> I believe that this will work at present, because setting $pc will not
>> go through write_pc.  There's some blind luck involved in this, though.

Or a lack of design, Arm needs to ensure that it doesn't define PC_REGNUM.

> In the past we've tried to distinguish R15 from PC.  This was especially 
> useful in the legacy 26-bit mode where the CPSR bits *were* in R15.
> 
> This would probably all have been much simpler if I'd been able to 
> complete my code for handling the banked register; sadly I never got far 
> enough, and I think the code is probably too bit-rotten to be worth trying 
> to resurrect directly at this point.

If there's an explicit "set_resume_address", separate to write_pc, this 
should happen:

	(gdb) set $r15 = 0x123
	- target sees:
		$r15=0x123
	(gdb) call foo()   OR (gdb) jump foo
	- target, via "set_resume_address", sees:
		$r15=&foo
		$ps&|=<magic-bits>

and significantly no other write_pc calls.

Andrew