[hatari-devel] Catching trap exit?

[Nicolas Pomarède]

Le 19/10/2010 22:07, Eero Tamminen a écrit :

>>    - you can have a simple step instruction that breaks each time PC is
>> changed
>
> Do you mean shortcut for:
> 	b pc ! "pc" :once
> or for:
> 	c 1
> debugger command?

this should be for the case where you execute one opcode at (pc) then 
stop, so I think it's more for "c 1"

Your 1st example would not work for the following code :

	move	#1567,d0
.loop	dbf	d0,loop

When pc = .loop, you will execute 1568 dbf, but pc won't change (from 
the point of view of the debugger, it will change at the 68000 level), 
so I think your 1set example will only break after the last dbf ?

I think the correst "step" command should be the one that breaks 1568 times.

>>    - you can have a more elaborate step instruction that breaks after
>> opcodes that changes PC by jumping elsewhere. This is handy to execute a
>> sub routine / trap handler as a whole, without going step by step in the
>> routine.
>
> Is the longest such instruction shorter than shortest such instruction?

?? Sorry, I don't understand what is "such instruction" ?

>
> If yes, then a shortcut for this is enough:
> 	b pc>  "pc"&&  pc<  "pc<maxlen + 1>" :once

what is maxlen ? If it's the maximum size of a branch instruction, then 
it won't work the way you propose.

jsr $123456.l will take 6 bytes
trap / bsr.s will take 2 bytes

so if you encounter a bsr.s and add a breakpoint at pc+6, the following 
code will fool your breakpoint :

	bsr.s	routine		; 2 bytes
	rts			; 2 bytes
routine
	nop			; 2 bytes
	...
	rts

-> this will break when reaching the nop, because it's at pc+4 (relative 
to the bsr.s)

The only safe way is to know how many byte take bsr.s/trap/jsr.l and at 
a breakpoint at pc+opcode_length.

>
> I quickly looked at these values but they didn't seem to match the next
> instruction address so I guess I was doing something wrong.  How you
> would get that value in Hatari C-code?

Have a look at m68k_do_rts in cpuemu.h :
m68k_setpc(get_long(m68k_areg(regs, 7)));

so get_long(m68k_areg(regs, 7)) is what you need to return from a non 
exception branch (for exceptions, return pc is at 2(a7), so 
get_long(m68k_areg(regs, 7)+2) will do the trick)


Nicolas