Re: Doubt using "set" command with registers

[joaoandreferro at sapo dot pt]

Hello Pete,

I didn't use ekgdboe (I'm using a serial connection between the target  
and the host). The only option I used to debug during boot was the  
"kgdbwait" option (in the GRUB configuration file), to hang the target  
at boot, and wait for a "cont" command from the host. I removed this  
option some weeks ago though, because I do no want to do debug during  
boot. But I remember it worked.

Regarding the original thread's subject, do you know anything on why  
some register values can be changed while others can't?

Regards,
JoÃo

Citando Pete Delaney <pdelaney@silver-peak.com>:

> Had any luck using KGDB during the early boot code?
>
> I haven't been able to get the ekgdboe working with the keyboard via  
> a serial interface, thought it does start early enough.
> Can't get ekgdboe working with tty0/ttyS0; starts way to late to  
> debug a boot problem I'm looking at.
>
> This use to be trivial with the old SunOS 4.1.4 kgdb patch, and I  
> think the pre-KDB version of KGDB seemed
> To do early boot code ok. With old 2 page SunOS 4.1.4 patch  
> switching between task was just a matter of
> Getting the SP and PC from the proc  structure, changing $PS and $PC  
> in gdb and restarting GDB so it would
> Reload it's cache.
>
> JTAG based debugging shouldn't be necessary to debug early boot code
> And JTAG isn't readily available for large PCs.
>
> -piet
>
> --
> Pete/Piet Delaney
> O: +1 408 935-1813
> C: +1 408 646-8557
> H: +1 408 243-8872
> Home Email: piet.delaney@gmail.com
>
>
>
>
> -----Original Message---though--
> From: gdb-owner@sourceware.org [mailto:gdb-owner@sourceware.org] On  
> Behalf Of joaoandreferro@sapo.pt
> Sent: Tuesday, October 28, 2014 9:02 AM
> To: gdb@sourceware.org
> Subject: Doubt using "set" command with registers
>
> Hello all,
>
> I've been using GDB (in fact, conjuged with KGDB) lately to do some  
> kernel debugging. I've been testing some GDB commands, and a doubt  
> arised while using the "set" command, trying to change the value of  
> some registers. I've been able to successfully change the value of  
> all the registers provided by the "info registers" command (i. e.,  
> in my case $eax, $ecx, $edx, $ebx, $esp, $ebp, $esi, $edi, $eip,  
> $eflags, $cs, $ss, $ds, $es, $fs and $gs) *except* for $esp, $ss,  
> $fs and $gs.though
> Why is it so? I've been making these tests after hitting a hardware  
> breakpoint, then I set the values with e. g. "set $es = 0xf1c2007c",  
> and verifiy the assignemt with "info reg $es". As I've pointed out,  
> it works with the majority of the registers, except for that four  
> (in this particular case, the first command doesn't output any  
> error, but the second command shows me that the registers remain  
> with the initial value). By the way, I've started GDB with the  
> -write option (checked it inside GDb with the "show write" command).  
> My GDB version is 7.2.
>
> If someone knows something about this situation and can share it  
> with me, it would be great.
>
> Best regards,
> JoÃo