Doubt using "set" command with registers

[joaoandreferro@sapo.pt]

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.  
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