i386: far jumps

i386: far jumps

[Emanuele Altieri]

Greetings,

I'm doing some experiments with far jumps to different code segments.
For example:

   ljmp $16, $hello

where $16 is the segment selector, and "hello" is a label that I set up
for testing. The code above works fine.

However, when I change the code as following:

   movw $16, %dx
   movw %dx, %fs
   ljmp *%fs:hello

I get a general protection error. I don't understand why. I believe it's
a notation error. Can anybody see it?

I'm interested in this because I need the segment argument of ljmp to be
variable.


Thanks!


 -Emanuele

Re: i386: far jumps

[h-peter recktenwald]

On 15 Jul 2002 22:21:26 -0400
Emanuele Altieri <ealtieri@hampshire.edu> wrote:

> Greetings,
> 
> I'm doing some experiments with far jumps to different code segments.
> For example:
> 
>    ljmp *%fs:hello
> 
> I get a general protection error. I don't understand why. I believe it's
> a notation error. Can anybody see it?
> 
> I'm interested in this because I need the segment argument of ljmp to be

	Linux?

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Re: i386: far jumps

[Emanuele Altieri]

On Fri, 2002-07-19 at 13:44, h-peter recktenwald wrote:
> On 15 Jul 2002 22:21:26 -0400
> Emanuele Altieri <ealtieri@hampshire.edu> wrote:
> 
> > Greetings,
> > 
> > I'm doing some experiments with far jumps to different code segments.
> > For example:
> > 
> >    ljmp *%fs:hello
> > 
> > I get a general protection error. I don't understand why. I believe it's
> > a notation error. Can anybody see it?
> > 
> > I'm interested in this because I need the segment argument of ljmp to be
> 
> 	Linux?

No (/ not yet). I'm just experimenting on Intel's segmentation and task
support with a small OS that I created for this purpose. I'm using GCC
and GAS to implement it.

By the way, I found a workaround to the ljmp problem:

    pushw <segment>
    pushl <offset>
    lret

For example

    pushw $16
    pushl $hello
    lret

This is equivalent to "ljmp $16, $hello", but now the segment and offset
arguments do not need to be constants.


  -Emanuele