Subj : NASM segment registers- a
To   : Jasen Betts
From : Robin Sheppard
Date : Tue Oct 30 2001 12:18 pm

 JB> You, can't always do it like that,  CS and DS can reference the same
 JB> memory but have different values if the descriptors they point to point
 JB> to the same memory. of course if CS == DS then they'll be pointing to
 JB> the same descriptor.

 JB> but I doubt that DJGPP uses an environment that complex.

You don't like DJGPP, do you?

Anyways, they have to be different descriptors, simply because one 
must be code, and the other data.  Data segments aren't executable, 
and code segments aren't writable.  Loading a data selector into CS 
would cause some sort of exception immediately, as it tried to execute 
the next instruction- or maybe the exception would be caught on 
loading CS with an invalid value.  Loading DS would probably be okay 
until you tried to write data to it, but I'm not sure- maybe the CPU 
verifies that it is indeed a valid data selector.  I don't know much 
about protected mode, but I'm pretty sure on these aspects.

.... Christian Science programming: "Let God debug it!"

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