A20, 8259,IDT help. 
Author Message
 A20, 8259,IDT help.

When should I enable the a20 and set the new IRQ vectors? before I
switch into RAW PM or after? And if I want to set up an interrupt on
vector FFh... do I have to have to set up all the ones in between?

something like...

idt     dq 255+16 DUP (?)       ; would that be all the ints and
exceptions? then could I go in and set up one on ffh?



Wed, 23 Dec 1998 03:00:00 GMT  
 A20, 8259,IDT help.

Quote:

>When should I enable the a20 and set the new IRQ vectors? before I
>switch into RAW PM or after? And if I want to set up an interrupt on
>vector FFh... do I have to have to set up all the ones in between?
>something like...
>idt dq 255+16 DUP (?)       ; would that be all the ints and
>exceptions? then could I go in and set up one on ffh?

You could enable the stuff any time you want.  I was writing in pmode
so it was easier for me to do it after the switch... but there is
nothing to stop you from doing it before the switch.

Yes, you have to have a table that is big enough to hold the last interrupt
vector you want to use.  Depending on how secure you want things, it might
be a good idea to fill in all unused vectors with a pointer to a iretd.

Be aware also that if you go to real mode (e.g. to access DOS or the BIOS)
you have to have bimodal interrupts.  In other words any hardware interrupt
that can come in might come in either in pmode or real-mode... so you
have to have handlers that can handle the interrupt both in pmode and in
real mode.  A lot of times people just put pointers to mode switch
code in any vector that could come in during the wrong mode.

David



Wed, 23 Dec 1998 03:00:00 GMT  
 
 [ 2 post ] 

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