How to read system variabl

How to read system variabl

Post by Vincent_Parting.. » Mon, 15 Mar 1993 16:32:32



In a message of <Fri  5 Mar 93 11:49>, Ulrich Kuehn (2:281/202.0) writes:

 UK> It is a very bad idea to grab system vectors without thinking of how
 UK> to get out of it again. A solution would be the following procedure:
 UK> if you has to grab a system vector from a program (of whatever kind),
 UK> DO USE xbra. When dechaining the vector, one searches through the xbra
 UK> list for the own entry point (check the xbra id AND the vector
adress!) UK> Then modify the old vector in the predecessor's xbra
structure (or the
 UK> system vector itself) to point to the old vector one has saved at
 UK> install time. If that is not possible, i.e. because a non-xbra program
 UK> has installed itself into the vector afterwards, there is no
 UK> possibility of unchaining oneself, so one has to stay in memory, at
least
 UK> is it necessary to leave a dummy trap jumper in memory.

I understand; it's a lot of hassle, especially if appl.B doesn't follow the
XBRA protocol. I think I'll just do it the simple way and call a routine
with Supexec periodically, that's a lot simpler and it will always work.

Ta!
    Vincent

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How to read system variabl

Post by Neil Forsy » Wed, 24 Mar 1993 19:34:58




>In a message of <Fri  5 Mar 93 11:49>, Ulrich Kuehn (2:281/202.0) writes:
> UK> (description of XBRA usage deleted but still _very_ valid)

>I understand; it's a lot of hassle, especially if appl.B doesn't follow the
>XBRA protocol. I think I'll just do it the simple way and call a routine
>with Supexec periodically, that's a lot simpler and it will always work.

I have rapidly been coming to the conclusion that in addition to XBRA, TSR
programs should uses Cookies too for vector dechaining. The Cookie could
point to say an array of three longwords (current, active, bypassed).
The 'current' vector would be the one called by the TSR and 'active' and
'bypassed' would be the two alternatives for switching the TSR on and off.
It would probably be wise to insist that the switch be done in supervisor
mode at IPL #7.
I think the use of vectors rather than flags would be more general and
allow some hooking too (preferably for patching and enhancment and not by
some virus!).

Perhaps a standard cookie name convention could be created for this usage.
As a first attempt I suggest the first three letters of the normal cookie
with the fourth a '*'.

Whadaya think?

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! Neil Forsyth                                                               !
! Department of Computing and Electrical Engineering                         !
! Heriot-Watt University                                                     !
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Dusty Reichwein

WWW   : http://www.engr.orst.edu/~reichwja

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