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From: bartonr@eecs.cs.pdx.edu (Robert L Barton)
Newsgroups: comp.sys.amiga
Subject: Re: BCPL vs. 2.0
Message-ID: <2915@psueea.UUCP>
Date: 28 May 90 11:11:25 GMT
Sender: news@psueea.UUCP
Reply-To: bartonr@eecs.cs.pdx.edu (Robert L Barton)
Organization: Portland State University, Portland, OR
Lines: 29


 bartonr@eecs.cs.pdx.edu (Robert L Barton) writes:
RLB> You don't need 2.0 to do this.  SetPatch 1.34, for example,
RLB> changes the Execute() vector to make use of a resident Run command.

 deven@rpi.edu (Deven T. Corzine) replies:
DTC> Which does not mean that the SetFunction() call works on it.  It means
DTC> SetPatch knows the bizarre jump format and patches that.

  No, SetFunction() works just fine on Execute().  SetFunction simply replaces
the first two bytes with a JMP instruction and the last four with an address.
Of course the return value won't be meaningful, so if you patch an entry with

  oldFunc = SetFunction(DOSBase, offset, newFunc)

and then try to undo that by following it later with

  newFunc = SetFunction(DOSBase, offset, oldFunc)

you would have a problem, since oldFunc would not be an absolute address, as it
would be with a non-DOS function.  But for one-time patching, such as in the
startup-sequence, it does the trick.  Also SetFunction does not change the entry
in the Global Vector, so any internal calls to Execute() would still go to the
original place, but I don't think there are any of these in dos.library.

---------------
Robert L Barton
Memorial Coliseum
Portland OR