Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!bbn!oberon!news
From: news@oberon.USC.EDU (USENET News)
Newsgroups: comp.sys.amiga.tech
Subject: Re: Why 6 bytes between pointe to functions in *Base
Summary: because they are executable machine code
Keywords: libraries, jmp, machine language
Message-ID: <16136@oberon.USC.EDU>
Date: 28 Mar 89 18:18:43 GMT
Expires: 14 Apr 89 08:00:00 GMT
References: <2486@scolex.sco.COM>
Reply-To: raulmill@aludra.usc.edu (Raul Miller)
Followup-To: comp.sys.amiga.tech
Distribution: comp
Organization: Narcoleptic Killer Apes
Lines: 28

In article <2486@scolex.sco.COM> brianm@sco.COM (Brian Moffet) writes:
>I noticed the other day that the distance between shared 
>library functions is 6 bytes.  The is at an offset less than 
>the base of the Shared Library pointer.  I would like to know,
>why 6 bytes?  4 of these bytes should be the pointer itself,
>but what are the other 2?  flags?

The other two bytes are the jump instruction which is necessary to
transfer control to the actual library routine (which has an unknown
length).  In other words, your program will need to execute the
instruction which in assembly language has the form:
        ...
        jsr     _LVOWhatever(a6)        ;invoke whatever
        ...
which then transfers control to the library stubs, which have a form
like:   ...
_LVFoo:         jmp     _Foo
_LVWhatever:    jmp     _Whatever
_LVBar:         jmp     _Bar
        ...

And, as it happens, the jmp instruction occupies two bytes.


Raul Miller                           |
USENET:     raulmill@aludra.usc.edu   |
U.S.SNAIL:  721 E Windsor #4          |  55 mph = 82 nc
            GLENDALE CA  91205        |