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From: steve@edm.UUCP (Stephen Samuel)
Newsgroups: comp.os.research
Subject: Re: Shared Libraries
Message-ID: <3095@sdcsvax.UCSD.EDU>
Date: Sat, 2-May-87 08:31:33 EDT
Article-I.D.: sdcsvax.3095
Posted: Sat May  2 08:31:33 1987
Date-Received: Sat, 9-May-87 01:00:51 EDT
Sender: darrell@sdcsvax.UCSD.EDU
Organization: Unexsys Systems Inc., Edmonton,AB.
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Approved: mod-os@sdcsvax.uucp


The way run-time loading is done on MTS requires 3 loader sections:
1) External Symbol Directories consist of the name of the routine and the number
2) ReLocation Directory consisted of the rotine # (from ESD), displacement into 
the loaded routine where it is used, and type of reference (2-byte/4-byte,
relative/absolute etc.)
3) The actual text contains any displacement from the 'external' location's
public address

 EG: pseudo-C for an absolute reference to _fread+16 from 0x7da into a subroutine,
with fread being the 6th symbol referenced) would consist of:

 ESD: ... 
  struct { char EsdNam[ENSIZE] ; THING * ES_ptr; } esd_sym[]
    = { <5 entries>,  {"_fread  ",NULL} ...}

 RLD: ...
  struct { int EsdNum; int disp ; FLAGS type }
rld_sym[] ={ ..., {5,0x7da,RT_16bit+RT_abs} ... }

 routine+0x7da: (thing *) 16

once all external references were found, the value of each referencing
location (assuming all pointers were the same length) was:

for(i=0;i<RLDcount;i++){
  proc[rld_sym[i].disp] += esd_sym[rld_sym[i].EsdNum].ES_ptr -
  ( (rld_sym[i].type & RT_abs) ? 0 : &(proc[rld_sym[i].disp] ) ) ;
};

We leave it as an exercise to the reader to add thing like: 1/2/4 byte pointers;
Symbols OTHER than simple external references; everything else I've forgotten.

For those of you who don't know: MTS is an operating system for /370 type
systems that's actually reasonable and nice to use (and definitely NOT
from IBM). The loader form I paraphrased above may actually originate with
an old IBM loader but, not having seen one, I can't really say.


--
 Stephen Samuel 			Disclaimer: You betcha! (done@4am)
  {ihnp4,ubc-vision,seismo!mnetor,vax135}!alberta!edm!steve