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From: dg@wrs.UUCP (David Goodenough)
Newsgroups: comp.lang.c
Subject: near / far branches
Message-ID: <420@wrs.UUCP>
Date: Thu, 15-Oct-87 20:26:42 EDT
Article-I.D.: wrs.420
Posted: Thu Oct 15 20:26:42 1987
Date-Received: Sun, 18-Oct-87 02:21:04 EDT
References: <117@nusdhub.UUCP>
Reply-To: dg@wrs.UUCP (David Goodenough)
Organization: Wind River Systems, Emeryville, CA
Lines: 52

In article <117@nusdhub.UUCP> rwhite@nusdhub.UUCP (Robert C. White Jr.) writes:
>	First, a linker is a "linking loader" and it's only REAL
>purpose is to resolve refrences and setup dynamic relocation
>information.  ALL "short" jumps [add/subtract value from the
>instruction pointer directly] are compleetly generated and closed
>in the assembler.  If there were to be an alteration in the size
>of a block of code-text, the linker would have to "reassemble" the
>code block to make up for the altered "short" jumps and such.  In
>order for the linker to do this it would either need the source
>code, or it would have to take a stab at disassembeling the object
>code and hope not to get it wrong.

Not true - every (useful) assembler out there can generate information
that a given word needs to be relocated in some way, so why not just
have two types or relocation: far (for unresolved, long distance) and
near (for resolved, short distance)

>	The assembler is designed to AUTOMATICALLY determine the
>"near"ness or "far"ness of a refrence.  This works beautfuly as
>long as you act like a pascal programmer and only use backward
>refrences.  For the forward refrences you must use a keyword like
>"near" or "far" or else take what you get, and hope what you get
>is close enough to work.

Wrong again - I have seen many assemblers in my time, and only one was
a single pass animal (and it was a real sharp piece of software - had to
maintain two symbol tables: one for defined labels, and another for
unresolved references). Think about it for half a second, you have to do
a second pass to resolve all the forward references anyway, so all you
do is add a link field to your symbol table, linking the symbols together
in address order, then whenever you munge a far / near branch, just run
up the chain adjusting all the references (this actually requires a third
pass or O N^2 time based on the number of symbols).

>	It should be obvious that the assemblers job is to assemble
>and the linkers job is to link.

Open to discussion, I added the -X flag to my assembler to do an
assemble and link all in one go on a source - saves some time as I
get away with three passes (two for the assembly and one for linkage)
as opposed to four (two each for the separate assembly and linkage)
--
		dg@wrs.UUCP - David Goodenough

		..... !rtech!--\
				>--!wrs!dg
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