* subsegs.c (subseg_change): allow and handle a change into SEG_BSS.

[deliverable/binutils-gdb.git] / gas / README.rich

		 The Code Pedigree of This Directory


This directory contains a big merge of several development lines of
gas as well as a few bug fixes and some configuration that I've added
in order to retain my own sanity.

A little history.

The only common baseline of all versions was gas-1.31.

From 1.31, Intel branched off and added:

	support for the Intel 80960 (i960) processor.
	support for b.out object files.
	some bug fixes.
	sloppy mac MPW support
	Intel gnu/960 makefiles and version numbering.

Many of the bug fixes found their way into the main development line
prior to 1.36.  ALL intel changes were ifdef'd I80960.  This was good
as it isolated the changes, but bad in that it connected the b.out
support to the i960 support, and bad in that the bug fixes were only
active in the i960+b.out executables of gas, (although most of these
were nicely marked with comments indicating that they were probably
general bug fixes.)

To pick up the main FSF development line again, along the way to 1.36,
several new processors were added, many bugs fixed, and the world was
a somewhat better place in general.

From gas-1.36, Loic at Axis Design (france!) encapsulated object
format specific actions, added coff versions of those encapsulations,
and a config.gas style configuration and Makefile.  This was a big
change and a lot of work.

Then along came the FIRST FSF release of gas-1.37.  I say this because
there have been at least two releases of gas-1.37.  Only two of them
do we care about for this story, so let's call them gas-1.37.1 and
gas-1.37.2.

Here starts the confusion.  Firstly, gas-1.37.1 did not compile.

In the meantime, John Gilmore at Cygnus Support had been hacking
gas-1.37.1.  He got it to compile.  He added support for the AMD 29000
processor.  AND he started encapsulating some of the a.out specific
pieces of code mostly into functions.  AND he rebuilt the relocation
info to be generic.  AND he restructured somewhat so that for a single
host, cross assemblers could be built for all targets in the same
directory.  Useful work but a considerable nuisance because the a29k
changes were not partitioned from the encapsulation changes, the
encapsulation changes were incomplete, and the encapsulation required
functions where alternate structuring might have used macros. Let's
call this version gas-1.37.1+a29k.

By the time gas-1.37.2 was "released", (remember that it TOO was
labelled by FSF as gas-1.37), it compiled, but it also added i860
support and ansi style const declarations.

At this point, Loic rolled his changes into gas-1.37.2.

What I've done.

I collected all the stray versions of gas that sounded relevant to my
goals of cross assembly and alternate object file formats and the FSF
releases from which the stray versions had branched.

I rolled the Intel i960 changes from 1.31 into versions that I call
1.34+i960, 1.36+i960, and then 1.37.1+i960.

Then I merged 1.37.1+i960 with 1.37.1+a29k to produce what I call
1.37.1+i960+a29k or 1.37.3.

From 1.37.3, I pulled in Loic's stuff.  This wasn't easy as Loic's
stuff hit all the same points as John's encapsulations.  Loic's goal
was to split the a.out from coff dependancies for native assembly on
coff, while John's was to split for multiple cross assembly from a
single host.

Loic's config arranged files much like emacs into m-*, etc.  I've
rearranged these somewhat.

Theory:

The goal of the new configuration scheme is to bury all object format,
target processor, and host machine dependancies in object, target, and
host specific files.  That is, to move all #ifdef's out of the gas
common code.

Here's how it works.  There is a .h and a .c file for each object file
format, a .h and a .c file for each target processor, and a .h for
each host.  config.gas creates {sym}links in the current directory to
the appropriate files in the config directory.  config.gas also serves
as a list of triplets {host, target, object-format} that have been
tested at one time or another.  I also recommend that config.gas be
used to document triplet specific notes as to purpose of the triplet,
etc.

Implementation:

host.h is a {sym}link to .../config/xm-yourhost.h.  It is intended to
be used to hide host compiler, system header file, and system library
differences between host machines.  If your host needs actual c source
files, then either: these are generally useful functions, in which
case you should probably build a local library outside of the gas
source tree, or someone, perhaps me, is confused about what is needed
by different hosts.

obj-format.h is a {sym}link to .../config/obj-something.h.  It is intended

All gas .c files include as.h.

as.h #define's "gas", includes host.h, defines a number of gas
specific structures and types, and then includes tp.h, obj.h, and
target-environment.h.

target-environment.h defines a target environment specific
preprocessor flag, eg, TE_SUN, and then includes obj-format.h.

obj-format.h defines an object format specific preprocessor flag, eg,
OBJ_AOUT, OBJ_BOUT, OBJ_COFF, includes "target-processor.h", and then
defines the object specific macros, functions, types, and structures.

target-processor.h 

target-processor.

Porting:

There appear to be four major types of ports; new hosts, new target
processors, new object file formats, and new target environments.


-----

reloc now stored internally as generic. (symbols too?) (segment types
vs. names?)

I don't mean to overlook anyone here.  There have also been several
other development lines here that I looked at and elected to bypass.
Specifically, xxx's stabs in coff stuff was particularly tempting.