Usenet 1994 January/usenetsourcesnewsgroupsinfomagicjanuary1994.iso/sources/unix/volume4/68kdisassem/part1/doc

Text File | 1986-11-30 | 6.3 KB | 191 lines

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.SA 1
.PH "''A Disassembler''"
.PF "'Issue %I%'- Page \\\\nP -'%G%'"
.H 1 "Introduction"
This document describes the first release of a disassembler for UNIX
executable files.
The key features are:
.AL
.LI
For object files the output can be assembled to generate the same
object module, (apart from minor variations in symbol table ordering) as the
input.
.LI
For stripped executable files object modules and libraries may be scanned,
modules in the main input identified and the appropriate names automatically
inserted into the output.
.LI
An option is available to convert most non-global names into local symbols,
which cuts down the symbols in the generated assembler file.
.LI
The disassembler copes reasonably with modules merged with the
.B "-r"
option to
.B "ld" ,
generating a warning message as to the number of modules involved.
.LE
.P
At present this is available for certain Motorola 68000 ports of UNIX
System III and System V. Dependencies on
.AL a
.LI
Instruction set.
.LI
Object module format.
.LI
Library module format.
.LI
Assembler output format.
.LE
.P
are hopefully sufficiently localised to make the product useful as a
basis for other disassemblers for other versions of UNIX.
.P
The product is thus distributed in source form at present.
.H 1 "Use"
The disassembler is run by entering:
.DS I
unc mainfile lib1 lib2 ...
.DE
.P
The first named file is the file to be disassembled, which should be
a single file, either an object module, a (possibly stripped) executable
file, or a library member. Library members are designated using a
parenthesis notation, thus:
.DS I
unc '/lib/libc.a(printf.o)'
.DE
.P
It is usually necessary to escape the arguments in this case to prevent
misinterpretation by the shell. Libraries in standard places such as
.I "/lib"
and
.I "/usr/lib"
may be specified in the same way as to
.B "ld" ,
thus
.DS I
unc '-lc(printf.o)'
unc '-lcurses(wmove.o)'
.DE
.P
As an additional facility, the list of directories searched for
libraries may be varied by setting the environment variable
.B "LDPATH" ,
which is interpreted similarly to the shell
.B "PATH"
variable, and of course defaults to
.DS I
LDPATH=/lib:/usr/lib
.DE
.P
As a further facility, the insertion of
.B "lib"
before and
.B ".a"
after the argument may be suppressed by using a capital
.B "-L"
argument, thus to print out the assembler for
.I "/lib/crt0.o" ,
then the command
.DS I
unc -Lcrt0.o
.DE
.P
should have the desired effect.
.P
Second and subsequent file arguments are only referenced for stripped
executable files, and may consist of single object files and library
members, using the same syntax as before, or whole libraries of object
files, thus:
.DS I
unc strippedfile -Lcrt0.o -lcurses -ltermcap '-lm(sqrt.o)' -lc
.DE
.P
It is advisable to make some effort to put the libraries to be searched
in the order in which they were originally loaded. This is because the
search for each module starts where the previously matched module ended.
However, no harm is done if this rule is not adhered to apart from
increased execution time except in the rare cases where the disassembler
is confused by object modules which are very nearly similar.
.H 1 "Additional options"
The following options are available to modify the behaviour of the
disassembler.
.VL 15 2
.LI "-o file"
Causes output to be sent to the specified file instead of the standard
output.
.LI "-t prefix"
Causes temporary files to be created with the given prefix. The default
prefix is
.B "split" ,
thus causing two temporary files to be created with this prefix in the
current directory. If it is desired, for example, to create the files as
.B "/tmp/xx*" ,
then the argument
.B "-t /tmp/xx"
should be given. Note that the temporary files may be very large as a
complete map of the text and data segments is generated.
.LI "-a"
Suppresses the generation of non-global absolute symbols from the
output. This saves output from C compilations without any obvious
problems, but the symbols are by default included in the name of
producing as nearly identical output as possible to the original source.
.LI "-s"
Causes an additional scan to take place where all possible labels are
replaced by local symbols. The local symbols are inserted in strictly
ascending order, starting at 1.
.LI "-v"
Causes a blow-by-blow account of activities to be output on the standard
error.
.LE
.H 1 "Diagnostics etc"
Truncated or garbled object and library files usually cause processing
to stop with an explanatory message.
.P
The only other kinds of message are some passing warnings concerning
obscure constructs not handled, such as the relocation of byte fields,
or the relocation of overlapping fields. Occasionally a message
.DS I
Library clash: message
.DE
.P
may appear and processing cease. This message is found where at a late
stage in processing libraries, the program discovers that due to the
extreme similarity of two or more library members, it has come to the
wrong conclusion about which one to use. The remedy here is to spell out
to the program which members to take in which order.
.H 1 "Future development"
In the future it is hoped to devise ways of making the disassembler
independent of all the above-mentioned version dependencies, by first
reading a files defining these things. This will probably be applied
after the Common Object Format becomes more standard.
.P
In the long term it would be desirable and useful to enhance the product
to produce compilable C in addition to assemblable assembler. Stages in
the process are seen as follows:
.AL
.LI
Better identification of basic blocks in the code. Switch statements are
a major problem here, as are constant data held in the text segment.
.LI
Marrying of data to the corresponding text. It is in various places hard
to divorce static references "on the fly" (e.g. strings, and switch
lists in some implementations) from static at the head of a module. This
is part of the problem of identifying basic blocks.
.LI
Compilation of header files to work out structure references within the
text. At this stage some interaction may be needed.
.LE
.P
Meanwhile the product is one which is a useful tool to the author in its
present form. Comments and suggestions as to the most practical method
of improving the product in the ways suggested or in other ways would be
gratefully considered.