CUG #430: M68KDIS Motorola 68000 family disassembler

Christopher G. Phillips (University of Texas at Austin) submits his "m68kdis" disassembler for the Motorola 68000 family of CPU chips. Disassemblers are system software that accepts a binary executable as input and produces assembly language source as output. Specifically, m68kdis supports the full instruction sets of the 68000, 68008, 68010, 68020, and 68030 CPU chips. Additionally, m68kdis decodes instructions for the 68851 Paged Memory Unit and the 68881/68882 Floating-Point Coprocessors. The Motorola 68000 family chips power millions of computers including the Macintosh, Atari, Amiga, and many embedded CPU industrial applications. The CUG Library edition of m68kdis includes full source in C (no executables are provided). The m68kdis disassembler is immediately available as CUG volume #441.

Since m68kdis is portable, it is actually a cross-disassembler. For example, you can disassemble 68000 programs on a variety of host CPUs from PCs to Unix machines. Phillips provides a very clean Makefile without any OS specific flags or options. Additionally, he takes care to avoid common pitfalls such as dependencies on the size of the int data type.

Using m68kdis

In its basic operation, m68kdis reads in a single binary file (.o) and produces an ASCII file containing an assembly language output (.s). Ideally, the ".s" file could be run through your assembler and would produce an ".o" file identical to the original. Strictly speaking, a disassembler is just a subset of functionality found in a debugger. The output of m68kdisk is divided into five distinct columns: program counter, file contents, label, instruction, and operands (see Fig. 1).

Programmers often use disassemblers in two specific modes of operation. First, a disassembler may be used as a one-time operation to check the operation of a compiler, the validity of an executable, the executable startup code, or similar examination. Second, the programmer may use a sophisticated disassembler such as m68kdis in several successive iterations. In this cycle, the programmer uses his own judgement to help separate blocks of instructions from blocks of data. Then he applies his knowledge or guesswork so that the next iteration provides clearer output. With persistence, a usable source file can result.

m68kdis supports this iterative method of use by providing options (see Fig. 2) for the import of separate files containing external knowledge of the input file. For example, the "-i file" option accepts an input ASCII file containing unsigned long integers that represent addresses at which instructions are known to start. Similarly, you can create files of addresses where data is known to reside. A small improvement to m68kdis would allow ranges of addresses instead of just singleton addresses in the file.

Macintosh Support

Although m68kdis works equally well against any target 68000 executable, the author includes special additional support for Macintosh executables. m68kdis understands enough of the executable to follow the resource fork and dump it in a readable fashion. This insures more reliable output because determining the difference between instructions and data is the prime job of a disassembler.

m68kdis supplies files containing Macintosh A-line instructions and their OS equivalent entry point names. More than 800 A-line instructions can be decoded in this manner. These entry points are somewhat analogous to BIOS interrupt (INT) instructions used on Intel-based MS-DOS PCs.

There is also a series of PERL scripts that perform post-processing on the output of the disassembler. PERL is not included in the CUG Library distribution, but is widely available on the Internet.

Documentation, Licensing, and Support

Documentation for m68kdis consists primarily of a 5-page summary that appears in Unix "man" page format. As such, it primarily details command line options and usage. For insights into disassembly algorithms, you'll need to follow the comments in the code. For details on the 68000 family instructions sets, please see the section named "Further Reading" below.

Phillips includes an extremely generous license that extends the permission to use, copy, modify, and distribute this software and its documentation for any purpose and without fee. The only restriction is that his supplied copyright notice appear must in all copies of software and that both the copyright notice and the permission notice appear in supporting documentation.

Since m68kdis is freeware, there are no formal support options available. Support is thus neccessarily limited to email correspondence regarding bugs and enhancement requests.

For Further Reading

For those interested in how 68000 family instructions are encoded, Prentice-Hall (Englewood Cliffs, NJ) publishes a complete line of databooks. Here are some of the titles that Phillips cites:

Motorola. M68000 8/16/32 Bit Microprocessors: Programmer's Reference Manual, 5th ed.

Motorola. M68030: Enhanced 32-Bit Microprocessor User's Manual, 2nd ed

Motorola. M68851: Paged Memory Management Unit User's Manual, 2nd ed.

Motorola. M68881/MC68882: Floating-Point Coprocessor User's Manual, 2nd ed.

------------ Sample Output of m68kdis options (FIG. 2) ----------
  PC              File  Contents      Label  Instruction + Operands
=========  ========================== ====== ======================
00000000   206f0004                          MOVEA.L (4,SP),A0
00000004   43fa0030                          LEA     L5,A1
00000008   322f0008                          MOVE.W  (8,SP),D1
0000000c   7000                              MOVEQ   #0,D0
0000000e   600a                              BRA     L2
00000010   b308                         L1   CMPM.B  (A0)+,(A1)+
00000012   6620                              BNE     L4
00000014   4a28ffff                          TST.B   (-1,A0)
00000018   6604                              BNE     L3
0000001a   51c9fff4                     L2   DBRA    D1,L1
0000001e   f23a4800001c                 L3   FMOVE.X L6,FP0
00000024   f2000004                          FSQRT.X FP0
00000028   f22f5438000a                      FCMP.D  (10,SP),FP0
0000002e   f29c0004                          FBNGE   L4
00000032   7001                              MOVEQ   #1,D0
00000034   4e75                         L4   RTS
00000036   6d36386b646973               L5   DC.B    'm68kdis'
0000003d   00                                DC.B    #0
0000003e   40000000c90fdaa22168c000     L6   DC.X    #3.14159265358979

------------ Quick Summary of m68kdis options (FIG. 2) ----------

 -ddd      Specify the chip and coprocessors (e.g. 020)
 -a file   Load file containing A-line opcodes
 -all[c]   Force single pass operation
 -b file   Load file of data addresses to force newlines
 -bad      Echo invalid instructions to stderr
 -f file   Load file containing F-line opcodes
 -f# #     Floating-point constant mantissa
 -i file   Load file of known instruction addresses
 -j file   Load file of A-line and F-Line jumps
 -l        Force output to lower case
 -lft      Affects fall-through instructions to LINK
 -n file   Load file of known data addresses
 -ns file  Load file of known addresses not starting instrs.
 -o file   Names output file
 -odd      Allow instructions to start on odd offset
 -pc #     Set program counter to this value on start
 -s #      Minimum length to consider data as string
 -slenp #  Maximum length of a string on output line
 -sp       Output A7 as SP except in MOVEM instructions

Other 68000 Family Tools from CUG Library

CUG Library has assemblers, cross-assemblers, C compilers, and other tools for use with the 68000 family of CPUs. Each of these volumes is available for only $4 per diskette. Alternately, all of these titles are included on the CUG Librayr CD-ROM. Order using the form provided in the center leaf of this issue or send mail to for more information.

CUG #      Title
------     ---------------
CUG190     Steve Passe's 68K Assembler
CUG204     68000 C Compiler
CUG261     68K Cross Assembler, MSDOS
CUG303     MC68K Disassembler*
CUG338     68000 C Compiler and Assembler (2 disks)
CUG363     68020 Cross Assembler (2 disks)

* The CUG303 MC68K Disassembler is considerably less advanced than M68KDIS. For example, it does not include instruction-level support for 68010, 68020, 68851, or 68881/68882 chips. Also MC68K omits Macintosh support and the ability to declare regions of the file as data or code.