A Quick and Easy Command Line Disassembler

I've written a very short program to disassemble 65816 instructions from SNES ROM files. It is not a fully equipped disassembler and it doesn't generate assembly output that necessarily can be assembled by your favorite assembler tool. However, it does work fine as a very quick and easy way to dump out a short listing, if you know the address where it starts and have the ROM file handy.

DISASM

Here is a session at the command line prompt to illustrate what it does. I wanted to dump out the division routine in Dragon Quest III. So I did this:

C:\>disasm DQ3.SMC -S $C0121C
C0121C:   08           php
C0121D:   78           sei
C0121E:   C2 20        rep #$20
C01220:   48           pha
C01221:   B5 00        lda $00,X
C01223:   8F 04 42 00  sta $004204
C01227:   68           pla
C01228:   E2 20        sep #$20
C0122A:   8F 06 42 00  sta $004206
C0122E:   C2 20        rep #$20
C01230:   EA           nop
C01231:   EA           nop
C01232:   EA           nop
C01233:   EA           nop
C01234:   EA           nop
C01235:   EA           nop
C01236:   EA           nop
C01237:   AF 14 42 00  lda $004214
C0123B:   95 00        sta $00,X
C0123D:   AF 16 42 00  lda $004216
C01241:   28           plp
C01242:   6B           rtl

As you can see, I merely specified the starting address and the ROM filename. The disassembler gave me a quick listing of the subroutine. It stopped when the subroutine was complete, too.

C:\>disasm src.smc -S $C0121C -E $C01242 -D
C01210:                                          08 78 C2 20
C01220:   48 B5 00 8F 04 42 00 68    E2 20 8F 06 42 00 C2 20
C01230:   EA EA EA EA EA EA EA AF    14 42 00 95 00 AF 16 42
C01240:   00 28 6B

Same information, different format. In this case, though, I had to tell the disassembler where to end the listing, too. So I needed to add that address to the command line, as you can see above. I also added the -D parameter to the command line in order to tell the program to "dump" out the hexadecimal bytes.

C:\>disasm
disasm -- 65816 Quick Disassembler
Disassembles short segments of 65C816 code from a binary ROM file.
usage: disasm <filename> /H /A /T /S <startaddr> /E <endaddr> /L <length> /R <tblfilename>
  (- or / may be used; start, end, and length values may be decimal or hex)

      <filename>     Specifies the ROM filename to examine
      -H             Requests this help
      -R <tablefile> Specifies the address table filename
      -S <addr>      Specifies the starting memory address to dump
      -E <addr>      Specifies the ending memory address to dump
      -L <length>    Specifies the length of the region to dump
      -A             Assembler source code format (default)
      -D             Hexadecimal format
      -T             Specifies using special termination logic

You may use -S with -L, -S with -E, and -E with -L, to specify the code bytes to examine. You may also specify just the -S and the code will automatically try and figure out when the best time to stop may be. (This doesn't work when using -D, though, as the code isn't analyzing the source bytes as instructions when it is just dumping out data.)

Finally, there is the -R option. This allows you to include a file which defines symbols that the disassembler will then apply to the listings in generates. The file format looks like:

DTMSKY    0xC90566 FUNC { DB, DW, ADR, DW, ADR } Table: Fetch masked data (Y index)
DTMSKX    0xC90572 FUNC { DB, DW, ADR, DW, ADR } Table: Fetch masked data (X index)
SETFLG    0xC908F0 FUNC { ADR, DW } Set Flag
GETFLG    0xC909AE FUNC { ADR, DW } Get Flag
DIVIDE    0xC0121C FUNC { } Divides two values
DIVIDEND  0x004204 DATA { } Dividend
DIVISOR   0x004206 DATA { } Divisor
QUOTIENT  0x004214 DATA { } Quotient
REMAINDER 0x004216 DATA { } Remainder

The above is an example. Note that two kinds of labels can be declared: FUNC and DATA. These are just 24-bit addresses that either declare a function entry point or else a data item. The open and close set signs allows you to inform the disassembler about parameters that may follow a function call, included as in-line data. The disassembler will then automatically figure out and display the additional information included as in-line parameter values to the function call. Lastly, there is some description text that may also be included. If included, the text will appear as a comment on the source line, as well.

The current code is still a work in progress. It's just a quick tool I developed to allow me to quickly explore the source code or data that is located at some address I'm considering at the moment.

Last updated 5/17/2016, 16:00 UT. You may contact me at jonk at infinitefactors dot org.

A Quick and Easy Command Line Disassembly Tool

DISASM