Disassembling 6502 code with Radare – Part II

Let’s crack a simple game. If you are not familiar with Radare, read Part I first.

Creating and opening a VICE Snapshot file

Let’s crack BC’s Quest For Tires since its copy-protection is easy to bypass.

Screen Shot 2015-12-09 at 3.03.39 PM
BC’s Quest For Tires copy protection
  • …the game has some kind of copy-protection. If we enter invalid codes, we won’t be able to play the game.

Since Radare supports VICE Snapshot File format, we can save an snapshot of the game, and analyze it with Radare.

  • In VICE, go to the menu, Snapshot -> Save Snapshot Image…
    • If we select “Save ROMs”, then the BASIC ROM and the KERNAL ROM will be saved inside the Snapshot file, and will be included as Radare sections.

save_snapshot_dialog

 

Radare VICE Snapshot File (VSF) support lets us inspect:

  • The 64k RAM of the computer at the moment the snapshot was saved
  • The BASIC and KERNAL ROMs in case they were saved.

To open a VSF file, just pass the VSF file as the first argument:

$ r2 bc_copy_protection_screen.vsf
[0x00005689]>

0x00005689 is the PC (program counter) at the moment the snapshot was saved.
Continue reading “Disassembling 6502 code with Radare – Part II”

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Disassembling 6502 code with Radare – Part I

Radare is an open source portable reversing framework that can do many things, among those things it can disassemble 6502 code.

Download and install radare

  • First, download radare from github. You need a recent version in order to disassemble 6502 code.
  • And then install it by running sys/install.sh (or sys/user.sh for local installation):
$ git clone https://github.com/radare/radare2.git
$ cd radare2
$ ./sys/install.sh

Loading a c64 .prg

Radare has many command line options. But in order to load 6502 programs we need just two:

  • -a6502 to specify the 6502 architecture.
  • -mMemoryAddress to map the file to a certain memory address. Use 2047 for “normal” programs. Usually they start at $0801 (2049), but we have to subtract 2 from the .prg header.

Example:

$ r2 -a6502 -m2047 mygame.prg

Disassembling

Radare doesn’t have a GUI, like IDA. Instead is has a powerful command line interface (think of GDB). Example:

$ r2 -a6502 -m2047 musicplayer.prg
[0x000007ff]>

And 0x7ff (2047) is the seek address, meaning that all commands will use that address as the base address. Let’s print the first 32 bytes. (px = print hexa):

[0x000007ff]> px 32
offset   0 1  2 3  4 5  6 7  8 9  A B  C D  E F  0123456789ABCDEF
0x07ff  0108 0b08 3905 9e32 3036 3100 0000 78ad  ....9..2061...x.
0x080f  0ddc a212 a000 b9d4 1a99 f020 c8d0 f7ce  ........... ....

The “2061” that we see, is part of the BASIC “SYS 2061” command that usually appears in all C64 programs. So, let’s disassemble the first 12 instructions from 2061. (pd = print disassemble):

[0x000007ff]> pd 12 @ 2061
            0x0000080d    78             sei
            0x0000080e    ad0ddc         lda 0xdc0d
            0x00000811    a212           ldx #0x12
            0x00000813    a000           ldy #0x00
       ┌┌─> 0x00000815    b9d41a         lda 0x1ad4,y
       ││   0x00000818    99f020         sta 0x20f0,y
       ││   0x0000081b    c8             iny
       └──< 0x0000081c    d0f7           bne 0xf7
        │   0x0000081e    ce1708         dec 0x0817
        │   0x00000821    ce1a08         dec 0x081a
        │   0x00000824    ca             dex
        └─< 0x00000825    d0ee           bne 0xee

In case we don’t know the meaning of a certain opcode, we can print its description with ?d:

[0x00000815]> ?d sei
set interrupt disable status

Or if we want to print the description in every disassembled line, we can do:

e asm.describe=true

And then disassemble again:

[0x0000080e]> pd 12 @2061
    0x080d  78        sei           ; set interrupt disable status
    0x080e  ad0ddc    lda 0xdc0d    ; load accumulator with memory
    0x0811  a212      ldx #0x12     ; load index x with memory
    0x0813  a000      ldy #0x00     ; load index y with memory
┌─> 0x0815  b9d41a    lda 0x1ad4,y  ; load accumulator with memory
│   0x0818  99f020    sta 0x20f0,y  ; store accumulator in memory
│   0x081b  c8        iny           ; increment index y by one
└─< 0x081c  d0f7      bne 0xf7      ; branch on result not zero
    0x081e  ce1708    dec 0x0817    ; decrement memory by one
    0x0821  ce1a08    dec 0x081a    ; decrement memory by one
    0x0824  ca        dex           ; decrement index x by one
    0x0825  d0ee      bne 0xee      ; branch on result not zero

For more disassembling options just type p?
Continue reading “Disassembling 6502 code with Radare – Part I”