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?



Searching

In order to search for something, like in Vi, we have to use the / command. Examples:

Search for asm opcodes: /c opcode. The following will search for sta $d020 , sta $d021 , sta $d022 , etc…

[0x0000080e]> /c sta 0xd02 0x00000829 # 3: sta 0xd020 0x0000082c # 3: sta 0xd021

Search for strings (although this is not very useful since most probably the strings are stored in screen codes and not in PETSCII):

[0x0000080e]> / hello

Search for a sequence of hexadecimal bytes: /x. The following searches for the MSB of the music frequency table:

[0x0000080e]> /x 010101010102 Searching 6 bytes... # 7 [0x80d-0x13d4] hits: 1 0x0000098b hit0_0 010101010102

We can use the flag hit0_0 to refer to that address. For example, in order to dump the first 32 bytes from hit0_0 we can do:

[0x080d]> px 32 @ hit0_0 offset 0 1 2 3 4 5 6 7 8 9 A B C D E F 0123456789ABCDEF 0x098c 0101 0101 0202 0202 0202 0203 0303 0303 ................ 0x099c 0404 0404 0505 0506 0607 0707 0808 0909 ................

For more search options just type /?

Visual Mode

Besides the Command Line Interface, Radare has another interface called the Visual Mode. It is similar to Vi, where each key has an associated function. In this mode, instead of entering commands, you just press one or two keys without pressing Enter.

In fact, some keys have the same Vi functionality:

hjkl: move around

gG: go top/bottom of page

: : Enter a command

Visual mode has 8 different view modes that can be activated by pressing p

hex, the hexadecimal view

disasm, the disassembly listing

debug, the debugger

words, the word-hexidecimal view

buf, the C-formatted buffer

annotated, the annotated op analysis color map

annotated, the annotated hexdump

Adding Comments

While analyzing code, sometimes it is useful to add comments. While in Visual Mode, we can add comments by pressing ; plus the comment.

Example:



Saving

After adding some comments, we should save the project in order not to loose the changes. To save a project just enter Ps projectName (Project save), and to open an existing project enter Po projectName (Project open). And enter Pl to list existing projects.

Example:

[0x0000080d]> Ps myproject myproject [0x0000080d]> Pl myproject [0x0000080d]> Po myproject Reloading project

And from the command line, we can open existing projects with the -p argument. Example:

$ r2 -p myproject

Getting help

Just append ? to each command to get more help about that command. Example:

? :to list all the possible commands

P? :to get help about the Project (P) command

p? :to get help about the Print (p) command

p8? :to get help about the Print 8bit hexpair command

and so on.

When in Visual Mode, also press ? to get help.

Other resources