Spice86 - A PC emulator for real mode reverse engineering

Spice86 is a tool to execute, reverse engineer and rewrite real mode DOS programs for which source code is not available.

Release are available on Nuget.

Pre-releases are also available on the Release page

NOTE: This is a port, and a continuation from the original Java Spice86.

It requires .NET 8 and runs on Windows, macOS, and Linux.

Approach

Rewriting a program from only the binary is a hard task.

Spice86 is a tool that helps you do so with a methodic divide and conquer approach.

General process:

• You start by emulating the program in the Spice86 emulator.
• At the end of each run, the emulator dumps some runtime data (memory dump and execution flow)
• You load those data into ghidra via the spice86-ghidra-plugin
• The plugin converts the assembly instructions in the memory dump to C# that can be loaded into spice86 and used either partially or completely instead of the assembly code.
• This allows you to gradually reimplement the assembly code with your C# methods
• This is helpful because:
  • Small sequences of assembly can be statically analyzed and are generally easy to translate to a higher level language.
  • You work all the time with a fully working version of the program so it is relatively easy to catch mistakes early.
  • Rewriting code function by function allows you to discover the intent of the author.

Running your exe

This is a .NET program, you run it with the regular command line or dotnet run. Example with running a program called file.exe:

Spice86 -e file.exe

COM files and BIOS files are also supported.

Dumping data

It is recommended to set SPICE86_DUMPS_FOLDER environment variable pointing to where the emulator should dump the runtime data. If the variable is set or if --RecordedDataDirectory parameter is passed, the emulator will dump a bunch of information about the run there. If nothing is set, data will be dumped in the current directory. If there is already data there the emulator will load it first and complete it, you don't need to start from zero each time!

More command line options

  --Ems                              (Default: false) Enables EMS memory. EMS adds 8 MB of memory accessible to DOS programs through the EMM Page Frame.
  --A20Gate                          (Default: false) Disables the 20th address line to support programs relying on the rollover of memory addresses above the HMA (slightly above 1 MB).
  -m, --Mt32RomsPath                 Zip file or directory containing the MT-32 ROM files
  -c, --CDrive                       Path to C drive, default is exe parent
  -r, --RecordedDataDirectory        Directory to dump data to when not specified otherwise. Working directory if blank
  -e, --Exe                          Required. Path to executable
  -a, --ExeArgs                      List of parameters to give to the emulated program
  -x, --ExpectedChecksum             Hexadecimal string representing the expected SHA256 checksum of the emulated program
  -f, --FailOnUnhandledPort          (Default: false) If true, will fail when encountering an unhandled IO port. Useful to check for unimplemented hardware. false by default.
  -g, --GdbPort                      gdb port, if empty gdb server will not be created. If not empty, application will pause until gdb connects
  -o, --OverrideSupplierClassName    Name of a class that will generate the initial function information. See documentation for more information.
  -p, --ProgramEntryPointSegment     (Default: 4096) Segment where to load the program. DOS PSP and MCB will be created before it.
  -u, --UseCodeOverride              (Default: true) <true or false> if false it will use the names provided by overrideSupplierClassName but not the code
  -i, --InstructionsPerSecond        <number of instructions that have to be executed by the emulator to consider a second passed> if blank will use time based timer.
  -t, --TimeMultiplier               (Default: 1) <time multiplier> if >1 will go faster, if <1 will go slower.
  -d, --DumpDataOnExit               (Default: true) When true, records data at runtime and dumps them at exit time
  -h, --HeadlessMode                 (Default: false) Headless mode. If true, no GUI is shown.
  -l, --VerboseLogs                  (Default: false) Enable verbose level logs
  -w, --WarningLogs                  (Default: false) Enable warning level logs
  -s, --SilencedLogs                 (Default: false) Disable all logs
  -i, --InitializeDOS                (Default: true) Install DOS interrupt vectors or not.

  --help                             Display this help screen.
  --version                          Display version information.

Dynamic analysis

Spice86 speaks the GDB remote protocol:

• it supports most of the commands you need to debug.
• it also provides custom GDB commands to do dynamic analysis.

Connecting

You need to specify a port for the GDB server to start when launching Spice86:

Spice86 --GdbPort=10000

Spice86 will wait for GDB to connect before starting execution so that you can setup breakpoints and so on.

Here is how to connect from GDB command line client and how to set the architecture:

(gdb) target remote localhost:10000
(gdb) set architecture i8086

Vanilla GDB

You can add breakpoints, step, view memory and so on.

Example with a breakpoint on VGA VRAM writes:

(gdb) watch *0xA0000

Viewing assembly:

(gdb) layout asm

Removing a breakpoint:

(gdb) remove 1

Searching for a sequence of bytes in memory (start address 0, length F0000, ascii bytes of 'Spice86' string):

(gdb) find /b 0x0, 0xF0000, 0x53, 0x70, 0x69, 0x63, 0x65, 0x38, 0x36

GDB does not support x86 real mode segmented addressing, so pointers need to refer to the actual physical address in memory. VRAM at address A000:0000 would be 0xA0000 in GDB.

Similarly, The $pc variable in GDB will be exposed by Spice86 as the physical address pointed by CS:IP.

Custom commands (where the magic happens)

The list of custom commands can be displayed like this:

(gdb) monitor help

Dump information about current run

(gdb) monitor dumpall

Dumps everything described below in one shot. Files are created in the dump folder as explained here Several files are produced:

• spice86dumpMemoryDump.bin: Snapshot of the real mode address space. Contains the instructions that are actually loaded and executed. They may differ from the exe you are running because DOS programs can rewrite some of their instructions / load additional modules in memory.
• spice86dumpExecutionFlow.json: Contains information used by the spice86-ghidra-plugin to make sense of the memory dump, like addresses of the functions, the labels, the instructions that have been executed ...

Special breakpoints

Break after x emulated CPU Cycles:

(gdb) monitor breakCycles 1000

Break at the end of the emulated program:

(gdb) monitor breakStop

#Refreshing screen or buffers while debugging

(gdb) monitor vbuffer refresh

For a pleasing and productive experience with GDB, the seerGDB client is highly recommended.

Reverse engineering process

Concrete example with Cryo Dune here.

First run your program and make sure everything works fine in Spice86. If you encounter issues it could be due to unimplemented hardware / DOS / BIOS features.

When Spice86 exits, it should dump data in current folder or in folder specified by env variable

Open the data in ghidra with the spice86-ghidra-plugin and generate code. You can import the generated files in a template project you generate via the spice86-dotnet-templates:

dotnet new spice86.project

Overriding emulated code with C# code

You can provide your own C# code to override the program original assembly code.

Defining overrides

Spice86 can take in input an instance of Spice86.Core.Emulator.Function.IOverrideSupplier that builds a mapping between the memory address of functions and their C# overrides.

For a complete example you can check the source code of Cryogenic.

Here is a simple example of how it would look like:

namespace My.Program;

// This class is responsible for providing the overrides to spice86.
// There is only one per program you reimplement.
public class MyProgramOverrideSupplier : IOverrideSupplier {
  public IDictionary<SegmentedAddress, FunctionInformation> GenerateFunctionInformations(int programStartSegment,
                                                                                 Machine machine) {
    Dictionary<SegmentedAddress, FunctionInformation> res = new();
    // In more complex examples, overrides may call each other
    new MyOverrides(res, programStartSegment, machine);
    return res;
  }

  public override string ToString() {
    return "Overrides My program exe. class is " + GetType().FullName;
  }
}

// This class contains the actual overrides. As the project grows, you will probably need to split the reverse engineered code in several classes.
public class MyOverrides : CSharpOverrideHelper {
  private MyOverridesGlobalsOnDs globalsOnDs;

  public MyOverrides(IDictionary<SegmentedAddress, FunctionInformation> functionInformations, int segment, Machine machine) {
    // "myOverides" is a prefix that will be appended to all the function names defined in this class
    base(functionInformations, "myOverides", machine);
    globalsOnDs = new MyOverridesGlobalsOnDs(machine);
    // incUnknown47A8_0x1ED_0xA1E8_0xC0B8 will get executed instead of the assembly code when a call to 1ED:A1E8 is performed.
    // Also when dumping functions, the name myOverides.incUnknown47A8 or instead of unknown
    // Note: the segment is provided in parameter as spice86 can load executables in different places depending on the configuration
    DefineFunction(segment, 0xA1E8, "incDialogueCount47A8", IncDialogueCount47A8_0x1ED_0xA1E8_0xC0B8);
    DefineFunction(segment, 0x0100, "addOneToAX", AddOneToAX_0x1ED_0x100_0x1FD0);
  }

  public Action IncDialogueCount47A8_0x1ED_0xA1E8_0xC0B8() {
    // Accessing the memory via accessors
    globalsOnDs.SetDialogueCount47A8(globalsOnDs.GetDialogueCount47A8() + 1);
    // Depends on the actual return instruction performed by the function, needed to be called from the emulated code as
    // some programs like to mess with the stack ...
    return NearRet();
  }

  private Action AddOneToAX_0x1ED_0x100_0x1FD0() {
    // Assembly for this would be
    // INC AX
    // RETF
    // Note that you can access the whole emulator to change the state in the overrides.
    state.AX++;
    return NearRet();
  }
}

// Memory accesses can be encapsulated into classes like this to give names to addresses and make the code shorter.
public class MyOverridesGlobalsOnDs : MemoryBasedDataStructureWithDsBaseAddress {
  public DialoguesGlobalsOnDs(Machine machine) {
    base(machine);
  }

  public void SetDialogueCount47A8(int value) {
    this.SetUint8(0x47A8, value);
  }

  public int GetDialogueCount47A8() {
    return this.GetUint8(0x47A8);
  }
}

Remember: You must tell Spice86 to use your assembly code overrides with the command line argument "--UseCodeOverride true" when debugging your project.

Along with the mandatory path to your DOS program, passed with the --ExePath argument.

Misc

C Drive

It is possible to provide a C: Drive for emulated DOS functions with the option --CDrive. Default is current folder. For some games you may need to set the C drive to the game folder.

Emulated program arguments

You can pass arguments (max 127 chars!) to the emulated program with the option --ExeArgs. Default is empty.

Time

The emulated Timer hardware of the PC (Intel 8259) supports measuring time from either:

• The real elapsed time. Speed can be altered with parameter --TimeMultiplier.
• The number of instructions the emulated CPU executed. This is the behaviour that is activated with parameter --InstructionsPerSecond and is forced when in GDB mode so that you can debug with peace of mind without the timer triggering.

Screen refresh

Screen is refreshed 30 times per second and each time a VGA retrace wait is detected (see Renderer.cs).

Emulator features

CPU:

• Only 16 bits instructions are fully supported, memory size is 1MB
• Most 32 bits instructions are implemented, but not validated via integration tests for now.
• The only supported addressing mode is real mode. 286/386 Protected mode and the related instructions are not implemented.
• Instruction set is (hopefully!) fully implemented for 8086, and validated via automated tests.
• For 80186, BOUND instruction is missing.
• For 80286, instructions related to protected mode are not implemented
• For 80386, protected mode is not implemented.
• No FPU instruction implemented apart those used for FPU detection.

Memory:

• Segmented addressing is implemented.
• The A20 Gate is supported.
• Helpers are available in order to convert a segmented address into a physical address, and vice-versa.
• EMS (Expanded Memory) 3.2 is partially implemented.
• XMS (Extended Memory) is not implemented.
• X86 Paging (virtual memory) is not implemented.

Graphics:

• Text modes, VGA, EGA, and CGA are implemented.

DOS:

• Part of int 21 is implemented. Identifies itself as dos 5.0 for now.

Input:

• Keyboard
• Mouse
• No joystick for now

CD-ROM:

• No MSCDEX support for now. Some games, like DUNE, can be copied entirely from the CD and run from the hard drive.

Sound:

On *nix systems, you'll need to have libportaudio installed. Without it, there will be no sound.

• PC Speaker is implemented.
• Adlib/SoundBlaster MIDI OPL is supported.
• SoundBlaster PCM is supported.
• MT-32 is supported. However, not on macOS, as a static build of MUNT is missing for that platform. (PRs welcome !)
• General MIDI is supported.

Compatibility list available here.

How to build on your machine

• Install the .NET 8 SDK (once)
• clone the repo
• run this where Spice86.sln is located:

   dotnet build

How to run

   Spice86 -e <path to executable>

or use this where Spice86.csproj is located:

   dotnet run -e <path to executable>

Ghidra plugin

This uses Ghidra and Java 17.

Before using it, define an environnement variable named SPICE86_DUMPS_FOLDER pointing to a folder where the Spice86 dumps are located. They are generated on exit.

General procedure, in order:

1.Ghidra's own script 'ImportSymbolScript.py' (input used is "spice86dumpGhidraSymbols.txt")

2.Ghidra's Auto-Analyze (only enable 'Dissasemble Entry Points')

3.Now, you can use the plugin.

Remember: if Ghidra displays SUBROUTINES, use the 'f' key to convert them into functions. The code generator only works with functions.

Also, if you have any weird behaviour, make sure you have Java 17 and ONLY Java 17. That's how Ghidra likes it.

Some screenshots

Cryo dune:

Prince of persia:

Stunts:

Credits

Some emulation code was adapted from the Aeon emulator by @gregdivis. Those are: The DMA Controller, the PC Speaker, the SoundBlaster, the MT-32, and General MIDI.

This project uses JetBrains Rider licenses, thanks to JetBrains' Open Source Community Support.

The UI is powered by Avalonia UI.