Sharp86 v1.0

Welcome to Sharp86

Sharp86 is an Intel 8086 emulator for .NET that is simple to host and includes a built-in "text based GUI" debugger.


Sharp86 is the CPU emulation used by Win3mu - a 16-bit Windows 3 emulator.

It supports the 8086 instruction but can also be run in a pseudo-protected mode where the host emulator provides selector support. This is possible because Sharp86's bus interface provides both segment/selector and offset for memory operations (rather than accessing a flat memory model like a real processor would at the bus level).

In other words, even though a host can emulate protected mode for user mode software, you can't run operating system level software that expects protected mode instructions to be available.

In the context of Win3mu this means the running software thinks it's running in protected mode, but Win3mu itself isn't running the real Windows 3 operating system.


Sharp86 is hosted on BitBucket:

or more typically you'd just add it as a submodule to an existing project:

> git submodule add

Hosting Sharp86

To host Sharp86, first create an instance of it, or derive a class from it and provide an implementation of the IMemoryBus and IPortBus interfaces:

class MyMachine : Sharp86.CPU, IMemoryBus, IPortBus
    public MyMachine()
        base.MemoryBus = this;
        base.PortBus = this;

The memory bus is a simple 8-bit bus used everytime the processor needs to read from or write to memory:

public interface IMemoryBus
    byte ReadByte(ushort seg, ushort offset);
    void WriteByte(ushort seg, ushort offset, byte value);
    bool IsExecutableSelector(ushort seg);

A minimal implementation for a flat (non-protected mode) memory model might look something like this:

// A chunk of memory
byte[] _memory = new byte[1024 * 1024];

// Read a byte from memory
byte IMemoryBus.ReadByte(ushort seg, ushort offset)
    return _mem[seg << 4 + offset]

// Write a byte to memory
void IMemoryBus.WriteByte(ushort seg, ushort offset, byte value)
    _mem[seg << 4 + offset] = value;

// Check if a selector is executable.  For non-protected mode
// just return true.  Called to validate selectors before 
// being loaded into the cs register
bool IMemoryBus.IsExecutableSelector(ushort seg)
    return true;

The port bus is used for port I/O operations:

public interface IPortBus
    byte ReadPortByte(ushort port);
    void WritePortByte(ushort port, byte value);

A minimal implementation of this could simply return 0 and ignore port writes.

Running the Processor

Before running the processor, you'll need to load some valid 8086 code somewhere into the processor's address space. How you do this is outside the scope of the document (although see the section "Minimal Host Implementation" below) and depends on what you're trying to emulate. Suffice to say when the processor starts executing you'll need to provide valid code to execute via the IMemoryBus.ReadByte() method.

You can set the address at which the processor will begin executing by directly accessing the IP register:

// Start execution after the interrupt vector table
cpu.IP = 0x0400;  

To make the processor run, call the Run() method indicating how many instructions to execute:

// Run the next 10,000 instructions

The processor will now run, reading instructions from the memory bus and executing them.

About Run Frames

The above call to Run() executes what's called a "run frame". A run frame is set of instructions executed as a batch. Executing instructions in batches like this is considerably faster than calling a Step() method (if it existed) many times.

Normally a run frame will execute until the specified number of instructions have been executed. You can cancel the current run frame however by calling the CPU.AbortRunFrame() method while in a callback from a bus operation or an interrupt handler (see below). This will cause the Run() method to return before executing the next instruction.

You might want to do this if the processor executes an instruction that should cause the machine to shut down (eg: calling a DOS exit process interupt) and no more instructions should be executed.

The current run frame is automatically aborted if the processor executes a halt instruction.

Implementing Interrupt Handlers

Aside from the memory bus and port bus, the other main way the CPU can interact with the hosting environment is via interrupts.

To install an interrupt handler, override the CPU class's RaiseInterrupt method:

public override void RaiseInterrupt(byte interruptNumber)
    // DOS interrupt?
    if (interruptNumber == 0x21)
        // Handle the interrupt by directly interacting
        // with the CPU registers and machine memory

        // Don't pass to the default handler

    // Do default (ie: invoke interrupt vector table)

By returning from RaiseInterrupt without calling the base method implementation the CPU will continue execution at the instruction immediately after the interrupt.

Normally an interrupt handler will handle the interrupt by directly interacting with the machine's memory and/or registers:

void HandleDosInterrupt(interruptNumber)
    switch (_cpu.ah)
        // handle DOS interrupt sub-op

    // Clear carry flag to indicate no error
    _cpu.FlagC = false;

Raising Hardware Interrupts

To raise a hardware interrupt, call the CPU.RaiseHardwareInterrupt method. This will cause the RaiseInterrupt method to be invoked after the next instruction has finished executing.

Note the processor doesn't include an interrupt controller nor have a concept of hardware interrupt priorities - that'll need to be provided by your hosting emulation. If you call RaiseInterrupt a second time before the first interrupt is detected and invoked, the first interrupt will be lost. Typically this is resolved by implementing an interrupt controller that requires interrupt acknowledgement before raising the next highest priority interrupt.

Minimal Host Implementation

For a minimal host implementation see the included "ConDos" program. ConDos provides a minimal implementation of a PC/DOS type machine that implements a single DOS interrupt - Int 21h, Sub-Function 9 - Write a string to the console.

In the sandbox subdirectory of the ConDos project, you'll also find a simple DOS .com program that only uses this one DOS interrupt that can be used for testing. (the program will also run on a real DOS machine)

C:\Users\Brad\Projects\Sharp86\ConDos>..\build\Debug\ConDos.exe sandbox\
Hello World from Sharp86 - (9)
Hello World from Sharp86 - (8)
Hello World from Sharp86 - (7)
Hello World from Sharp86 - (6)
Hello World from Sharp86 - (5)
Hello World from Sharp86 - (4)
Hello World from Sharp86 - (3)
Hello World from Sharp86 - (2)
Hello World from Sharp86 - (1)
Hello World from Sharp86 - (0)

Here's the test program assembly source which can be built using the build.bat command in the same directory (you'll need YASM installed).

org 100h

        mov     cx,10
        mov     al,cl
        add     al,'0'-1
        mov     [counter],al
        mov     ah,09h
        mov     dx,hello
        int     21h
        loop    loop1

        db      "Hello World from Sharp86 - ("
        db      "0"
        db      ")", 13, 10, "$"

Built-in Debugger

Sharp86 includes a built in debugger. To enable the debugger, create an instance of it and set its CPU property:

// Create a debugger and attach it to the CPU
_debugger = new Sharp86.TextGuiDebugger();
_debugger.CPU = _cpu;

To break the debugger on the next executed instruction, call it's Break() method:

if (IsKeyPress && KeyCode == KeyCode.F9)

Calling Break() will cause execution to stop just before the next instruction is executed. (ie: the processor needs to be running for the break to occur - either by calling Run() again, or by calling Break() while a call to Run() is in progress).

Using the Debugger

The built in debugger is displayed as a Windows console mode window with a GUIish retro 90's style text mode user-interface. The GUI is somewhat limited (ie: unfortunately no mouse support) but considerably better than a scrolling console mode debugger.

The basics of using the debugger are as follows:

  • F5 = Continue
  • F8 = Step
  • Shift+F8 = Step Out
  • F9 = Set breakpoint (in code window)
  • F10 = Step Over
  • Ctrl+Tab and Ctrl+Shift+Tab move focus between windows (aka: panels)
  • Type over an address to change position (code and data windows)
  • Shift + Up/Down in console window to scroll (but, see notes below)
  • Type "help" in the console window for list of other commands.

By default, some of the above keys won't work in newer versions of Windows. To fix this, click the debugger window's system menu, choose Properties and turn off: "Quick Edit Mode", "Insert Mode", "Enable Ctrl Key Shortcuts", "Filter clipboard contents on paste", "Enable line wrapping selection" and "Extended text selection keys".

Debugger Commands

The debugger supports the following set of commands that can be typed into the console window (from the help command):

                  bp - Set a break point at a code location
            bp break - Attach a break condition to a break point
            bp clear - Remove all break points
          bp cputime - Set a CPU time break point
              bp del - Delete break point
             bp edit - Edit a break point
             bp expr - Set a expression change break point
              bp int - Set a interrupt break point
             bp list - List break points
            bp match - Attach a match condition to a break point
              bp mem - Set a memory change break point
             bp memr - Set a memory read break point
             bp memw - Set a memory write break point
              bp off - Disable break point
               bp on - Enable break point
            bp reset - Reset break point trip counts
           close log - Close log file
              disasm - Disassemble (format=b,w,dw,i,l)
            dump mem - Dump memory (format=b,w,dw,i,l)
                   e - Evaluate an expression
                help - List all available commands
                 log - Log output to a file
                   o - Step Over
                   r - Run
               r for - Run for CPU cycles
              r time - Run to CPU time
                r to - Run to address
                   s - Step
                   t - Step Out
               trace - Dump execution trace
         trace clear - Clear execution trace
           trace off - Disable execution tracing
            trace on - Enable execution tracing
           trace set - Set the execution trace buffer size
           view code - View address in the code window
            view mem - View address in the memory window
                   w - Add a watch expression
             w clear - Remove all watch expression
               w del - Remove a watch expression
              w edit - Edit a watch point

Some commands expect arguments in which case you can use expressions. Most C# style operators are supported.

eg: This would move the memory window to watch 16 bytes before the current stack pointer.

view mem ss:sp-0x10

Note that commands that expect strings should be quoted:

log "dump.txt"

eg: watch to the current top of stack:

w word ptr [ss:sp]

eg: set a break point at code location:

bp cs:0x1000

eg: set a break point on a write to memory:

bp memw ds:0x1234,16

Aside from watch expressions, all expressions are evaluted at the time the command is invoked (ie: the above break point wouldn't move if the DS register changed).

Note too that break points are evaluated for exact matches on the segment/offset. For memory models where multiple addresses refer to the same memory location, break points only work on a matching segment address.


Sharp86 - 8086 Emulator Copyright © 2017-2018 Topten Software.

Sharp86 is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

Sharp86 is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with Sharp86. If not, see