Add live debugger frontend and mutex stall recovery (#383)

src/SharpEmu.Debugger/Breakpoints/Breakpoint.cs

@@ -0,0 +1,48 @@
+// Copyright (C) 2026 SharpEmu Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+namespace SharpEmu.Debugger.Breakpoints;
+
+/// <summary>
+/// A single breakpoint or watchpoint. Instances are immutable; the owning
+/// <see cref="BreakpointStore"/> replaces an entry to change its enabled state.
+/// </summary>
+public sealed class Breakpoint
+{
+    public Breakpoint(int id, BreakpointKind kind, ulong address, ulong length = 1, bool enabled = true)
+    {
+        if (length == 0)
+        {
+            throw new ArgumentOutOfRangeException(nameof(length), "Breakpoint length must be at least one byte.");
+        }
+
+        Id = id;
+        Kind = kind;
+        Address = address;
+        Length = length;
+        Enabled = enabled;
+    }
+
+    /// <summary>The store-assigned identifier used by clients to reference it.</summary>
+    public int Id { get; }
+
+    public BreakpointKind Kind { get; }
+
+    /// <summary>The first guest address the breakpoint covers.</summary>
+    public ulong Address { get; }
+
+    /// <summary>
+    /// The number of bytes the breakpoint covers. Always one for
+    /// <see cref="BreakpointKind.Execute"/>; the watch kinds may span a range.
+    /// </summary>
+    public ulong Length { get; }
+
+    public bool Enabled { get; }
+
+    /// <summary>True when <paramref name="address"/> falls within this breakpoint.</summary>
+    public bool Covers(ulong address) => address >= Address && address < Address + Length;
+
+    /// <summary>Returns a copy with a different enabled state.</summary>
+    public Breakpoint WithEnabled(bool enabled)
+        => enabled == Enabled ? this : new Breakpoint(Id, Kind, Address, Length, enabled);
+}

src/SharpEmu.Debugger/Breakpoints/BreakpointKind.cs

@@ -0,0 +1,26 @@
+// Copyright (C) 2026 SharpEmu Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+namespace SharpEmu.Debugger.Breakpoints;
+
+/// <summary>
+/// The kind of stop a breakpoint requests. Execution breakpoints are honoured
+/// at the frame-boundary seam that exists today; the data-watch kinds are part
+/// of the surface so client protocols and tooling can be built against them,
+/// and are armed once the execution backend can report the corresponding
+/// accesses.
+/// </summary>
+public enum BreakpointKind
+{
+    /// <summary>Stop when the instruction pointer reaches the address.</summary>
+    Execute,
+
+    /// <summary>Stop when the guest reads from the address range.</summary>
+    ReadWatch,
+
+    /// <summary>Stop when the guest writes to the address range.</summary>
+    WriteWatch,
+
+    /// <summary>Stop when the guest reads from or writes to the address range.</summary>
+    AccessWatch,
+}

src/SharpEmu.Debugger/Breakpoints/BreakpointStore.cs

@@ -0,0 +1,90 @@
+// Copyright (C) 2026 SharpEmu Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+namespace SharpEmu.Debugger.Breakpoints;
+
+/// <summary>
+/// A thread-safe registry of breakpoints. The debug server mutates it from
+/// client-servicing threads while the emulation thread queries it at frame
+/// boundaries, so every operation takes the same lock.
+/// </summary>
+public sealed class BreakpointStore
+{
+    private readonly object _sync = new();
+    private readonly Dictionary<int, Breakpoint> _breakpoints = new();
+    private int _nextId = 1;
+
+    /// <summary>Adds a breakpoint and returns the created entry with its id.</summary>
+    public Breakpoint Add(BreakpointKind kind, ulong address, ulong length = 1)
+    {
+        lock (_sync)
+        {
+            var effectiveLength = kind == BreakpointKind.Execute ? 1UL : Math.Max(1UL, length);
+            var breakpoint = new Breakpoint(_nextId++, kind, address, effectiveLength);
+            _breakpoints[breakpoint.Id] = breakpoint;
+            return breakpoint;
+        }
+    }
+
+    /// <summary>Removes a breakpoint by id. Returns false when it did not exist.</summary>
+    public bool Remove(int id)
+    {
+        lock (_sync)
+        {
+            return _breakpoints.Remove(id);
+        }
+    }
+
+    /// <summary>Enables or disables a breakpoint by id.</summary>
+    public bool SetEnabled(int id, bool enabled)
+    {
+        lock (_sync)
+        {
+            if (!_breakpoints.TryGetValue(id, out var breakpoint))
+            {
+                return false;
+            }
+
+            _breakpoints[id] = breakpoint.WithEnabled(enabled);
+            return true;
+        }
+    }
+
+    /// <summary>Removes every breakpoint.</summary>
+    public void Clear()
+    {
+        lock (_sync)
+        {
+            _breakpoints.Clear();
+        }
+    }
+
+    /// <summary>Returns a point-in-time copy of all breakpoints.</summary>
+    public IReadOnlyList<Breakpoint> Snapshot()
+    {
+        lock (_sync)
+        {
+            return _breakpoints.Values.ToArray();
+        }
+    }
+
+    /// <summary>
+    /// Finds the first enabled execution breakpoint covering <paramref name="address"/>,
+    /// or null when none applies.
+    /// </summary>
+    public Breakpoint? FindExecuteHit(ulong address)
+    {
+        lock (_sync)
+        {
+            foreach (var breakpoint in _breakpoints.Values)
+            {
+                if (breakpoint.Enabled && breakpoint.Kind == BreakpointKind.Execute && breakpoint.Covers(address))
+                {
+                    return breakpoint;
+                }
+            }
+
+            return null;
+        }
+    }
+}

src/SharpEmu.Debugger/DebugRegisterFile.cs

@@ -0,0 +1,73 @@
+// Copyright (C) 2026 SharpEmu Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+using SharpEmu.Core.Cpu.Debugging;
+using SharpEmu.HLE;
+
+namespace SharpEmu.Debugger;
+
+/// <summary>
+/// An immutable snapshot of the guest integer register state at a stop. XMM/YMM
+/// state is intentionally omitted here and read on demand through the target to
+/// keep the common register-dump path cheap.
+/// </summary>
+public readonly struct DebugRegisterFile
+{
+    private readonly ulong[] _generalPurpose;
+
+    public DebugRegisterFile(
+        ulong[] generalPurpose,
+        ulong rip,
+        ulong rflags,
+        ulong fsBase,
+        ulong gsBase)
+    {
+        ArgumentNullException.ThrowIfNull(generalPurpose);
+        if (generalPurpose.Length != 16)
+        {
+            throw new ArgumentException("Expected 16 general-purpose registers.", nameof(generalPurpose));
+        }
+
+        _generalPurpose = generalPurpose;
+        Rip = rip;
+        Rflags = rflags;
+        FsBase = fsBase;
+        GsBase = gsBase;
+    }
+
+    public ulong Rip { get; }
+
+    public ulong Rflags { get; }
+
+    public ulong FsBase { get; }
+
+    public ulong GsBase { get; }
+
+    /// <summary>Reads a register by identifier.</summary>
+    public ulong this[DebugRegisterId id] => id switch
+    {
+        DebugRegisterId.Rip => Rip,
+        DebugRegisterId.Rflags => Rflags,
+        DebugRegisterId.FsBase => FsBase,
+        DebugRegisterId.GsBase => GsBase,
+        _ when id.IsGeneralPurpose() => _generalPurpose[(int)id],
+        _ => throw new ArgumentOutOfRangeException(nameof(id), id, null),
+    };
+
+    /// <summary>Reads a general-purpose register.</summary>
+    public ulong this[CpuRegister register] => _generalPurpose[(int)register];
+
+    /// <summary>Captures the integer register state of a live debug frame.</summary>
+    public static DebugRegisterFile Capture(ICpuDebugFrame frame)
+    {
+        ArgumentNullException.ThrowIfNull(frame);
+
+        var gpr = new ulong[16];
+        for (var i = 0; i < gpr.Length; i++)
+        {
+            gpr[i] = frame.GetRegister((CpuRegister)i);
+        }
+
+        return new DebugRegisterFile(gpr, frame.Rip, frame.Rflags, frame.FsBase, frame.GsBase);
+    }
+}

src/SharpEmu.Debugger/DebugRegisterId.cs

@@ -0,0 +1,62 @@
+// Copyright (C) 2026 SharpEmu Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+using SharpEmu.HLE;
+
+namespace SharpEmu.Debugger;
+
+/// <summary>
+/// The registers a debugger can name. The first sixteen values line up with
+/// <see cref="CpuRegister"/> so a general-purpose register can be converted
+/// between the two enums by casting; the remaining values cover the special
+/// registers a debug frame exposes.
+/// </summary>
+public enum DebugRegisterId
+{
+    Rax = 0,
+    Rcx = 1,
+    Rdx = 2,
+    Rbx = 3,
+    Rsp = 4,
+    Rbp = 5,
+    Rsi = 6,
+    Rdi = 7,
+    R8 = 8,
+    R9 = 9,
+    R10 = 10,
+    R11 = 11,
+    R12 = 12,
+    R13 = 13,
+    R14 = 14,
+    R15 = 15,
+
+    Rip = 16,
+    Rflags = 17,
+    FsBase = 18,
+    GsBase = 19,
+}
+
+/// <summary>Helpers for mapping between debug and CPU register identifiers.</summary>
+public static class DebugRegisterIdExtensions
+{
+    /// <summary>
+    /// True when the identifier names one of the sixteen general-purpose
+    /// registers and can be cast to <see cref="CpuRegister"/>.
+    /// </summary>
+    public static bool IsGeneralPurpose(this DebugRegisterId id)
+        => id is >= DebugRegisterId.Rax and <= DebugRegisterId.R15;
+
+    /// <summary>
+    /// Converts a general-purpose identifier to its <see cref="CpuRegister"/>.
+    /// Throws when <paramref name="id"/> is a special register.
+    /// </summary>
+    public static CpuRegister ToCpuRegister(this DebugRegisterId id)
+    {
+        if (!id.IsGeneralPurpose())
+        {
+            throw new ArgumentOutOfRangeException(nameof(id), id, "Not a general-purpose register.");
+        }
+
+        return (CpuRegister)(int)id;
+    }
+}

src/SharpEmu.Debugger/DebuggerServerHost.cs

@@ -0,0 +1,59 @@
+// Copyright (C) 2026 SharpEmu Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+using System.Net;
+using SharpEmu.Core.Cpu.Debugging;
+using SharpEmu.Debugger.Server;
+using SharpEmu.Debugger.Session;
+
+namespace SharpEmu.Debugger;
+
+/// <summary>
+/// One-call wiring of the live debugger: it owns a <see cref="DebuggerSession"/>
+/// and a <see cref="DebuggerServer"/>, exposes the <see cref="Hook"/> to attach
+/// to <c>SharpEmuRuntimeOptions.DebugHook</c>, and starts/stops the network
+/// front-end. A host constructs one, hands <see cref="Hook"/> to the runtime,
+/// calls <see cref="Start"/>, and calls <see cref="NotifyRunCompleted"/> once the
+/// runtime returns.
+/// </summary>
+public sealed class DebuggerServerHost : IAsyncDisposable
+{
+    private readonly DebuggerSession _session;
+    private readonly DebuggerServer _server;
+
+    public DebuggerServerHost(
+        DebuggerServerOptions? serverOptions = null,
+        DebuggerSessionOptions? sessionOptions = null)
+    {
+        _session = new DebuggerSession(sessionOptions);
+        _server = new DebuggerServer(_session, serverOptions);
+    }
+
+    /// <summary>The session driving the target.</summary>
+    public IDebuggerSession Session => _session;
+
+    /// <summary>
+    /// The dispatcher hook to hand to the runtime so guest frames route through
+    /// the debugger.
+    /// </summary>
+    public ICpuDebugHook Hook => _session.Hook;
+
+    /// <summary>The endpoint the server bound to, or null before <see cref="Start"/>.</summary>
+    public IPEndPoint? Endpoint => _server.Endpoint;
+
+    /// <summary>Begins accepting debugger clients.</summary>
+    public void Start() => _server.Start();
+
+    /// <summary>
+    /// Releases a parked emulation thread and marks the target terminated. Call
+    /// after the runtime's run returns so any attached client is notified and the
+    /// guest thread is never left blocked in the debugger.
+    /// </summary>
+    public void NotifyRunCompleted() => _session.NotifyTerminated();
+
+    public async ValueTask DisposeAsync()
+    {
+        _session.NotifyTerminated();
+        await _server.DisposeAsync().ConfigureAwait(false);
+    }
+}

src/SharpEmu.Debugger/Protocol/DebugCommandDispatcher.cs

@@ -0,0 +1,340 @@
+// Copyright (C) 2026 SharpEmu Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+using SharpEmu.Debugger.Breakpoints;
+using SharpEmu.Debugger.Session;
+using SharpEmu.HLE;
+
+namespace SharpEmu.Debugger.Protocol;
+
+/// <summary>
+/// Translates parsed <see cref="DebugRequest"/> verbs into operations on an
+/// <see cref="IDebuggerSession"/> and packages the outcome as a
+/// <see cref="DebugResponse"/>. This is the single place command semantics live,
+/// so it is shared by every connection and independent of the wire format.
+/// </summary>
+public sealed class DebugCommandDispatcher
+{
+    private readonly IDebuggerSession _session;
+
+    public DebugCommandDispatcher(IDebuggerSession session)
+    {
+        _session = session ?? throw new ArgumentNullException(nameof(session));
+    }
+
+    public DebugResponse Dispatch(DebugRequest request)
+    {
+        return request.Command switch
+        {
+            JsonLineDebugProtocol.ParseErrorCommand => ParseError(request),
+            "ping" => DebugResponse.Success(request.Command),
+            "status" or "info" => Status(request),
+            "state" => DebugResponse.Success(request.Command, new Dictionary<string, object?>
+            {
+                ["state"] = _session.State.ToString(),
+            }),
+            "registers" or "regs" => Registers(request),
+            "set-register" or "set-reg" => SetRegister(request),
+            "read-memory" or "read-mem" => ReadMemory(request),
+            "write-memory" or "write-mem" => WriteMemory(request),
+            "list-breakpoints" or "breakpoints" => ListBreakpoints(request),
+            "add-breakpoint" or "break" => AddBreakpoint(request),
+            "remove-breakpoint" or "delete-breakpoint" => RemoveBreakpoint(request),
+            "enable-breakpoint" => EnableBreakpoint(request),
+            "continue" or "cont" or "c" => Simple(request, _session.Continue(), "Target is not paused."),
+            "step" or "s" => Simple(request, _session.StepFrame(), "Target is not paused."),
+            "pause" => Pause(request),
+            _ => DebugResponse.Failure(request.Command, $"Unknown command '{request.Command}'."),
+        };
+    }
+
+    private static DebugResponse ParseError(DebugRequest request)
+    {
+        var message = request.TryGetString("message", out var text) ? text : "Malformed request.";
+        return DebugResponse.Failure(request.Command, message);
+    }
+
+    private DebugResponse Status(DebugRequest request)
+    {
+        var data = new Dictionary<string, object?>
+        {
+            ["state"] = _session.State.ToString(),
+            ["breakpoints"] = _session.Breakpoints.Snapshot().Count,
+        };
+
+        if (_session.LastStop is { } stop)
+        {
+            data["lastStop"] = DescribeStop(stop);
+        }
+
+        return DebugResponse.Success(request.Command, data);
+    }
+
+    private DebugResponse Registers(DebugRequest request)
+    {
+        if (!_session.TryGetRegisters(out var registers))
+        {
+            return NotPaused(request);
+        }
+
+        return DebugResponse.Success(request.Command, new Dictionary<string, object?>
+        {
+            ["registers"] = DescribeRegisters(registers),
+        });
+    }
+
+    private DebugResponse SetRegister(DebugRequest request)
+    {
+        if (!request.TryGetString("register", out var name) || !TryParseRegister(name, out var id))
+        {
+            return DebugResponse.Failure(request.Command, "Expected a valid 'register' name.");
+        }
+
+        if (!request.TryGetUInt64("value", out var value))
+        {
+            return DebugResponse.Failure(request.Command, "Expected a 'value'.");
+        }
+
+        return _session.TrySetRegister(id, value)
+            ? DebugResponse.Success(request.Command)
+            : DebugResponse.Failure(request.Command, "Register is not writable or target is not paused.");
+    }
+
+    private DebugResponse ReadMemory(DebugRequest request)
+    {
+        if (!request.TryGetUInt64("address", out var address))
+        {
+            return DebugResponse.Failure(request.Command, "Expected an 'address'.");
+        }
+
+        if (!request.TryGetInt32("length", out var length) || length <= 0 || length > MaxMemoryChunk)
+        {
+            return DebugResponse.Failure(request.Command, $"Expected a 'length' between 1 and {MaxMemoryChunk}.");
+        }
+
+        var buffer = new byte[length];
+        if (!_session.TryReadMemory(address, buffer))
+        {
+            return DebugResponse.Failure(request.Command, "Memory is unreadable or target is not paused.");
+        }
+
+        return DebugResponse.Success(request.Command, new Dictionary<string, object?>
+        {
+            ["address"] = FormatAddress(address),
+            ["length"] = length,
+            ["bytes"] = Convert.ToHexString(buffer),
+        });
+    }
+
+    private DebugResponse WriteMemory(DebugRequest request)
+    {
+        if (!request.TryGetUInt64("address", out var address))
+        {
+            return DebugResponse.Failure(request.Command, "Expected an 'address'.");
+        }
+
+        if (!request.TryGetString("bytes", out var hex) || hex.Length == 0 || (hex.Length & 1) != 0)
+        {
+            return DebugResponse.Failure(request.Command, "Expected 'bytes' as an even-length hex string.");
+        }
+
+        byte[] data;
+        try
+        {
+            data = Convert.FromHexString(hex);
+        }
+        catch (FormatException)
+        {
+            return DebugResponse.Failure(request.Command, "'bytes' is not valid hex.");
+        }
+
+        if (data.Length > MaxMemoryChunk)
+        {
+            return DebugResponse.Failure(request.Command, $"Cannot write more than {MaxMemoryChunk} bytes at once.");
+        }
+
+        return _session.TryWriteMemory(address, data)
+            ? DebugResponse.Success(request.Command, new Dictionary<string, object?> { ["written"] = data.Length })
+            : DebugResponse.Failure(request.Command, "Memory is unwritable or target is not paused.");
+    }
+
+    private DebugResponse ListBreakpoints(DebugRequest request)
+    {
+        var breakpoints = _session.Breakpoints.Snapshot()
+            .OrderBy(breakpoint => breakpoint.Id)
+            .Select(DescribeBreakpoint)
+            .ToArray();
+
+        return DebugResponse.Success(request.Command, new Dictionary<string, object?>
+        {
+            ["breakpoints"] = breakpoints,
+        });
+    }
+
+    private DebugResponse AddBreakpoint(DebugRequest request)
+    {
+        if (!request.TryGetUInt64("address", out var address))
+        {
+            return DebugResponse.Failure(request.Command, "Expected an 'address'.");
+        }
+
+        var kind = BreakpointKind.Execute;
+        if (request.TryGetString("kind", out var kindText) && !TryParseBreakpointKind(kindText, out kind))
+        {
+            return DebugResponse.Failure(request.Command, $"Unknown breakpoint kind '{kindText}'.");
+        }
+
+        var length = 1UL;
+        if (request.TryGetUInt64("length", out var requestedLength) && requestedLength > 0)
+        {
+            length = requestedLength;
+        }
+
+        var breakpoint = _session.Breakpoints.Add(kind, address, length);
+        return DebugResponse.Success(request.Command, new Dictionary<string, object?>
+        {
+            ["breakpoint"] = DescribeBreakpoint(breakpoint),
+        });
+    }
+
+    private DebugResponse RemoveBreakpoint(DebugRequest request)
+    {
+        if (!request.TryGetInt32("id", out var id))
+        {
+            return DebugResponse.Failure(request.Command, "Expected an 'id'.");
+        }
+
+        return _session.Breakpoints.Remove(id)
+            ? DebugResponse.Success(request.Command)
+            : DebugResponse.Failure(request.Command, $"No breakpoint with id {id}.");
+    }
+
+    private DebugResponse EnableBreakpoint(DebugRequest request)
+    {
+        if (!request.TryGetInt32("id", out var id))
+        {
+            return DebugResponse.Failure(request.Command, "Expected an 'id'.");
+        }
+
+        var enabled = !request.TryGetBool("enabled", out var requested) || requested;
+        return _session.Breakpoints.SetEnabled(id, enabled)
+            ? DebugResponse.Success(request.Command)
+            : DebugResponse.Failure(request.Command, $"No breakpoint with id {id}.");
+    }
+
+    private DebugResponse Pause(DebugRequest request)
+    {
+        _session.RequestPause();
+        return DebugResponse.Success(request.Command);
+    }
+
+    private static DebugResponse Simple(DebugRequest request, bool succeeded, string failureMessage)
+        => succeeded ? DebugResponse.Success(request.Command) : DebugResponse.Failure(request.Command, failureMessage);
+
+    private static DebugResponse NotPaused(DebugRequest request)
+        => DebugResponse.Failure(request.Command, "Target is not paused.");
+
+    internal static IReadOnlyDictionary<string, object?> DescribeStop(DebugStopEvent stop)
+    {
+        var data = new Dictionary<string, object?>
+        {
+            ["reason"] = stop.Reason.ToString(),
+            ["address"] = FormatAddress(stop.Address),
+            ["frameKind"] = stop.FrameKind.ToString(),
+            ["frameLabel"] = stop.FrameLabel,
+            ["registers"] = DescribeRegisters(stop.Registers),
+        };
+
+        if (stop.Breakpoint is { } breakpoint)
+        {
+            data["breakpoint"] = DescribeBreakpoint(breakpoint);
+        }
+
+        if (stop.Result is { } result)
+        {
+            data["result"] = result.ToString();
+        }
+
+        if (stop.Detail is { } detail)
+        {
+            data["detail"] = detail;
+        }
+
+        if (stop.OpcodeBytes is { } opcodeBytes)
+        {
+            data["opcodeBytes"] = opcodeBytes;
+        }
+
+        if (stop.StallInfo is { } stall)
+        {
+            data["stall"] = new Dictionary<string, object?>
+            {
+                ["kind"] = stall.Kind.ToString(),
+                ["nid"] = stall.Nid,
+                ["instructionPointer"] = FormatAddress(stall.InstructionPointer),
+                ["dispatchIndex"] = stall.DispatchIndex,
+                ["argument0"] = FormatAddress(stall.Argument0),
+                ["argument1"] = FormatAddress(stall.Argument1),
+                ["resolved"] = stall.IsResolved,
+                ["library"] = stall.LibraryName,
+                ["function"] = stall.FunctionName,
+            };
+        }
+
+        return data;
+    }
+
+    private static IReadOnlyDictionary<string, object?> DescribeRegisters(DebugRegisterFile registers)
+    {
+        var result = new Dictionary<string, object?>(20);
+        for (var i = 0; i < 16; i++)
+        {
+            result[((CpuRegister)i).ToString().ToLowerInvariant()] = FormatAddress(registers[(CpuRegister)i]);
+        }
+
+        result["rip"] = FormatAddress(registers.Rip);
+        result["rflags"] = FormatAddress(registers.Rflags);
+        result["fs_base"] = FormatAddress(registers.FsBase);
+        result["gs_base"] = FormatAddress(registers.GsBase);
+        return result;
+    }
+
+    private static IReadOnlyDictionary<string, object?> DescribeBreakpoint(Breakpoint breakpoint)
+        => new Dictionary<string, object?>
+        {
+            ["id"] = breakpoint.Id,
+            ["kind"] = breakpoint.Kind.ToString(),
+            ["address"] = FormatAddress(breakpoint.Address),
+            ["length"] = breakpoint.Length,
+            ["enabled"] = breakpoint.Enabled,
+        };
+
+    private static string FormatAddress(ulong value) => $"0x{value:X16}";
+
+    private static bool TryParseRegister(string name, out DebugRegisterId id)
+    {
+        var normalized = name.Trim().ToLowerInvariant();
+        switch (normalized)
+        {
+            case "rip":
+                id = DebugRegisterId.Rip;
+                return true;
+            case "rflags":
+                id = DebugRegisterId.Rflags;
+                return true;
+            case "fs_base" or "fsbase":
+                id = DebugRegisterId.FsBase;
+                return true;
+            case "gs_base" or "gsbase":
+                id = DebugRegisterId.GsBase;
+                return true;
+        }
+
+        return Enum.TryParse(normalized, ignoreCase: true, out id) && Enum.IsDefined(id);
+    }
+
+    private static bool TryParseBreakpointKind(string text, out BreakpointKind kind)
+        => Enum.TryParse(text.Trim(), ignoreCase: true, out kind) && Enum.IsDefined(kind);
+
+    private const int MaxMemoryChunk = 64 * 1024;
+}

src/SharpEmu.Debugger/Protocol/DebugRequest.cs

@@ -0,0 +1,152 @@
+// Copyright (C) 2026 SharpEmu Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+using System.Globalization;
+using System.Text.Json;
+
+namespace SharpEmu.Debugger.Protocol;
+
+/// <summary>
+/// A parsed client request: a <see cref="Command"/> verb plus a bag of named
+/// arguments backed by the original JSON. Numeric arguments accept either JSON
+/// numbers or <c>"0x"</c>-prefixed hex strings so addresses read naturally on
+/// the wire.
+/// </summary>
+public sealed class DebugRequest
+{
+    private readonly JsonElement _root;
+
+    private DebugRequest(string command, JsonElement root)
+    {
+        Command = command;
+        _root = root;
+    }
+
+    /// <summary>The lower-cased command verb.</summary>
+    public string Command { get; }
+
+    /// <summary>
+    /// Parses a single JSON object into a request. Returns false when the text is
+    /// not a JSON object or is missing a string <c>command</c> field.
+    /// </summary>
+    public static bool TryParse(string json, out DebugRequest request, out string error)
+    {
+        request = null!;
+        error = string.Empty;
+        try
+        {
+            using var document = JsonDocument.Parse(json);
+            var root = document.RootElement.Clone();
+            if (root.ValueKind != JsonValueKind.Object)
+            {
+                error = "Request must be a JSON object.";
+                return false;
+            }
+
+            if (!root.TryGetProperty("command", out var commandElement) ||
+                commandElement.ValueKind != JsonValueKind.String)
+            {
+                error = "Request is missing a string 'command'.";
+                return false;
+            }
+
+            var command = commandElement.GetString() ?? string.Empty;
+            request = new DebugRequest(command.Trim().ToLowerInvariant(), root);
+            return true;
+        }
+        catch (JsonException ex)
+        {
+            error = $"Malformed JSON: {ex.Message}";
+            return false;
+        }
+    }
+
+    public bool TryGetString(string name, out string value)
+    {
+        if (_root.TryGetProperty(name, out var element) && element.ValueKind == JsonValueKind.String)
+        {
+            value = element.GetString() ?? string.Empty;
+            return true;
+        }
+
+        value = string.Empty;
+        return false;
+    }
+
+    public bool TryGetUInt64(string name, out ulong value)
+    {
+        value = 0;
+        if (!_root.TryGetProperty(name, out var element))
+        {
+            return false;
+        }
+
+        switch (element.ValueKind)
+        {
+            case JsonValueKind.Number:
+                return element.TryGetUInt64(out value);
+            case JsonValueKind.String:
+                return TryParseNumber(element.GetString(), out value);
+            default:
+                return false;
+        }
+    }
+
+    public bool TryGetInt32(string name, out int value)
+    {
+        value = 0;
+        if (!_root.TryGetProperty(name, out var element))
+        {
+            return false;
+        }
+
+        switch (element.ValueKind)
+        {
+            case JsonValueKind.Number:
+                return element.TryGetInt32(out value);
+            case JsonValueKind.String when TryParseNumber(element.GetString(), out var parsed) && parsed <= int.MaxValue:
+                value = (int)parsed;
+                return true;
+            default:
+                return false;
+        }
+    }
+
+    public bool TryGetBool(string name, out bool value)
+    {
+        value = false;
+        if (!_root.TryGetProperty(name, out var element))
+        {
+            return false;
+        }
+
+        switch (element.ValueKind)
+        {
+            case JsonValueKind.True:
+                value = true;
+                return true;
+            case JsonValueKind.False:
+                value = false;
+                return true;
+            default:
+                return false;
+        }
+    }
+
+    private static bool TryParseNumber(string? text, out ulong value)
+    {
+        value = 0;
+        if (string.IsNullOrWhiteSpace(text))
+        {
+            return false;
+        }
+
+        text = text.Trim();
+        if (text.StartsWith("0x", StringComparison.OrdinalIgnoreCase))
+        {
+            return ulong.TryParse(text.AsSpan(2), NumberStyles.HexNumber, CultureInfo.InvariantCulture, out value);
+        }
+
+        return ulong.TryParse(text, NumberStyles.Integer, CultureInfo.InvariantCulture, out value);
+    }
+}

src/SharpEmu.Debugger/Protocol/DebugResponse.cs

@@ -0,0 +1,34 @@
+// Copyright (C) 2026 SharpEmu Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+namespace SharpEmu.Debugger.Protocol;
+
+/// <summary>
+/// The reply to a <see cref="DebugRequest"/>: either success with an optional
+/// data payload, or a failure with a human-readable message.
+/// </summary>
+public sealed class DebugResponse
+{
+    private DebugResponse(bool ok, string? command, IReadOnlyDictionary<string, object?>? data, string? error)
+    {
+        Ok = ok;
+        Command = command;
+        Data = data;
+        Error = error;
+    }
+
+    public bool Ok { get; }
+
+    /// <summary>Echoes the command the reply answers, when known.</summary>
+    public string? Command { get; }
+
+    public IReadOnlyDictionary<string, object?>? Data { get; }
+
+    public string? Error { get; }
+
+    public static DebugResponse Success(string command, IReadOnlyDictionary<string, object?>? data = null)
+        => new(ok: true, command, data, error: null);
+
+    public static DebugResponse Failure(string command, string error)
+        => new(ok: false, command, data: null, error);
+}

src/SharpEmu.Debugger/Protocol/IDebugProtocol.cs

@@ -0,0 +1,33 @@
+// Copyright (C) 2026 SharpEmu Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+namespace SharpEmu.Debugger.Protocol;
+
+/// <summary>
+/// Frames debugger traffic on a connection. A protocol turns bytes into
+/// <see cref="DebugRequest"/> objects and serialises <see cref="DebugResponse"/>
+/// replies plus asynchronous events (stops, resumes, termination) back to the
+/// client. Swapping the implementation (line-delimited JSON today, a GDB remote
+/// serial stub later) leaves the session and server untouched.
+/// </summary>
+public interface IDebugProtocol
+{
+    /// <summary>A short protocol name reported in the handshake.</summary>
+    string Name { get; }
+
+    /// <summary>
+    /// Reads the next request, or null at end of stream. Parse failures are
+    /// surfaced as a request with a reserved error command rather than throwing.
+    /// </summary>
+    Task<DebugRequest?> ReadRequestAsync(TextReader reader, CancellationToken cancellationToken);
+
+    /// <summary>Writes a reply to a request.</summary>
+    Task WriteResponseAsync(TextWriter writer, DebugResponse response, CancellationToken cancellationToken);
+
+    /// <summary>Writes an unsolicited event (for example a stop notification).</summary>
+    Task WriteEventAsync(
+        TextWriter writer,
+        string eventName,
+        IReadOnlyDictionary<string, object?> data,
+        CancellationToken cancellationToken);
+}

src/SharpEmu.Debugger/Protocol/JsonLineDebugProtocol.cs

@@ -0,0 +1,112 @@
+// Copyright (C) 2026 SharpEmu Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+using System.Text.Json;
+
+namespace SharpEmu.Debugger.Protocol;
+
+/// <summary>
+/// A newline-delimited JSON protocol: one JSON object per line in each
+/// direction. Requests carry a <c>command</c>; replies carry <c>ok</c> plus
+/// <c>data</c>/<c>error</c>; events carry an <c>event</c> name. It is trivial to
+/// drive from a socket, <c>nc</c>, or a small script, which suits bring-up and
+/// tooling while a richer protocol is layered on later.
+/// </summary>
+public sealed class JsonLineDebugProtocol : IDebugProtocol
+{
+    /// <summary>The command assigned to a request that failed to parse.</summary>
+    public const string ParseErrorCommand = "$parse-error";
+
+    private static readonly JsonSerializerOptions SerializerOptions = new()
+    {
+        WriteIndented = false,
+    };
+
+    public string Name => "json-lines/1";
+
+    public async Task<DebugRequest?> ReadRequestAsync(TextReader reader, CancellationToken cancellationToken)
+    {
+        while (true)
+        {
+            cancellationToken.ThrowIfCancellationRequested();
+            var line = await reader.ReadLineAsync(cancellationToken).ConfigureAwait(false);
+            if (line is null)
+            {
+                return null;
+            }
+
+            if (string.IsNullOrWhiteSpace(line))
+            {
+                continue;
+            }
+
+            if (DebugRequest.TryParse(line, out var request, out var error))
+            {
+                return request;
+            }
+
+            // Surface the parse failure as a synthetic request so the connection
+            // loop can reply with an error rather than dropping the client.
+            var envelope = $"{{\"command\":\"{ParseErrorCommand}\",\"message\":{JsonSerializer.Serialize(error)}}}";
+            if (DebugRequest.TryParse(envelope, out var errorRequest, out _))
+            {
+                return errorRequest;
+            }
+        }
+    }
+
+    public async Task WriteResponseAsync(TextWriter writer, DebugResponse response, CancellationToken cancellationToken)
+    {
+        var payload = new Dictionary<string, object?>
+        {
+            ["ok"] = response.Ok,
+        };
+
+        if (response.Command is not null)
+        {
+            payload["command"] = response.Command;
+        }
+
+        if (response.Data is not null)
+        {
+            payload["data"] = response.Data;
+        }
+
+        if (response.Error is not null)
+        {
+            payload["error"] = response.Error;
+        }
+
+        await WriteLineAsync(writer, payload, cancellationToken).ConfigureAwait(false);
+    }
+
+    public async Task WriteEventAsync(
+        TextWriter writer,
+        string eventName,
+        IReadOnlyDictionary<string, object?> data,
+        CancellationToken cancellationToken)
+    {
+        var payload = new Dictionary<string, object?>(data.Count + 1)
+        {
+            ["event"] = eventName,
+        };
+
+        foreach (var (key, value) in data)
+        {
+            payload[key] = value;
+        }
+
+        await WriteLineAsync(writer, payload, cancellationToken).ConfigureAwait(false);
+    }
+
+    private static async Task WriteLineAsync(
+        TextWriter writer,
+        IReadOnlyDictionary<string, object?> payload,
+        CancellationToken cancellationToken)
+    {
+        cancellationToken.ThrowIfCancellationRequested();
+        var json = JsonSerializer.Serialize(payload, SerializerOptions);
+        await writer.WriteLineAsync(json.AsMemory(), cancellationToken).ConfigureAwait(false);
+        await writer.FlushAsync(cancellationToken).ConfigureAwait(false);
+    }
+}

src/SharpEmu.Debugger/Server/DebuggerClientConnection.cs

@@ -0,0 +1,158 @@
+// Copyright (C) 2026 SharpEmu Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+using System.Net.Sockets;
+using System.Text;
+using SharpEmu.Debugger.Protocol;
+using SharpEmu.Debugger.Session;
+using SharpEmu.Logging;
+
+namespace SharpEmu.Debugger.Server;
+
+/// <summary>
+/// Services a single connected client: reads requests, dispatches them against
+/// the shared session, and pushes session lifecycle events. Writes from the
+/// request loop and from event callbacks are serialised through one lock so the
+/// two never interleave a half-written line.
+/// </summary>
+internal sealed class DebuggerClientConnection : IAsyncDisposable
+{
+    private static readonly SharpEmuLogger Log = SharpEmuLog.For("SharpEmu.Debugger");
+    private static readonly UTF8Encoding Utf8NoBom = new(encoderShouldEmitUTF8Identifier: false);
+
+    private readonly TcpClient _client;
+    private readonly IDebuggerSession _session;
+    private readonly IDebugProtocol _protocol;
+    private readonly DebugCommandDispatcher _dispatcher;
+    private readonly SemaphoreSlim _writeLock = new(1, 1);
+
+    private TextWriter? _writer;
+    private CancellationToken _cancellationToken;
+
+    public DebuggerClientConnection(TcpClient client, IDebuggerSession session, IDebugProtocol protocol)
+    {
+        _client = client;
+        _session = session;
+        _protocol = protocol;
+        _dispatcher = new DebugCommandDispatcher(session);
+    }
+
+    public async Task RunAsync(CancellationToken cancellationToken)
+    {
+        _cancellationToken = cancellationToken;
+        var endpoint = _client.Client.RemoteEndPoint?.ToString() ?? "unknown";
+        Log.Info($"Debugger client connected: {endpoint}");
+
+        using var stream = _client.GetStream();
+        using var reader = new StreamReader(stream, Utf8NoBom);
+        await using var writer = new StreamWriter(stream, Utf8NoBom) { AutoFlush = false };
+        _writer = writer;
+
+        _session.Stopped += OnStopped;
+        _session.Resumed += OnResumed;
+        _session.Terminated += OnTerminated;
+        try
+        {
+            await SendEventAsync("hello", new Dictionary<string, object?>
+            {
+                ["protocol"] = _protocol.Name,
+                ["state"] = _session.State.ToString(),
+            }).ConfigureAwait(false);
+
+            while (!cancellationToken.IsCancellationRequested)
+            {
+                var request = await _protocol.ReadRequestAsync(reader, cancellationToken).ConfigureAwait(false);
+                if (request is null)
+                {
+                    break;
+                }
+
+                var response = _dispatcher.Dispatch(request);
+                await WriteResponseAsync(response).ConfigureAwait(false);
+            }
+        }
+        catch (OperationCanceledException)
+        {
+            // Server shutting down.
+        }
+        catch (IOException)
+        {
+            // Client dropped the connection.
+        }
+        catch (Exception ex)
+        {
+            Log.Warn($"Debugger client error ({endpoint}): {ex.Message}");
+        }
+        finally
+        {
+            _session.Stopped -= OnStopped;
+            _session.Resumed -= OnResumed;
+            _session.Terminated -= OnTerminated;
+            _writer = null;
+            Log.Info($"Debugger client disconnected: {endpoint}");
+        }
+    }
+
+    private void OnStopped(object? sender, DebugStopEvent stop)
+        => _ = SendEventAsync("stopped", DebugCommandDispatcher.DescribeStop(stop));
+
+    private void OnResumed(object? sender, EventArgs e)
+        => _ = SendEventAsync("resumed", EmptyData);
+
+    private void OnTerminated(object? sender, EventArgs e)
+        => _ = SendEventAsync("terminated", EmptyData);
+
+    private async Task WriteResponseAsync(DebugResponse response)
+    {
+        var writer = _writer;
+        if (writer is null)
+        {
+            return;
+        }
+
+        await _writeLock.WaitAsync(_cancellationToken).ConfigureAwait(false);
+        try
+        {
+            await _protocol.WriteResponseAsync(writer, response, _cancellationToken).ConfigureAwait(false);
+        }
+        finally
+        {
+            _writeLock.Release();
+        }
+    }
+
+    private async Task SendEventAsync(string name, IReadOnlyDictionary<string, object?> data)
+    {
+        var writer = _writer;
+        if (writer is null)
+        {
+            return;
+        }
+
+        try
+        {
+            await _writeLock.WaitAsync(_cancellationToken).ConfigureAwait(false);
+            try
+            {
+                await _protocol.WriteEventAsync(writer, name, data, _cancellationToken).ConfigureAwait(false);
+            }
+            finally
+            {
+                _writeLock.Release();
+            }
+        }
+        catch (Exception ex) when (ex is IOException or OperationCanceledException or ObjectDisposedException)
+        {
+            // The client went away between the event firing and the write.
+        }
+    }
+
+    public ValueTask DisposeAsync()
+    {
+        _writeLock.Dispose();
+        _client.Dispose();
+        return ValueTask.CompletedTask;
+    }
+
+    private static readonly IReadOnlyDictionary<string, object?> EmptyData = new Dictionary<string, object?>();
+}

src/SharpEmu.Debugger/Server/DebuggerServer.cs

@@ -0,0 +1,136 @@
+// Copyright (C) 2026 SharpEmu Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+using System.Collections.Concurrent;
+using System.Net;
+using System.Net.Sockets;
+using SharpEmu.Debugger.Protocol;
+using SharpEmu.Debugger.Session;
+using SharpEmu.Logging;
+
+namespace SharpEmu.Debugger.Server;
+
+/// <summary>
+/// A TCP server that exposes an <see cref="IDebuggerSession"/> to remote
+/// clients over a pluggable <see cref="IDebugProtocol"/>. Every connection sees
+/// the same session, so multiple clients (for example a UI and a scripted
+/// probe) observe a consistent view of the target.
+/// </summary>
+public sealed class DebuggerServer : IDebuggerServer
+{
+    private static readonly SharpEmuLogger Log = SharpEmuLog.For("SharpEmu.Debugger");
+
+    private readonly IDebuggerSession _session;
+    private readonly DebuggerServerOptions _options;
+    private readonly Func<IDebugProtocol> _protocolFactory;
+    private readonly ConcurrentDictionary<DebuggerClientConnection, Task> _connections = new();
+    private readonly CancellationTokenSource _shutdown = new();
+
+    private TcpListener? _listener;
+    private Task? _acceptLoop;
+
+    public DebuggerServer(
+        IDebuggerSession session,
+        DebuggerServerOptions? options = null,
+        Func<IDebugProtocol>? protocolFactory = null)
+    {
+        _session = session ?? throw new ArgumentNullException(nameof(session));
+        _options = options ?? new DebuggerServerOptions();
+        _protocolFactory = protocolFactory ?? (static () => new JsonLineDebugProtocol());
+    }
+
+    public bool IsListening => _listener is not null;
+
+    public IPEndPoint? Endpoint { get; private set; }
+
+    public void Start()
+    {
+        if (_listener is not null)
+        {
+            return;
+        }
+
+        var listener = new TcpListener(_options.BindAddress, _options.Port);
+        listener.Start(_options.MaxClients);
+        _listener = listener;
+        Endpoint = (IPEndPoint?)listener.LocalEndpoint;
+        Log.Info($"Debug server listening on {Endpoint} (protocol {_protocolFactory().Name})");
+        _acceptLoop = Task.Run(() => AcceptLoopAsync(listener, _shutdown.Token));
+    }
+
+    private async Task AcceptLoopAsync(TcpListener listener, CancellationToken cancellationToken)
+    {
+        while (!cancellationToken.IsCancellationRequested)
+        {
+            TcpClient client;
+            try
+            {
+                client = await listener.AcceptTcpClientAsync(cancellationToken).ConfigureAwait(false);
+            }
+            catch (OperationCanceledException)
+            {
+                break;
+            }
+            catch (SocketException) when (cancellationToken.IsCancellationRequested)
+            {
+                break;
+            }
+            catch (ObjectDisposedException)
+            {
+                break;
+            }
+
+            var connection = new DebuggerClientConnection(client, _session, _protocolFactory());
+            var task = Task.Run(() => ServeAsync(connection, cancellationToken), cancellationToken);
+            _connections[connection] = task;
+        }
+    }
+
+    private async Task ServeAsync(DebuggerClientConnection connection, CancellationToken cancellationToken)
+    {
+        try
+        {
+            await connection.RunAsync(cancellationToken).ConfigureAwait(false);
+        }
+        finally
+        {
+            _connections.TryRemove(connection, out _);
+            await connection.DisposeAsync().ConfigureAwait(false);
+        }
+    }
+
+    public async Task StopAsync()
+    {
+        if (_listener is null)
+        {
+            return;
+        }
+
+        await _shutdown.CancelAsync().ConfigureAwait(false);
+        _listener.Stop();
+        _listener = null;
+
+        try
+        {
+            if (_acceptLoop is not null)
+            {
+                await _acceptLoop.ConfigureAwait(false);
+            }
+
+            await Task.WhenAll(_connections.Values).ConfigureAwait(false);
+        }
+        catch (Exception ex) when (ex is OperationCanceledException or SocketException or ObjectDisposedException)
+        {
+            // Expected while tearing connections down.
+        }
+
+        _connections.Clear();
+        Log.Info("Debug server stopped.");
+    }
+
+    public async ValueTask DisposeAsync()
+    {
+        await StopAsync().ConfigureAwait(false);
+        _shutdown.Dispose();
+    }
+}

src/SharpEmu.Debugger/Server/DebuggerServerOptions.cs

@@ -0,0 +1,78 @@
+// Copyright (C) 2026 SharpEmu Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+using System.Net;
+
+namespace SharpEmu.Debugger.Server;
+
+/// <summary>Network configuration for a <see cref="DebuggerServer"/>.</summary>
+public sealed class DebuggerServerOptions
+{
+    /// <summary>The default TCP port the debug server listens on.</summary>
+    public const int DefaultPort = 5714;
+
+    /// <summary>
+    /// The address to bind. Defaults to loopback so the debug surface is not
+    /// exposed off-box; a caller must opt in to a routable address explicitly.
+    /// </summary>
+    public IPAddress BindAddress { get; init; } = IPAddress.Loopback;
+
+    /// <summary>The TCP port to listen on.</summary>
+    public int Port { get; init; } = DefaultPort;
+
+    /// <summary>
+    /// The maximum number of simultaneous client connections. Additional
+    /// connections wait in the accept backlog.
+    /// </summary>
+    public int MaxClients { get; init; } = 4;
+
+    /// <summary>
+    /// Parses a <c>host:port</c>, bare <c>port</c>, or bare host into options.
+    /// Returns false when the text cannot be interpreted.
+    /// </summary>
+    public static bool TryParseEndpoint(string? text, out DebuggerServerOptions options, out string error)
+    {
+        options = new DebuggerServerOptions();
+        error = string.Empty;
+        if (string.IsNullOrWhiteSpace(text))
+        {
+            return true;
+        }
+
+        var value = text.Trim();
+        var host = value;
+        var port = DefaultPort;
+
+        var separator = value.LastIndexOf(':');
+        if (separator >= 0)
+        {
+            var portText = value[(separator + 1)..];
+            if (portText.Length > 0)
+            {
+                if (!int.TryParse(portText, out port) || port is <= 0 or > 65535)
+                {
+                    error = $"Invalid port '{portText}'.";
+                    return false;
+                }
+            }
+
+            host = value[..separator];
+        }
+
+        var address = IPAddress.Loopback;
+        if (!string.IsNullOrWhiteSpace(host) &&
+            !string.Equals(host, "localhost", StringComparison.OrdinalIgnoreCase) &&
+            !IPAddress.TryParse(host, out address!))
+        {
+            error = $"Invalid bind address '{host}'.";
+            return false;
+        }
+
+        options = new DebuggerServerOptions
+        {
+            BindAddress = address ?? IPAddress.Loopback,
+            Port = port,
+        };
+        return true;
+    }
+}

src/SharpEmu.Debugger/Server/IDebuggerServer.cs

@@ -0,0 +1,24 @@
+// Copyright (C) 2026 SharpEmu Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+using System.Net;
+
+namespace SharpEmu.Debugger.Server;
+
+/// <summary>
+/// A network front-end that exposes a debugger session to remote clients.
+/// </summary>
+public interface IDebuggerServer : IAsyncDisposable
+{
+    /// <summary>True once the listener is accepting connections.</summary>
+    bool IsListening { get; }
+
+    /// <summary>The endpoint the server is bound to, or null before start.</summary>
+    IPEndPoint? Endpoint { get; }
+
+    /// <summary>Binds and begins accepting client connections.</summary>
+    void Start();
+
+    /// <summary>Stops accepting connections and closes active clients.</summary>
+    Task StopAsync();
+}

src/SharpEmu.Debugger/Session/DebugStopEvent.cs

@@ -0,0 +1,69 @@
+// Copyright (C) 2026 SharpEmu Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+using SharpEmu.Core.Cpu.Debugging;
+using SharpEmu.Debugger.Breakpoints;
+using SharpEmu.HLE;
+
+namespace SharpEmu.Debugger.Session;
+
+/// <summary>
+/// Describes a stop delivered to debugger clients: why the target stopped,
+/// where, and the register snapshot at that point.
+/// </summary>
+public sealed class DebugStopEvent
+{
+    public DebugStopEvent(
+        DebugStopReason reason,
+        DebugRegisterFile registers,
+        CpuDebugFrameKind frameKind,
+        string frameLabel,
+        Breakpoint? breakpoint = null,
+        OrbisGen2Result? result = null,
+        string? detail = null,
+        string? opcodeBytes = null,
+        CpuStallInfo? stallInfo = null)
+    {
+        Reason = reason;
+        Registers = registers;
+        FrameKind = frameKind;
+        FrameLabel = frameLabel ?? string.Empty;
+        Breakpoint = breakpoint;
+        Result = result;
+        Detail = detail;
+        OpcodeBytes = opcodeBytes;
+        StallInfo = stallInfo;
+    }
+
+    public DebugStopReason Reason { get; }
+
+    /// <summary>The instruction pointer where the target stopped.</summary>
+    public ulong Address => Registers.Rip;
+
+    public DebugRegisterFile Registers { get; }
+
+    public CpuDebugFrameKind FrameKind { get; }
+
+    public string FrameLabel { get; }
+
+    /// <summary>The breakpoint responsible for the stop, when applicable.</summary>
+    public Breakpoint? Breakpoint { get; }
+
+    /// <summary>
+    /// The frame result for a <see cref="DebugStopReason.Fault"/> stop; null for
+    /// non-fault stops.
+    /// </summary>
+    public OrbisGen2Result? Result { get; }
+
+    /// <summary>A human-readable summary of a fault, when applicable.</summary>
+    public string? Detail { get; }
+
+    /// <summary>
+    /// A hex preview of the bytes at <see cref="Address"/> (the faulting
+    /// instruction), when the stop is a fault and the bytes were readable.
+    /// </summary>
+    public string? OpcodeBytes { get; }
+
+    /// <summary>Structured backend evidence for a stall stop.</summary>
+    public CpuStallInfo? StallInfo { get; }
+}

src/SharpEmu.Debugger/Session/DebugStopReason.cs

@@ -0,0 +1,32 @@
+// Copyright (C) 2026 SharpEmu Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+namespace SharpEmu.Debugger.Session;
+
+/// <summary>Why the target stopped and handed control to the debugger.</summary>
+public enum DebugStopReason
+{
+    /// <summary>Stopped at the configured entry point before running any frame.</summary>
+    EntryPoint,
+
+    /// <summary>An execution breakpoint was hit.</summary>
+    Breakpoint,
+
+    /// <summary>A data watchpoint was hit.</summary>
+    Watchpoint,
+
+    /// <summary>A single-step (frame step) request completed.</summary>
+    Step,
+
+    /// <summary>A client-requested pause took effect.</summary>
+    Pause,
+
+    /// <summary>The guest raised a fault or trap.</summary>
+    Fault,
+
+    /// <summary>
+    /// The backend detected an execution stall (for example a mutex spin loop /
+    /// livelock) with no forward progress.
+    /// </summary>
+    Stall,
+}

src/SharpEmu.Debugger/Session/DebuggerRunState.cs

@@ -0,0 +1,23 @@
+// Copyright (C) 2026 SharpEmu Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+namespace SharpEmu.Debugger.Session;
+
+/// <summary>The execution state of a debugged target as seen by the debugger.</summary>
+public enum DebuggerRunState
+{
+    /// <summary>No guest frame has entered the debugger yet.</summary>
+    Detached,
+
+    /// <summary>The guest is executing and cannot be inspected safely.</summary>
+    Running,
+
+    /// <summary>
+    /// The guest is parked at a frame boundary. Registers and memory can be
+    /// read and written, and breakpoints can be edited.
+    /// </summary>
+    Paused,
+
+    /// <summary>The guest has finished; no further frames will run.</summary>
+    Terminated,
+}

src/SharpEmu.Debugger/Session/DebuggerSession.cs

@@ -0,0 +1,425 @@
+// Copyright (C) 2026 SharpEmu Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+using SharpEmu.Core.Cpu.Debugging;
+using SharpEmu.Debugger.Breakpoints;
+using SharpEmu.HLE;
+using SharpEmu.Logging;
+
+namespace SharpEmu.Debugger.Session;
+
+/// <summary>
+/// The default <see cref="IDebuggerSession"/>. It plugs into the CPU dispatcher
+/// as an <see cref="ICpuDebugHook"/>: when a frame boundary warrants a stop it
+/// parks the emulation thread inside <see cref="ICpuDebugHook.OnFrameEnter"/>
+/// while a debug client inspects and edits state, then releases it on
+/// continue/step.
+/// </summary>
+/// <remarks>
+/// Pausing works by blocking the emulation thread on <see cref="_resumeGate"/>
+/// from within the hook call. Because that thread is the one that owns the guest
+/// context, register and memory accessors are safe to serve from other threads
+/// only while it is parked — which is exactly the <see cref="DebuggerRunState.Paused"/>
+/// window the accessors gate on.
+/// </remarks>
+public sealed class DebuggerSession : IDebuggerSession, ICpuDebugHook
+{
+    private static readonly SharpEmuLogger Log = SharpEmuLog.For("SharpEmu.Debugger");
+
+    private readonly object _sync = new();
+    private readonly ManualResetEventSlim _resumeGate = new(initialState: false);
+    private readonly DebuggerSessionOptions _options;
+
+    private ICpuDebugFrame? _currentFrame;
+    private DebuggerRunState _state = DebuggerRunState.Detached;
+    private DebugStopEvent? _lastStop;
+    private bool _seenFirstFrame;
+    private bool _pausePending;
+    private bool _stepPending;
+
+    public DebuggerSession(DebuggerSessionOptions? options = null)
+    {
+        _options = options ?? new DebuggerSessionOptions();
+        Breakpoints = new BreakpointStore();
+    }
+
+    public BreakpointStore Breakpoints { get; }
+
+    public ICpuDebugHook Hook => this;
+
+    public event EventHandler<DebugStopEvent>? Stopped;
+
+    public event EventHandler? Resumed;
+
+    public event EventHandler? Terminated;
+
+    public DebuggerRunState State
+    {
+        get
+        {
+            lock (_sync)
+            {
+                return _state;
+            }
+        }
+    }
+
+    public DebugStopEvent? LastStop
+    {
+        get
+        {
+            lock (_sync)
+            {
+                return _lastStop;
+            }
+        }
+    }
+
+    void ICpuDebugHook.OnFrameEnter(ICpuDebugFrame frame)
+    {
+        DebugStopEvent? stop;
+        lock (_sync)
+        {
+            _currentFrame = frame;
+            var firstFrame = !_seenFirstFrame;
+            _seenFirstFrame = true;
+
+            var reason = ResolveStopReason(frame, firstFrame, out var breakpoint);
+            if (reason is null)
+            {
+                _state = DebuggerRunState.Running;
+                return;
+            }
+
+            _state = DebuggerRunState.Paused;
+            _lastStop = new DebugStopEvent(
+                reason.Value,
+                DebugRegisterFile.Capture(frame),
+                frame.Kind,
+                frame.Label,
+                breakpoint);
+            stop = _lastStop;
+            _resumeGate.Reset();
+        }
+
+        Log.Debug($"Debugger stop: {stop!.Reason} at 0x{stop.Address:X16} ({stop.FrameLabel})");
+        Stopped?.Invoke(this, stop);
+
+        // Park the emulation thread until a client resumes the target. The frame
+        // stays live and inspectable for the whole wait.
+        _resumeGate.Wait();
+
+        lock (_sync)
+        {
+            if (_state != DebuggerRunState.Terminated)
+            {
+                _state = DebuggerRunState.Running;
+            }
+        }
+
+        Resumed?.Invoke(this, EventArgs.Empty);
+    }
+
+    void ICpuDebugHook.OnFrameExit(ICpuDebugFrame frame, OrbisGen2Result result)
+    {
+        DebugStopEvent? stop = null;
+        lock (_sync)
+        {
+            if (_options.BreakOnFault &&
+                result != OrbisGen2Result.ORBIS_GEN2_OK &&
+                _state != DebuggerRunState.Terminated)
+            {
+                // Parking here keeps the post-fault frame inspectable.
+                _currentFrame = frame;
+                _state = DebuggerRunState.Paused;
+                _lastStop = BuildFaultStop(frame, result);
+                stop = _lastStop;
+                _resumeGate.Reset();
+            }
+        }
+
+        if (stop is not null)
+        {
+            Log.Debug($"Debugger fault stop: {stop.Result} at 0x{stop.Address:X16} ({stop.FrameLabel})");
+            Stopped?.Invoke(this, stop);
+            _resumeGate.Wait();
+            Resumed?.Invoke(this, EventArgs.Empty);
+        }
+
+        lock (_sync)
+        {
+            if (ReferenceEquals(_currentFrame, frame))
+            {
+                _currentFrame = null;
+            }
+
+            if (_state != DebuggerRunState.Terminated)
+            {
+                _state = DebuggerRunState.Running;
+            }
+        }
+    }
+
+    void ICpuDebugHook.OnStall(ICpuDebugFrame frame, CpuStallInfo info)
+    {
+        if (!_options.BreakOnStall)
+        {
+            return;
+        }
+
+        DebugStopEvent? stop = null;
+        lock (_sync)
+        {
+            if (_state == DebuggerRunState.Terminated)
+            {
+                return;
+            }
+
+            _currentFrame = frame;
+            _state = DebuggerRunState.Paused;
+            _lastStop = new DebugStopEvent(
+                DebugStopReason.Stall,
+                DebugRegisterFile.Capture(frame),
+                frame.Kind,
+                frame.Label,
+                breakpoint: null,
+                result: null,
+                detail: info.Detail,
+                opcodeBytes: ReadOpcodePreview(frame, info.InstructionPointer, 16),
+                stallInfo: info);
+            stop = _lastStop;
+            _resumeGate.Reset();
+        }
+
+        Log.Debug($"Debugger stall stop: {info.Kind} nid={info.Nid} at 0x{info.InstructionPointer:X16}");
+        Stopped?.Invoke(this, stop);
+        _resumeGate.Wait();
+
+        lock (_sync)
+        {
+            if (ReferenceEquals(_currentFrame, frame))
+            {
+                _currentFrame = null;
+            }
+
+            if (_state != DebuggerRunState.Terminated)
+            {
+                _state = DebuggerRunState.Running;
+            }
+        }
+
+        Resumed?.Invoke(this, EventArgs.Empty);
+    }
+
+    private static DebugStopEvent BuildFaultStop(ICpuDebugFrame frame, OrbisGen2Result result)
+    {
+        var opcodeBytes = ReadOpcodePreview(frame, frame.Rip, 16);
+        var detail = $"result={result}";
+        if (opcodeBytes is not null)
+        {
+            detail += $", bytes={opcodeBytes}";
+        }
+
+        return new DebugStopEvent(
+            DebugStopReason.Fault,
+            DebugRegisterFile.Capture(frame),
+            frame.Kind,
+            frame.Label,
+            breakpoint: null,
+            result: result,
+            detail: detail,
+            opcodeBytes: opcodeBytes);
+    }
+
+    private static string? ReadOpcodePreview(ICpuDebugFrame frame, ulong address, int maxBytes)
+    {
+        Span<byte> buffer = stackalloc byte[maxBytes];
+        var count = 0;
+        for (; count < maxBytes; count++)
+        {
+            if (!frame.Memory.TryRead(address + (ulong)count, buffer.Slice(count, 1)))
+            {
+                break;
+            }
+        }
+
+        return count == 0 ? null : Convert.ToHexString(buffer[..count]);
+    }
+
+    /// <summary>
+    /// Signals that the whole guest run has finished. Releases any parked
+    /// emulation thread and moves the session to
+    /// <see cref="DebuggerRunState.Terminated"/>.
+    /// </summary>
+    public void NotifyTerminated()
+    {
+        lock (_sync)
+        {
+            _state = DebuggerRunState.Terminated;
+            _currentFrame = null;
+        }
+
+        _resumeGate.Set();
+        Terminated?.Invoke(this, EventArgs.Empty);
+    }
+
+    private DebugStopReason? ResolveStopReason(ICpuDebugFrame frame, bool firstFrame, out Breakpoint? breakpoint)
+    {
+        breakpoint = null;
+        if (_pausePending)
+        {
+            _pausePending = false;
+            return DebugStopReason.Pause;
+        }
+
+        if (_stepPending)
+        {
+            _stepPending = false;
+            return DebugStopReason.Step;
+        }
+
+        var hit = Breakpoints.FindExecuteHit(frame.EntryPoint);
+        if (hit is not null)
+        {
+            breakpoint = hit;
+            return DebugStopReason.Breakpoint;
+        }
+
+        if (_options.StopAtEntry && firstFrame)
+        {
+            return DebugStopReason.EntryPoint;
+        }
+
+        return null;
+    }
+
+    public bool TryGetRegisters(out DebugRegisterFile registers)
+    {
+        lock (_sync)
+        {
+            if (!IsPausedWithFrame(out var frame))
+            {
+                registers = default;
+                return false;
+            }
+
+            registers = DebugRegisterFile.Capture(frame);
+            return true;
+        }
+    }
+
+    public bool TrySetRegister(DebugRegisterId id, ulong value)
+    {
+        lock (_sync)
+        {
+            if (!IsPausedWithFrame(out var frame))
+            {
+                return false;
+            }
+
+            if (id.IsGeneralPurpose())
+            {
+                frame.SetRegister(id.ToCpuRegister(), value);
+                return true;
+            }
+
+            switch (id)
+            {
+                case DebugRegisterId.Rip:
+                    frame.Rip = value;
+                    return true;
+                case DebugRegisterId.Rflags:
+                    frame.Rflags = value;
+                    return true;
+                default:
+                    // FS/GS bases are owned by the TLS setup and are read-only here.
+                    return false;
+            }
+        }
+    }
+
+    public bool TryReadMemory(ulong address, Span<byte> destination)
+    {
+        lock (_sync)
+        {
+            return IsPausedWithFrame(out var frame) && frame.Memory.TryRead(address, destination);
+        }
+    }
+
+    public bool TryWriteMemory(ulong address, ReadOnlySpan<byte> source)
+    {
+        lock (_sync)
+        {
+            return IsPausedWithFrame(out var frame) && frame.Memory.TryWrite(address, source);
+        }
+    }
+
+    public bool TryReadXmm(int registerIndex, out ulong low, out ulong high)
+    {
+        lock (_sync)
+        {
+            if (!IsPausedWithFrame(out var frame) || (uint)registerIndex >= 16)
+            {
+                low = 0;
+                high = 0;
+                return false;
+            }
+
+            frame.GetXmm(registerIndex, out low, out high);
+            return true;
+        }
+    }
+
+    public bool Continue()
+    {
+        lock (_sync)
+        {
+            if (_state != DebuggerRunState.Paused)
+            {
+                return false;
+            }
+
+            _resumeGate.Set();
+            return true;
+        }
+    }
+
+    public bool StepFrame()
+    {
+        lock (_sync)
+        {
+            if (_state != DebuggerRunState.Paused)
+            {
+                return false;
+            }
+
+            _stepPending = true;
+            _resumeGate.Set();
+            return true;
+        }
+    }
+
+    public void RequestPause()
+    {
+        lock (_sync)
+        {
+            if (_state == DebuggerRunState.Running)
+            {
+                _pausePending = true;
+            }
+        }
+    }
+
+    private bool IsPausedWithFrame(out ICpuDebugFrame frame)
+    {
+        // Callers must hold _sync.
+        if (_state == DebuggerRunState.Paused && _currentFrame is not null)
+        {
+            frame = _currentFrame;
+            return true;
+        }
+
+        frame = null!;
+        return false;
+    }
+}

src/SharpEmu.Debugger/Session/DebuggerSessionOptions.cs

@@ -0,0 +1,31 @@
+// Copyright (C) 2026 SharpEmu Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+namespace SharpEmu.Debugger.Session;
+
+/// <summary>Configuration for a <see cref="DebuggerSession"/>.</summary>
+public sealed class DebuggerSessionOptions
+{
+    /// <summary>
+    /// When true, the session pauses at the first frame it observes so a client
+    /// can attach breakpoints before the guest runs. Defaults to true, matching
+    /// the "stop at entry" behaviour most debuggers expose.
+    /// </summary>
+    public bool StopAtEntry { get; init; } = true;
+
+    /// <summary>
+    /// When true, the session pauses when a frame ends with a non-OK result (a
+    /// CPU trap, memory fault, or unimplemented path) so a client can inspect the
+    /// post-fault register/memory state before the frame is torn down. Defaults
+    /// to true. The stop reports <see cref="DebugStopReason.Fault"/>.
+    /// </summary>
+    public bool BreakOnFault { get; init; } = true;
+
+    /// <summary>
+    /// When true, the session pauses when the backend detects an execution stall
+    /// (a mutex spin loop / livelock) before the guest is forced out of the loop,
+    /// so a client can inspect the stalled state. Defaults to true. The stop
+    /// reports <see cref="DebugStopReason.Stall"/>.
+    /// </summary>
+    public bool BreakOnStall { get; init; } = true;
+}

src/SharpEmu.Debugger/Session/IDebugTarget.cs

@@ -0,0 +1,51 @@
+// Copyright (C) 2026 SharpEmu Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+namespace SharpEmu.Debugger.Session;
+
+/// <summary>
+/// The inspection and control surface a debugger front-end (for example a
+/// network server) drives. Register and memory accessors succeed only while the
+/// target is <see cref="DebuggerRunState.Paused"/>; they return <c>false</c>
+/// otherwise so callers never read torn state from a running guest.
+/// </summary>
+public interface IDebugTarget
+{
+    /// <summary>The current execution state.</summary>
+    DebuggerRunState State { get; }
+
+    /// <summary>The most recent stop, or null if the target has not stopped yet.</summary>
+    DebugStopEvent? LastStop { get; }
+
+    /// <summary>Reads the integer register file. Fails unless paused.</summary>
+    bool TryGetRegisters(out DebugRegisterFile registers);
+
+    /// <summary>Writes a single register. Fails unless paused.</summary>
+    bool TrySetRegister(DebugRegisterId id, ulong value);
+
+    /// <summary>Reads guest memory into <paramref name="destination"/>. Fails unless paused.</summary>
+    bool TryReadMemory(ulong address, Span<byte> destination);
+
+    /// <summary>Writes guest memory from <paramref name="source"/>. Fails unless paused.</summary>
+    bool TryWriteMemory(ulong address, ReadOnlySpan<byte> source);
+
+    /// <summary>Reads a 128-bit XMM register. Fails unless paused.</summary>
+    bool TryReadXmm(int registerIndex, out ulong low, out ulong high);
+
+    /// <summary>
+    /// Resumes a paused target. Returns false when the target was not paused.
+    /// </summary>
+    bool Continue();
+
+    /// <summary>
+    /// Resumes a paused target and stops again at the next frame boundary.
+    /// Returns false when the target was not paused.
+    /// </summary>
+    bool StepFrame();
+
+    /// <summary>
+    /// Requests that a running target stop at the next frame boundary. Has no
+    /// effect if the target is already paused or terminated.
+    /// </summary>
+    void RequestPause();
+}

src/SharpEmu.Debugger/Session/IDebuggerSession.cs

@@ -0,0 +1,43 @@
+// Copyright (C) 2026 SharpEmu Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+using SharpEmu.Core.Cpu.Debugging;
+using SharpEmu.Debugger.Breakpoints;
+
+namespace SharpEmu.Debugger.Session;
+
+/// <summary>
+/// The debugger's coordination point. It bridges the CPU dispatcher seam
+/// (<see cref="Hook"/>) to the inspection surface (<see cref="IDebugTarget"/>),
+/// owns breakpoint state, and raises lifecycle events that a server relays to
+/// connected clients.
+/// </summary>
+public interface IDebuggerSession : IDebugTarget
+{
+    /// <summary>The breakpoints armed for this session.</summary>
+    BreakpointStore Breakpoints { get; }
+
+    /// <summary>
+    /// The dispatcher-facing hook. Assign this to
+    /// <c>SharpEmuRuntimeOptions.DebugHook</c> so guest frames are routed through
+    /// the session.
+    /// </summary>
+    ICpuDebugHook Hook { get; }
+
+    /// <summary>Raised on the emulation thread each time the target stops.</summary>
+    event EventHandler<DebugStopEvent>? Stopped;
+
+    /// <summary>Raised when a paused target resumes.</summary>
+    event EventHandler? Resumed;
+
+    /// <summary>Raised once the target has terminated.</summary>
+    event EventHandler? Terminated;
+
+    /// <summary>
+    /// Signals that the guest run has finished. Releases any parked emulation
+    /// thread and transitions the session to
+    /// <see cref="DebuggerRunState.Terminated"/>. Hosts call this after the
+    /// runtime returns.
+    /// </summary>
+    void NotifyTerminated();
+}

src/SharpEmu.Debugger/SharpEmu.Debugger.csproj

@@ -0,0 +1,16 @@
+<!--
+Copyright (C) 2026 SharpEmu Emulator Project
+SPDX-License-Identifier: GPL-2.0-or-later
+-->
+
+<Project Sdk="Microsoft.NET.Sdk">
+  <ItemGroup>
+    <ProjectReference Include="..\SharpEmu.Core\SharpEmu.Core.csproj" />
+    <ProjectReference Include="..\SharpEmu.HLE\SharpEmu.HLE.csproj" />
+    <ProjectReference Include="..\SharpEmu.Logging\SharpEmu.Logging.csproj" />
+  </ItemGroup>
+
+  <PropertyGroup>
+    <NoWarn>$(NoWarn);1591</NoWarn>
+  </PropertyGroup>
+</Project>