feat: add gdb support to zba

deduplicate slowmem backup read handler

.gitmodules

@@ -13,3 +13,6 @@
 [submodule "lib/zig-toml"]
 	path = lib/zig-toml
 	url = https://github.com/aeronavery/zig-toml
+[submodule "lib/zba-gdbstub"]
+	path = lib/zba-gdbstub
+	url = https://git.musuka.dev/paoda/zba-gdbstub

build.zig

@@ -1,6 +1,8 @@
 const std = @import("std");
 const builtin = @import("builtin");
+
 const Sdk = @import("lib/SDL.zig/Sdk.zig");
+const Gdbstub = @import("lib/zba-gdbstub/build.zig");
 
 pub fn build(b: *std.build.Builder) void {
     // Minimum Zig Version
@@ -43,6 +45,9 @@ pub fn build(b: *std.build.Builder) void {
     // OpenGL 3.3 Bindings
     exe.addPackagePath("gl", "lib/gl.zig");
 
+    // gdbstub
+    Gdbstub.link(exe);
+
     // Zig SDL Bindings: https://github.com/MasterQ32/SDL.zig
     const sdk = Sdk.init(b);
     sdk.link(exe, .dynamic);

lib/zba-gdbstub

@@ -0,0 +1 @@
+Subproject commit 1bd9964f58b21aecfb39675258370c68ffeb0593

src/core/Bus.zig

@@ -197,8 +197,29 @@ fn fillTableExternalMemory(bus: *Self, addr: usize) ?*anyopaque {
     return &bus.pak.buf[masked_addr];
 }
 
-// TODO: Take advantage of fastmem here too?
 pub fn dbgRead(self: *const Self, comptime T: type, unaligned_address: u32) T {
+    const bits = @typeInfo(std.math.IntFittingRange(0, page_size - 1)).Int.bits;
+    const page = unaligned_address >> bits;
+    const offset = unaligned_address & (page_size - 1);
+
+    // We're doing some serious out-of-bounds open-bus reads
+    if (page >= table_len) return self.openBus(T, unaligned_address);
+
+    if (self.read_table[page]) |some_ptr| {
+        // We have a pointer to a page, cast the pointer to it's underlying type
+        const Ptr = [*]const T;
+        const alignment = @alignOf(std.meta.Child(Ptr));
+        const ptr = @ptrCast(Ptr, @alignCast(alignment, some_ptr));
+
+        // Note: We don't check array length, since we force align the
+        // lower bits of the address as the GBA would
+        return ptr[forceAlign(T, offset) / @sizeOf(T)];
+    }
+
+    return self.dbgSlowRead(T, unaligned_address);
+}
+
+fn dbgSlowRead(self: *const Self, comptime T: type, unaligned_address: u32) T {
     const page = @truncate(u8, unaligned_address >> 24);
     const address = forceAlign(T, unaligned_address);
 
@@ -210,29 +231,18 @@ pub fn dbgRead(self: *const Self, comptime T: type, unaligned_address: u32) T {
 
             break :blk self.openBus(T, address);
         },
-        0x02 => self.ewram.read(T, address),
+        0x02 => unreachable, // handled by fastmem
-        0x03 => self.iwram.read(T, address),
+        0x03 => unreachable, // handled by fastmem
         0x04 => self.readIo(T, address),
 
         // Internal Display Memory
-        0x05 => self.ppu.palette.read(T, address),
+        0x05 => unreachable, // handled by fastmem
-        0x06 => self.ppu.vram.read(T, address),
+        0x06 => unreachable, // handled by fastmem
-        0x07 => self.ppu.oam.read(T, address),
+        0x07 => unreachable, // handled by fastmem
 
         // External Memory (Game Pak)
         0x08...0x0D => self.pak.dbgRead(T, address),
-        0x0E...0x0F => blk: {
+        0x0E...0x0F => self.readBackup(T, unaligned_address),
-            const value = self.pak.backup.read(unaligned_address);
-
-            const multiplier = switch (T) {
-                u32 => 0x01010101,
-                u16 => 0x0101,
-                u8 => 1,
-                else => @compileError("Backup: Unsupported read width"),
-            };
-
-            break :blk @as(T, value) * multiplier;
-        },
         else => self.openBus(T, address),
     };
 }
@@ -352,7 +362,12 @@ fn slowRead(self: *Self, comptime T: type, unaligned_address: u32) T {
 
         // External Memory (Game Pak)
         0x08...0x0D => self.pak.read(T, address),
-        0x0E...0x0F => blk: {
+        0x0E...0x0F => self.readBackup(T, unaligned_address),
+        else => self.openBus(T, address),
+    };
+}
+
+fn readBackup(self: *const Self, comptime T: type, unaligned_address: u32) T {
     const value = self.pak.backup.read(unaligned_address);
 
     const multiplier = switch (T) {
@@ -362,10 +377,7 @@ fn slowRead(self: *Self, comptime T: type, unaligned_address: u32) T {
         else => @compileError("Backup: Unsupported read width"),
     };
 
-            break :blk @as(T, value) * multiplier;
+    return @as(T, value) * multiplier;
-        },
-        else => self.openBus(T, address),
-    };
 }
 
 pub fn write(self: *Self, comptime T: type, unaligned_address: u32, value: T) void {

src/core/emu.zig

@@ -163,3 +163,59 @@ fn sleep(timer: *Timer, wake_time: u64) ?u64 {
 fn spinLoop(timer: *Timer, wake_time: u64) void {
     while (true) if (timer.read() > wake_time) break;
 }
+
+pub const EmuThing = struct {
+    const Self = @This();
+    const Interface = @import("gdbstub").Emulator;
+
+    cpu: *Arm7tdmi,
+    scheduler: *Scheduler,
+
+    pub fn init(cpu: *Arm7tdmi, scheduler: *Scheduler) Self {
+        return .{ .cpu = cpu, .scheduler = scheduler };
+    }
+
+    pub fn interface(self: *Self) Interface {
+        return Interface.init(self);
+    }
+
+    pub fn read(self: *const Self, addr: u32) u8 {
+        return self.cpu.bus.dbgRead(u8, addr);
+    }
+
+    pub fn write(self: *Self, addr: u32, value: u8) void {
+        _ = value;
+        _ = self;
+        _ = addr;
+
+        std.debug.panic("TODO: Implement Debug Writes?", .{});
+    }
+
+    pub fn registers(self: *const Self) *[16]u32 {
+        return &self.cpu.r;
+    }
+
+    pub fn cpsr(self: *const Self) u32 {
+        return self.cpu.cpsr.raw;
+    }
+
+    pub fn step(self: *Self) void {
+        const cpu = self.cpu;
+        const sched = self.scheduler;
+
+        // TODO: How can I make it easier to keep this in lock-step with runFrame?
+        while (true) {
+            if (!cpu.stepDmaTransfer()) {
+                if (cpu.isHalted()) {
+                    // Fast-forward to next Event
+                    sched.tick = sched.queue.peek().?.tick;
+                } else {
+                    cpu.step();
+                    break; // this function won't return until we've actually stepped once
+                }
+            }
+
+            if (sched.tick >= sched.nextTimestamp()) sched.handleEvent(cpu);
+        }
+    }
+};

src/main.zig

@@ -22,6 +22,7 @@ const params = clap.parseParamsComptime(
     \\-h, --help            Display this help and exit.
     \\-s, --skip            Skip BIOS.
     \\-b, --bios <str>      Optional path to a GBA BIOS ROM.
+    \\ --gdb                Run ZBA from the context of a GDB Server
     \\<str>                 Path to the GBA GamePak ROM.
     \\
 );
@@ -87,11 +88,38 @@ pub fn main() void {
         cpu.fastBoot();
     }
 
+    if (result.args.gdb) {
+        const Server = @import("gdbstub").Server;
+        const EmuThing = @import("core/emu.zig").EmuThing;
+
+        var emu_thing = EmuThing.init(&cpu, &scheduler);
+        const emulator = emu_thing.interface();
+
+        {
+            const frames_per_second: usize = 60;
+            const emu = @import("core/emu.zig");
+
+            var i: usize = 0;
+            while (i < frames_per_second * 120) : (i += 1) {
+                emu.runFrame(&scheduler, &cpu);
+
+                std.debug.print("Frame {:0>3}/{:0>3}\r", .{ i, frames_per_second * 120 });
+            }
+        }
+
+        log.info("Ready to connect", .{});
+
+        var server = Server.init(emulator) catch |e| exitln("failed to init gdb server: {}", .{e});
+        defer server.deinit(allocator);
+
+        server.run(allocator) catch |e| exitln("gdb server crashed: {}", .{e});
+    } else {
         var gui = Gui.init(&bus.pak.title, &bus.apu, width, height) catch |e| exitln("failed to init gui: {}", .{e});
         defer gui.deinit();
 
         gui.run(&cpu, &scheduler) catch |e| exitln("failed to run gui thread: {}", .{e});
     }
+}
 
 pub fn handleArguments(allocator: Allocator, data_path: []const u8, result: *const clap.Result(clap.Help, &params, clap.parsers.default)) !FilePaths {
     const rom_path = romPath(result);