feat(bus): emu is now able to read from user-provided BIOS

src/bus.zig

@@ -1,21 +1,25 @@
 const std = @import("std");
 
-const GamePak = @import("pak.zig").GamePak;
+const Bios = @import("bus/bios.zig").Bios;
+const GamePak = @import("bus/pak.zig").GamePak;
 const Allocator = std.mem.Allocator;
 
 pub const Bus = struct {
     pak: GamePak,
+    bios: Bios,
 
     pub fn withPak(alloc: Allocator, path: []const u8) !@This() {
         return @This(){
             .pak = try GamePak.fromPath(alloc, path),
+            // TODO: don't hardcode this + bundle open-sorce Boot ROM
+            .bios = try Bios.fromPath(alloc, "./bin/gba_bios.bin"),
         };
     }
 
     pub fn read32(self: *const @This(), addr: u32) u32 {
         return switch (addr) {
             // General Internal Memory
-            0x0000_0000...0x0000_3FFF => std.debug.panic("read32 from 0x{X:} in BIOS", .{addr}),
+            0x0000_0000...0x0000_3FFF => self.bios.get32(@as(usize, addr)),
             0x0200_0000...0x0203FFFF => std.debug.panic("read32 from 0x{X:} in IWRAM", .{addr}),
             0x0300_0000...0x0300_7FFF => std.debug.panic("read32 from 0x{X:} in EWRAM", .{addr}),
             0x0400_0000...0x0400_03FE => std.debug.panic("read32 from 0x{X:} in I/O", .{addr}),
@@ -58,7 +62,7 @@ pub const Bus = struct {
     pub fn read16(self: *const @This(), addr: u32) u16 {
         return switch (addr) {
             // General Internal Memory
-            0x0000_0000...0x0000_3FFF => std.debug.panic("read16 from 0x{X:} in BIOS", .{addr}),
+            0x0000_0000...0x0000_3FFF => self.bios.get16(@as(usize, addr)),
             0x0200_0000...0x0203FFFF => std.debug.panic("read16 from 0x{X:} in IWRAM", .{addr}),
             0x0300_0000...0x0300_7FFF => std.debug.panic("read16 from 0x{X:} in EWRAM", .{addr}),
             0x0400_0000...0x0400_03FE => std.debug.panic("read16 from 0x{X:} in I/O", .{addr}),
@@ -101,7 +105,7 @@ pub const Bus = struct {
     pub fn read8(self: *const @This(), addr: u32) u8 {
         return switch (addr) {
             // General Internal Memory
-            0x0000_0000...0x0000_3FFF => std.debug.panic("read8 from 0x{X:} in BIOS", .{addr}),
+            0x0000_0000...0x0000_3FFF => self.bios.get8(@as(usize, addr)),
             0x0200_0000...0x0203FFFF => std.debug.panic("read8 from 0x{X:} in IWRAM", .{addr}),
             0x0300_0000...0x0300_7FFF => std.debug.panic("read8 from 0x{X:} in EWRAM", .{addr}),
             0x0400_0000...0x0400_03FE => std.debug.panic("read8 from 0x{X:} in I/O", .{addr}),

src/bus/bios.zig

@@ -0,0 +1,30 @@
+const std = @import("std");
+
+const Allocator = std.mem.Allocator;
+
+pub const Bios = struct {
+    buf: []u8,
+
+    pub fn fromPath(alloc: Allocator, path: []const u8) !@This() {
+        const file = try std.fs.cwd().openFile(path, .{ .read = true });
+        defer file.close();
+
+        const len = try file.getEndPos();
+
+        return @This(){
+            .buf = try file.readToEndAlloc(alloc, len),
+        };
+    }
+
+    pub inline fn get32(self: *const @This(), idx: usize) u32 {
+        return (@as(u32, self.buf[idx + 3]) << 24) | (@as(u32, self.buf[idx + 2]) << 16) | (@as(u32, self.buf[idx + 1]) << 8) | (@as(u32, self.buf[idx]));
+    }
+
+    pub inline fn get16(self: *const @This(), idx: usize) u16 {
+        return (@as(u16, self.buf[idx + 1]) << 8) | @as(u16, self.buf[idx]);
+    }
+
+    pub inline fn get8(self: *const @This(), idx: usize) u8 {
+        return self.buf[idx];
+    }
+};

src/cpu.zig

@@ -26,6 +26,7 @@ pub const Arm7tdmi = struct {
     }
 
     pub inline fn step(self: *@This()) u64 {
+        std.debug.print("PC: 0x{X:} ", .{self.r[15]});
         const opcode = self.fetch();
         std.debug.print("opcode: 0x{X:}\n", .{opcode}); // Debug