From caaa60d1a87d030ac6d8c268b69be762d45b5b23 Mon Sep 17 00:00:00 2001
From: Rekai Musuka <rekai@musuka.dev>
Date: Tue, 27 Dec 2022 05:41:19 -0600
Subject: [PATCH] fix: rotate unaligned reads on BIOS open-bus

---
 src/core/Bus.zig      |  8 ++++----
 src/core/bus/Bios.zig | 31 +++++++++++++++++++++++++------
 2 files changed, 29 insertions(+), 10 deletions(-)

diff --git a/src/core/Bus.zig b/src/core/Bus.zig
index 2613a80..0c06f83 100644
--- a/src/core/Bus.zig
+++ b/src/core/Bus.zig
@@ -227,7 +227,7 @@ fn dbgSlowRead(self: *const Self, comptime T: type, unaligned_address: u32) T {
         // General Internal Memory
         0x00 => blk: {
             if (address < Bios.size)
-                break :blk self.bios.dbgRead(T, self.cpu.r[15], address);
+                break :blk self.bios.dbgRead(T, self.cpu.r[15], unaligned_address);
 
             break :blk self.openBus(T, address);
         },
@@ -347,7 +347,7 @@ fn slowRead(self: *Self, comptime T: type, unaligned_address: u32) T {
         // General Internal Memory
         0x00 => blk: {
             if (address < Bios.size)
-                break :blk self.bios.read(T, self.cpu.r[15], address);
+                break :blk self.bios.read(T, self.cpu.r[15], unaligned_address);
 
             break :blk self.openBus(T, address);
         },
@@ -437,11 +437,11 @@ inline fn rotateBy(comptime T: type, address: u32) u32 {
         u32 => address & 3,
         u16 => address & 1,
         u8 => 0,
-        else => @compileError("Backup: Unsupported write width"),
+        else => @compileError("Unsupported write width"),
     };
 }
 
-inline fn forceAlign(comptime T: type, address: u32) u32 {
+pub inline fn forceAlign(comptime T: type, address: u32) u32 {
     return switch (T) {
         u32 => address & ~@as(u32, 3),
         u16 => address & ~@as(u32, 1),
diff --git a/src/core/bus/Bios.zig b/src/core/bus/Bios.zig
index 89677a0..80dae9f 100644
--- a/src/core/bus/Bios.zig
+++ b/src/core/bus/Bios.zig
@@ -3,6 +3,9 @@ const std = @import("std");
 const Allocator = std.mem.Allocator;
 const log = std.log.scoped(.Bios);
 
+const rotr = @import("../../util.zig").rotr;
+const forceAlign = @import("../Bus.zig").forceAlign;
+
 /// Size of the BIOS in bytes
 pub const size = 0x4000;
 const Self = @This();
@@ -12,19 +15,35 @@ allocator: Allocator,
 
 addr_latch: u32,
 
-pub fn read(self: *Self, comptime T: type, r15: u32, addr: u32) T {
+// https://github.com/ITotalJustice/notorious_beeg/issues/106
+pub fn read(self: *Self, comptime T: type, r15: u32, address: u32) T {
     if (r15 < Self.size) {
+        const addr = forceAlign(T, address);
+
         self.addr_latch = addr;
         return self._read(T, addr);
     }
 
-    log.debug("Rejected read since r15=0x{X:0>8}", .{r15});
-    return @truncate(T, self._read(T, self.addr_latch));
+    log.warn("Open Bus! Read from 0x{X:0>8}, but PC was 0x{X:0>8}", .{ address, r15 });
+    const value = self._read(u32, self.addr_latch);
+
+    return @truncate(T, rotr(u32, value, 8 * rotateBy(T, address)));
 }
 
-pub fn dbgRead(self: *const Self, comptime T: type, r15: u32, addr: u32) T {
-    if (r15 < Self.size) return self._read(T, addr);
-    return @truncate(T, self._read(T, self.addr_latch + 8));
+fn rotateBy(comptime T: type, address: u32) u32 {
+    return switch (T) {
+        u8 => address & 3,
+        u16 => address & 2,
+        u32 => 0,
+        else => @compileError("bios: unsupported read width"),
+    };
+}
+
+pub fn dbgRead(self: *const Self, comptime T: type, r15: u32, address: u32) T {
+    if (r15 < Self.size) return self._read(T, forceAlign(T, address));
+
+    const value = self._read(u32, self.addr_latch);
+    return @truncate(T, rotr(u32, value, 8 * rotateBy(T, address)));
 }
 
 /// Read without the GBA safety checks