From 74b6fa2ecc96656453264f1d51a4aea5515e83f1 Mon Sep 17 00:00:00 2001
From: Rekai Musuka <rekai@musuka.dev>
Date: Fri, 21 Oct 2022 05:12:31 -0300
Subject: [PATCH] chore: mirror VRAM

---
 src/Bus.zig | 10 +++++-----
 src/ppu.zig | 23 ++++++++++++++++-------
 2 files changed, 21 insertions(+), 12 deletions(-)

diff --git a/src/Bus.zig b/src/Bus.zig
index 91162ea..5c5a493 100644
--- a/src/Bus.zig
+++ b/src/Bus.zig
@@ -61,7 +61,7 @@ pub fn read32(self: *const Self, addr: u32) u32 {
 
         // Internal Display Memory
         0x0500_0000...0x05FF_FFFF => self.ppu.palette.get32(addr & 0x3FF),
-        0x0600_0000...0x0601_7FFF => self.ppu.vram.read(u32, addr - 0x0600_0000),
+        0x0600_0000...0x06FF_FFFF => self.ppu.vram.read(u32, addr),
         0x0700_0000...0x07FF_FFFF => self.ppu.oam.get32(addr & 0x3FF),
 
         // External Memory (Game Pak)
@@ -84,7 +84,7 @@ pub fn write32(self: *Self, addr: u32, word: u32) void {
 
         // Internal Display Memory
         0x0500_0000...0x05FF_FFFF => self.ppu.palette.set32(addr & 0x3FF, word),
-        0x0600_0000...0x0601_7FFF => self.ppu.vram.write(u32, addr - 0x0600_0000, word),
+        0x0600_0000...0x06FF_FFFF => self.ppu.vram.write(u32, addr, word),
         0x0700_0000...0x07FF_FFFF => self.ppu.oam.set32(addr & 0x3FF, word),
 
         else => undWrite("Tried to write 0x{X:0>8} to 0x{X:0>8}", .{ word, addr }),
@@ -101,7 +101,7 @@ pub fn read16(self: *const Self, addr: u32) u16 {
 
         // Internal Display Memory
         0x0500_0000...0x05FF_FFFF => self.ppu.palette.get16(addr & 0x3FF),
-        0x0600_0000...0x0601_7FFF => self.ppu.vram.read(u16, addr - 0x0600_0000),
+        0x0600_0000...0x06FF_FFFF => self.ppu.vram.read(u16, addr),
         0x0700_0000...0x07FF_FFFF => self.ppu.oam.get16(addr & 0x3FF),
 
         // External Memory (Game Pak)
@@ -123,7 +123,7 @@ pub fn write16(self: *Self, addr: u32, halfword: u16) void {
 
         // Internal Display Memory
         0x0500_0000...0x05FF_FFFF => self.ppu.palette.set16(addr & 0x3FF, halfword),
-        0x0600_0000...0x0601_7FFF => self.ppu.vram.write(u16, addr - 0x0600_0000, halfword),
+        0x0600_0000...0x06FF_FFFF => self.ppu.vram.write(u16, addr, halfword),
         0x0700_0000...0x07FF_FFFF => self.ppu.oam.set16(addr & 0x3FF, halfword),
         0x0800_00C4, 0x0800_00C6, 0x0800_00C8 => log.warn("Tried to write 0x{X:0>4} to GPIO", .{halfword}),
 
@@ -141,7 +141,7 @@ pub fn read8(self: *const Self, addr: u32) u8 {
 
         // Internal Display Memory
         0x0500_0000...0x05FF_FFFF => self.ppu.palette.get8(addr & 0x3FF),
-        0x0600_0000...0x0601_7FFF => self.ppu.vram.read(u8, addr - 0x0600_0000),
+        0x0600_0000...0x06FF_FFFF => self.ppu.vram.read(u8, addr),
         0x0700_0000...0x07FF_FFFF => self.ppu.oam.get8(addr & 0x3FF),
 
         // External Memory (Game Pak)
diff --git a/src/ppu.zig b/src/ppu.zig
index 5fb37d3..64f8efc 100644
--- a/src/ppu.zig
+++ b/src/ppu.zig
@@ -479,16 +479,20 @@ const Vram = struct {
         self.alloc.free(self.buf);
     }
 
-    pub fn read(self: *const Self, comptime T: type, addr: usize) T {
-        switch (T) {
-            u32 => return (@as(T, self.buf[addr + 3]) << 24) | (@as(T, self.buf[addr + 2]) << 16) | (@as(T, self.buf[addr + 1]) << 8) | (@as(T, self.buf[addr])),
-            u16 => return (@as(T, self.buf[addr + 1]) << 8) | @as(T, self.buf[addr]),
-            u8 => return self.buf[addr],
+    pub fn read(self: *const Self, comptime T: type, address: usize) T {
+        const addr = Self.mirror(address);
+
+        return switch (T) {
+            u32 => (@as(T, self.buf[addr + 3]) << 24) | (@as(T, self.buf[addr + 2]) << 16) | (@as(T, self.buf[addr + 1]) << 8) | (@as(T, self.buf[addr])),
+            u16 => (@as(T, self.buf[addr + 1]) << 8) | @as(T, self.buf[addr]),
+            u8 => self.buf[addr],
             else => @compileError("VRAM: Unsupported read width"),
-        }
+        };
     }
 
-    pub fn write(self: *Self, comptime T: type, addr: usize, value: T) void {
+    pub fn write(self: *Self, comptime T: type, address: usize, value: T) void {
+        const addr = Self.mirror(address);
+
         switch (T) {
             u32 => {
                 self.buf[addr + 3] = @truncate(u8, value >> 24);
@@ -503,6 +507,11 @@ const Vram = struct {
             else => @compileError("VRAM: Unsupported write width"),
         }
     }
+
+    fn mirror(address: usize) usize {
+        const addr = address & 0x1FFFF; // repeated in steps of 128KiB
+        return if (addr >= 0x18000) addr & 0x7FFF else addr; // 64K + 32K + 32K (abcc)
+    }
 };
 
 const Oam = struct {