From 102b2c946b044c0eb3d850f1ba1b5a7d83dec48e Mon Sep 17 00:00:00 2001
From: Rekai Musuka <rekai@musuka.dev>
Date: Wed, 2 Nov 2022 07:54:06 -0300
Subject: [PATCH] fix(io): respect read-only bits in DISPSTAT

Superstar Saga now renders correctly
---
 src/core/bus/io.zig | 9 +++++++++
 src/core/ppu.zig    | 6 +++---
 2 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/src/core/bus/io.zig b/src/core/bus/io.zig
index d59c4c4..364c2aa 100644
--- a/src/core/bus/io.zig
+++ b/src/core/bus/io.zig
@@ -317,14 +317,23 @@ pub const DisplayControl = extern union {
 
 /// Read / Write
 pub const DisplayStatus = extern union {
+    /// read-only
     vblank: Bit(u16, 0),
+    /// read-only
     hblank: Bit(u16, 1),
+    // read-only
     coincidence: Bit(u16, 2),
     vblank_irq: Bit(u16, 3),
     hblank_irq: Bit(u16, 4),
     vcount_irq: Bit(u16, 5),
     vcount_trigger: Bitfield(u16, 8, 8),
     raw: u16,
+
+    pub fn set(self: *DisplayStatus, value: u16) void {
+        const mask: u16 = 0x00C7; // set bits are read-only
+
+        self.raw = (self.raw & mask) | (value & ~mask);
+    }
 };
 
 /// Read Only
diff --git a/src/core/ppu.zig b/src/core/ppu.zig
index 7f35f3c..24a943f 100644
--- a/src/core/ppu.zig
+++ b/src/core/ppu.zig
@@ -94,7 +94,7 @@ pub fn write(comptime T: type, ppu: *Ppu, addr: u32, value: T) void {
         u32 => switch (byte_addr) {
             0x00 => ppu.dispcnt.raw = @truncate(u16, value),
             0x04 => {
-                ppu.dispstat.raw = @truncate(u16, value);
+                ppu.dispstat.set(@truncate(u16, value));
                 ppu.vcount.raw = @truncate(u16, value >> 16);
             },
             0x08 => ppu.setAdjCnts(0, value),
@@ -130,7 +130,7 @@ pub fn write(comptime T: type, ppu: *Ppu, addr: u32, value: T) void {
         u16 => switch (byte_addr) {
             0x00 => ppu.dispcnt.raw = value,
             0x02 => {}, // Green Swap
-            0x04 => ppu.dispstat.raw = value,
+            0x04 => ppu.dispstat.set(value),
             0x06 => {}, // VCOUNT
 
             0x08 => ppu.bg[0].cnt.raw = value,
@@ -178,7 +178,7 @@ pub fn write(comptime T: type, ppu: *Ppu, addr: u32, value: T) void {
         u8 => switch (byte_addr) {
             0x00, 0x01 => ppu.dispcnt.raw = setHalf(u16, ppu.dispcnt.raw, byte_addr, value),
             0x02, 0x03 => {}, // Green Swap
-            0x04, 0x05 => ppu.dispstat.raw = setHalf(u16, ppu.dispstat.raw, byte_addr, value),
+            0x04, 0x05 => ppu.dispstat.set(setHalf(u16, ppu.dispstat.raw, byte_addr, value)),
             0x06, 0x07 => {}, // VCOUNT
 
             // BGXCNT