From e6b0248c5010492d5d7d33bc52258c16856ad99f Mon Sep 17 00:00:00 2001
From: Nathaniel Walizer <robots.are.love@gmail.com>
Date: Thu, 20 Mar 2025 21:24:25 -0700
Subject: [PATCH] Blargg PPU test fixes

---
 src/f6502.c | 22 +++++++++++++++++-----
 src/nes.c   | 18 ++++++++++++++++--
 2 files changed, 33 insertions(+), 7 deletions(-)

diff --git a/src/f6502.c b/src/f6502.c
index f5defa1..90800a4 100644
--- a/src/f6502.c
+++ b/src/f6502.c
@@ -76,14 +76,21 @@ static inline uint8_t f6502_read(nes_Memory* mem,
         }
 
         case ppu_reg_oam_data:
-            return mem->ppu.oam[mem->ppu.oam_addr++];
+            return mem->ppu.oam[mem->ppu.oam_addr];
 
         case ppu_reg_data:
         {   addr = mem->ppu.addr & 0x3FFFU;
             mem->ppu.addr += mem->ppu.addr_inc;
-            uint8_t val = mem->ppu.data;
-            mem->ppu.data = mem->ppu.bank[addr >> 10][addr & 0x3FFU];
-            return val;
+            uint8_t data = mem->ppu.bank[addr >> 10][addr & 0x3FFU];
+            if (addr >= 0x3F00) {
+                addr &= 0x2FFFU;
+                mem->ppu.data = mem->ppu.bank[addr >> 10][addr & 0x3FFU];
+                return data;
+            } else {
+                uint8_t val = mem->ppu.data;
+                mem->ppu.data = data;
+                return val;
+            }
         }
         }
         break;
@@ -317,7 +324,12 @@ static inline int f6502_write(nes_Memory* mem,
                 break;
             }
             if (NULL != src) {
-                memcpy(mem->ppu.oam, src, NES_PPU_OAM_SIZE);
+                int top_size = NES_PPU_OAM_SIZE -
+                               mem->ppu.oam_addr;
+                memcpy(mem->ppu.oam + mem->ppu.oam_addr,
+                       src, top_size);
+                memcpy(mem->ppu.oam, src + top_size,
+                       mem->ppu.oam_addr);
             }
             // TODO: Why does this throw off MMC3?
 //            ret = 513;
diff --git a/src/nes.c b/src/nes.c
index 36ac1df..3a018c0 100644
--- a/src/nes.c
+++ b/src/nes.c
@@ -108,16 +108,24 @@ static int nes_hsync(nes* sys, void* plat) {
 
 #define mapper_hsync_dot    (300U)
 #define render_dot          (256U)
+#define vblank_clear_dot    (320U)
 
 int nes_loop(nes* sys, void* plat) {
     int status = 0;
     int dot = 0;
     bool mapper_hsync = false;
+    bool vbl_clear = false;
 
     while (0 == status) {
         // TODO: Move to inline PPU function?
         int target_dot = nes_ppu_scanline_dots;
 
+        if (    nes_ppu_frame_end_line - 1 == sys->ppu.scanline &&
+                sys->core.memory.ppu.status & ppu_Status_VBlank) {
+            target_dot = vblank_clear_dot;
+            vbl_clear = true;
+        }
+
         if (    sys->core.memory.mapper.hsync &&
                 sys->ppu.scanline > nes_ppu_prerender_line &&
                 sys->ppu.scanline < nes_ppu_postrender_line &&
@@ -139,11 +147,17 @@ int nes_loop(nes* sys, void* plat) {
             }
         }
 
-        if (target_dot > dot) {
-            int cpu_cycles = (target_dot - dot + 2) / 3;
+        int dots_to_run = target_dot - dot;
+        if (dots_to_run > 0) {
+            int cpu_cycles = (dots_to_run + 2) / 3;
             dot += 3 * f6502_step(&sys->core, cpu_cycles);
         }
 
+        if (vbl_clear && dot >= vblank_clear_dot) {
+            sys->core.memory.ppu.status &= ~ppu_Status_VBlank;
+            vbl_clear = false;
+        }
+
         if (mapper_hsync && dot >= mapper_hsync_dot) {
             nes_Memory* mem = &sys->core.memory;
             mem->mapper.hsync(&mem->mapper);