Simulate ARM9 RAM starvation to fix DSi PictoChat

src/ARM.cpp

@@ -1154,13 +1154,16 @@ void ARMv4::DataRead32S(u32 addr, u32* val)
 
 void ARMv4::DataWrite8(u32 addr, u8 val)
 {
+    // decrease ARM7 write count if the write targets the main RAM, which is starving bus
+    if (NDS.ARM7BytesToWrite && (addr >> 24) == 0x02) NDS.DecreaseARM7BytesToWrite(1);
     BusWrite8(addr, val);
     DataRegion = addr;
     DataCycles = NDS.ARM7MemTimings[addr >> 15][0];
 }
 
 void ARMv4::DataWrite16(u32 addr, u16 val)
 {
+    if (NDS.ARM7BytesToWrite && (addr >> 24) == 0x02) NDS.DecreaseARM7BytesToWrite(2);
     addr &= ~1;
 
     BusWrite16(addr, val);
@@ -1170,6 +1173,7 @@ void ARMv4::DataWrite16(u32 addr, u16 val)
 
 void ARMv4::DataWrite32(u32 addr, u32 val)
 {
+    if (NDS.ARM7BytesToWrite && (addr >> 24) == 0x02) NDS.DecreaseARM7BytesToWrite(4);
     addr &= ~3;
 
     BusWrite32(addr, val);
@@ -1179,6 +1183,7 @@ void ARMv4::DataWrite32(u32 addr, u32 val)
 
 void ARMv4::DataWrite32S(u32 addr, u32 val)
 {
+    if (NDS.ARM7BytesToWrite && (addr >> 24) == 0x02) NDS.DecreaseARM7BytesToWrite(4);
     addr &= ~3;
 
     BusWrite32(addr, val);

src/ARMInterpreter.cpp

@@ -266,6 +266,10 @@ void A_MRC(ARM* cpu)
 
 void A_SVC(ARM* cpu)
 {
+    // update ARM9 starvation status on a BIOS call
+    if (cpu->Num == 1)
+        cpu->NDS.UpdateARM9Starve((cpu->CurInstr >> 16) & 0xFF, cpu->R[1], cpu->R[2]);
+
     u32 oldcpsr = cpu->CPSR;
     cpu->CPSR &= ~0xBF;
     cpu->CPSR |= 0x93;
@@ -278,6 +282,9 @@ void A_SVC(ARM* cpu)
 
 void T_SVC(ARM* cpu)
 {
+    if (cpu->Num == 1)
+        cpu->NDS.UpdateARM9Starve(cpu->CurInstr & 0xFF, cpu->R[1], cpu->R[2]);
+
     u32 oldcpsr = cpu->CPSR;
     cpu->CPSR &= ~0xBF;
     cpu->CPSR |= 0x93;

src/NDS.cpp

@@ -508,6 +508,8 @@ void NDS::Reset()
     IPCFIFO9.Clear();
     IPCFIFO7.Clear();
 
+    ARM7BytesToWrite = 0;
+
     DivCnt = 0;
     SqrtCnt = 0;
 
@@ -683,6 +685,7 @@ bool NDS::DoSavestate(Savestate* file)
     file->Var16(&SqrtCnt);
 
     file->Var32(&CPUStop);
+    file->Var32(&ARM7BytesToWrite);
 
     for (int i = 0; i < 8; i++)
     {
@@ -1002,6 +1005,11 @@ u32 NDS::RunFrame()
                         dsi.RunNDMAs(0);
                     }
                 }
+                else if (CPUStop & CPUStop_ARM9BusStarve)
+                {
+                    // starved of main RAM by the ARM7, see UpdateARM9Starve()
+                    ARM9Timestamp = ARM9Target;
+                }
                 else
                 {
                     ARM9.Execute<cpuMode>();
@@ -1092,6 +1100,46 @@ u32 NDS::RunFrame()
     }
 }
 
+void NDS::UpdateARM9Starve(u32 swi, u32 dst, u32 cnt)
+{
+    // Update the number of bytes that ARM7 will transfer to main RAM.
+    // DecreaseARM7BytesToWrite() will consumes it as the ARM7 writes;
+    // at zero the ARM9 is released.
+
+    // accept only CpuSet and CpuFastSet call
+    // see: https://problemkaputt.de/gbatek-bios-memory-copy.htm
+    if (swi != 0x0B && swi != 0x0C) return;
+
+    // destination must be in main RAM
+    if ((dst >> 24) != 0x02) return;
+
+    // bus starvation occurs only when ARM7 has higher priority
+    if (!(ExMemCnt[0] & (1<<15))) return;
+
+    u32 units = cnt & 0x1FFFFF;
+    u32 bytes = (swi == 0x0C || (cnt & (1<<26))) ? (units << 2) : (units << 1);
+
+    // The original intention of simulating bus starvation is for some problematic games,
+    // like DSi PictoChat, whose ARM7 fills a wide range of main RAM. Transfers below 4KB
+    // are too short for the starvation to matter and are ignored.
+    if (bytes < 0x1000) return;
+
+    ARM7BytesToWrite = bytes;
+    CPUStop |= CPUStop_ARM9BusStarve;
+}
+
+void NDS::DecreaseARM7BytesToWrite(u32 bytes)
+{
+    if (bytes < ARM7BytesToWrite)
+    {
+        ARM7BytesToWrite -= bytes;
+        return;
+    }
+
+    ARM7BytesToWrite = 0;
+    CPUStop &= ~CPUStop_ARM9BusStarve;
+}
+
 void NDS::Reschedule(u64 target)
 {
     if (CurCPU == 0)

src/NDS.h

@@ -171,6 +171,8 @@ enum
     CPUStop_NDMA9_3 = (1<<7),
     CPUStop_DMA9 = 0xFFF,
 
+    CPUStop_ARM9BusStarve = (1<<14),
+
     CPUStop_DMA7_0 = (1<<16),
     CPUStop_DMA7_1 = (1<<17),
     CPUStop_DMA7_2 = (1<<18),
@@ -289,6 +291,7 @@ class NDS
     u16 PowerControl9;
 
     u16 ExMemCnt[2];
+    u32 ARM7BytesToWrite;
 
 protected:
     // These BIOS arrays should be declared *before* the component objects (JIT, SPI, etc.)
@@ -443,6 +446,11 @@ class NDS
     bool HaltInterrupted(u32 cpu) const;
     void StopCPU(u32 cpu, u32 mask);
     void ResumeCPU(u32 cpu, u32 mask);
+
+    // ARM9 starvation while the ARM7 has main RAM priority
+    void UpdateARM9Starve(u32 swi, u32 dst, u32 cnt);
+    void DecreaseARM7BytesToWrite(u32 bytes);
+
     void GXFIFOStall();
     void GXFIFOUnstall();
 

src/Savestate.h

@@ -24,7 +24,7 @@
 #include <stdio.h>
 #include "types.h"
 
-#define SAVESTATE_MAJOR 14
+#define SAVESTATE_MAJOR 15
 #define SAVESTATE_MINOR 0
 
 // bitmask for the savestate config word