Core: Add `Volume::GetSimulatedMemorySize()`.

The data structure of the `bi2.bin` file in which the simulated memory
size is set:

> **13.2  Disk header Information**
>
> this is loaded to the Address in `0x800000f4` when a disc is initialized by the IPL
>
> |  offset  | end | size | Description           |
> | :------: | --- | :--: | --------------------- |
> | `0x0000` |     |   4  | Debug-monitor Size    |
> | `0x0004` |     |   4  | Simulated Memory Size |
> | `0x0008` |     |   4  | Argument offset       |
> | `0x000c` |     |   4  | Debug flag            |
> | `0x0010` |     |   4  | Track Location        |
> | `0x0014` |     |   4  | Track size            |
> | `0x0018` |     |   4  | Countrycode           |
> | `0x001c` |     |   4  | ?                     |

See https://hitmen.c02.at/files/yagcd/yagcd/chap13.html#sec13.2.

Most GameCube games set the value to `24 MiB`, which matches the
physical memory size. Most Wii games set the value to `0 MiB` (unset).
The debug build of _Mario Kart: Double Dash!!_ is one game that sets the
value to `48 MiB`.

4 files changed, 11 insertions, 0 deletions

diff --git a/Source/Core/DiscIO/Volume.h b/Source/Core/DiscIO/Volume.h
index 7dfabb3e97..61df7e032f 100644
--- a/Source/Core/DiscIO/Volume.h
+++ b/Source/Core/DiscIO/Volume.h
@@ -126,6 +126,7 @@ public:
   }
   virtual Region GetRegion() const = 0;
   virtual Country GetCountry(const Partition& partition = PARTITION_NONE) const = 0;
+  virtual u32 GetSimulatedMemorySize() const = 0;
   virtual BlobType GetBlobType() const = 0;
   // Size of virtual disc (may be inaccurate depending on the blob type)
   virtual u64 GetDataSize() const = 0;
diff --git a/Source/Core/DiscIO/VolumeDisc.cpp b/Source/Core/DiscIO/VolumeDisc.cpp
index e842540376..4dd16c4cef 100644
--- a/Source/Core/DiscIO/VolumeDisc.cpp
+++ b/Source/Core/DiscIO/VolumeDisc.cpp
@@ -179,6 +179,14 @@ std::optional<u8> VolumeDisc::GetDiscNumber(const Partition& partition) const
   return ReadSwapped<u8>(6, partition);
 }
 
+u32 VolumeDisc::GetSimulatedMemorySize() const
+{
+  // Simulated memory size is a 32-bit unsigned integer in the `bi2.bin` file at offset `0x04`.
+  // Apploader reads this and "simulates" a smaller memory size. Originally useful for simulating
+  // 24 MiB of memory on devkits with 48 MiB.
+  return ReadSwapped<u32>(BI2_ADDRESS + 0x04, GetGamePartition()).value_or(0);
+}
+
 bool VolumeDisc::IsNKit() const
 {
   constexpr u32 NKIT_MAGIC = 0x4E4B4954;  // "NKIT"
diff --git a/Source/Core/DiscIO/VolumeDisc.h b/Source/Core/DiscIO/VolumeDisc.h
index f630522e68..1b3774a87d 100644
--- a/Source/Core/DiscIO/VolumeDisc.h
+++ b/Source/Core/DiscIO/VolumeDisc.h
@@ -47,6 +47,7 @@ public:
   std::string GetInternalName(const Partition& partition = PARTITION_NONE) const override;
   std::string GetApploaderDate(const Partition& partition) const override;
   std::optional<u8> GetDiscNumber(const Partition& partition = PARTITION_NONE) const override;
+  u32 GetSimulatedMemorySize() const override;
   bool IsNKit() const override;
 
 protected:
diff --git a/Source/Core/DiscIO/VolumeWad.h b/Source/Core/DiscIO/VolumeWad.h
index 2c383c0254..88a0847f80 100644
--- a/Source/Core/DiscIO/VolumeWad.h
+++ b/Source/Core/DiscIO/VolumeWad.h
@@ -61,6 +61,7 @@ public:
   bool IsNKit() const override;
   Region GetRegion() const override;
   Country GetCountry(const Partition& partition = PARTITION_NONE) const override;
+  u32 GetSimulatedMemorySize() const override { return 0; }
 
   BlobType GetBlobType() const override;
   u64 GetDataSize() const override;