Save Game -- SRAM Persistence

Screenshot

What This Example Shows

How to save and load game data using the SNES cartridge SRAM (Static RAM). The SRAM is battery-backed – data persists even when the console is powered off. This is how classic SNES games like Zelda and Final Fantasy save progress.

Prerequisites

Read text/hello_world first (console setup, text display).

Controls

Button	Action
A	Write test data to Slot 1
B	Read and display Slot 1
X	Write test data to Slot 2
Y	Read and display Slot 2

Build & Run

cd $OPENSNES_HOME
make -C examples/memory/save_game

Then open save_game.sfc in your emulator (Mesen2 recommended).

How It Works

1. Define a save structure

typedef struct {
    s16 posX, posY;
    u16 camX, camY;
} SaveState;

The structure is 8 bytes. In a real game, this would hold player position, inventory, level progress, etc.

2. Save to SRAM

sramSaveOffset((u8 *)&vts, SAVE_SIZE, SAVE_SIZE * SLOT1);

sramSaveOffset() copies SAVE_SIZE bytes from RAM to SRAM at the given offset. Multiple save slots use different offsets (Slot 0 at offset 0, Slot 1 at offset 8, etc.).

3. Load from SRAM

sramLoadOffset((u8 *)&vtl, SAVE_SIZE, SAVE_SIZE * SLOT1);

sramLoadOffset() copies bytes back from SRAM into a RAM structure. The loaded values are then displayed as hex on screen using textPrintHex().

4. Enable SRAM in the Makefile

USE_SRAM = 1
SRAM_SIZE = 3

USE_SRAM sets the ROM header flag telling the emulator (or real hardware) that the cartridge has battery-backed SRAM. SRAM_SIZE = 3 means 8 KB (2^3 = 8 KB).

5. Text display

The example uses the text system on BG1 in Mode 0 (2bpp):

textInit(TEXT_DEFAULT_TILEMAP_ADDR, TEXT_DEFAULT_FONT_TILE, TEXT_DEFAULT_PALETTE);
textLoadFont(0x0000);
bgSetGfxPtr(0, 0x0000);
bgSetMapPtr(0, 0x3800, BG_MAP_32x32);

After each save/load operation, textPrintAt() updates the on-screen feedback and textFlush() sends the updated text buffer to VRAM.

SNES Concepts

SRAM Address Space

In LoROM mode, SRAM is mapped at $70:0000-$71:FFFF (up to 64 KB, though most cartridges have 8 KB). In HiROM mode, it appears at $20:6000-$3F:7FFF. The sramSaveOffset() and sramLoadOffset() functions handle the addressing – you just provide a byte offset.

Save Slots

Save slots are simply offsets into SRAM. With 8 KB of SRAM and 8-byte saves, you could theoretically have 1024 slots. Real games use 3-4 slots with larger structures (100-500 bytes for RPG save states).

Checksum Validation

This example does not validate saves. A real game should store a checksum alongside the save data to detect corrupted SRAM (weak battery, first boot with uninitialized RAM, etc.). A simple XOR or CRC-8 of the save bytes suffices.

Battery Persistence

On real SNES hardware, a CR2032 lithium battery in the cartridge powers the SRAM when the console is off. Batteries last 15-25 years. In emulators, the SRAM is saved to a .srm file on disk.

Project Structure

File	Purpose
main.c	Save/load logic, text display, input handling
Makefile	USE_SRAM := 1, LIB_MODULES := console dma text background sprite input

Going Further

• Add a checksum: Before saving, compute a checksum of the data bytes and store it alongside the save. On load, verify the checksum and display "CORRUPT" if it does not match.
• Larger save structure: Add fields for health, inventory, level number, and play time. This is how real RPG save systems work.
• Explore related examples:
  • memory/hirom_demo – Understand LoROM vs HiROM memory mapping
  • games/breakout – See game state management in a complete game