sram.h File Reference

SNES SRAM (Save RAM) Functions. More...

#include <snes/types.h>

Go to the source code of this file.

Macros
#define	SRAM_SIZE_16KB   0x04
	16KB SRAM (128Kbit)
#define	SRAM_SIZE_2KB   0x01
	2KB SRAM (16Kbit)
#define	SRAM_SIZE_32KB   0x05
	32KB SRAM (256Kbit)
#define	SRAM_SIZE_4KB   0x02
	4KB SRAM (32Kbit)
#define	SRAM_SIZE_8KB   0x03
	8KB SRAM (64Kbit) - Most common
#define	SRAM_SIZE_NONE   0x00
	No SRAM (0KB)

Functions
u8	sramChecksum (u8 *data, u16 size)
	Calculate simple checksum.
void	sramClear (u16 size)
	Clear SRAM to zero.
void	sramLoad (u8 *data, u16 size)
	Load data from SRAM.
void	sramLoadOffset (u8 *data, u16 size, u16 offset)
	Load data from SRAM at offset.
void	sramSave (u8 *data, u16 size)
	Save data to SRAM.
void	sramSaveOffset (u8 *data, u16 size, u16 offset)
	Save data to SRAM at offset.

Detailed Description

SNES SRAM (Save RAM) Functions.

Functions for saving and loading game data to battery-backed SRAM.

SRAM Overview

SRAM (Static RAM) on SNES cartridges is battery-backed RAM that persists when the console is powered off. It's used for save games.

Memory Layout (LoROM)

SRAM is mapped at bank $70, addresses $0000-$7FFF (32KB max). Most games use 2KB-8KB of SRAM.

Usage Example

// Define your save data structure
typedef struct {
    u8  magic[4];      // "SAVE" to detect valid save
    u16 score;
    u8  level;
    u8  lives;
    u8  checksum;
} SaveData;
 
SaveData save;
 
// Load save data at game start
sramLoad((u8*)&save, sizeof(SaveData));
if (save.magic[0] != 'S' || save.magic[1] != 'A') {
    // No valid save, initialize defaults
    save.score = 0;
    save.level = 1;
    save.lives = 3;
}
 
// Save when needed (e.g., at checkpoint)
save.magic[0] = 'S'; save.magic[1] = 'A';
save.magic[2] = 'V'; save.magic[3] = 'E';
sramSave((u8*)&save, sizeof(SaveData));

ROM Header Requirement

To enable SRAM, your ROM header (hdr.asm) must set:

• CARTRIDGETYPE $02 (ROM + SRAM)
• SRAMSIZE $03 (8KB) or appropriate size

Author

OpenSNES Team

Copyright

MIT License

Macro Definition Documentation

SRAM_SIZE_16KB

#define SRAM_SIZE_16KB   0x04

16KB SRAM (128Kbit)

SRAM_SIZE_2KB

#define SRAM_SIZE_2KB   0x01

2KB SRAM (16Kbit)

SRAM_SIZE_32KB

#define SRAM_SIZE_32KB   0x05

32KB SRAM (256Kbit)

SRAM_SIZE_4KB

#define SRAM_SIZE_4KB   0x02

4KB SRAM (32Kbit)

SRAM_SIZE_8KB

#define SRAM_SIZE_8KB   0x03

8KB SRAM (64Kbit) - Most common

SRAM_SIZE_NONE

#define SRAM_SIZE_NONE   0x00

No SRAM (0KB)

Function Documentation

sramChecksum()

u8 sramChecksum	(	u8 *	data,
		u16	size
	)

Calculate simple checksum.

Calculates a simple 8-bit checksum of data. Use this to verify save data integrity.

Parameters

data	Pointer to data
size	Number of bytes

Returns

8-bit checksum (XOR of all bytes)

// Before saving:
save.checksum = 0;
save.checksum = sramChecksum((u8*)&save, sizeof(save));
sramSave((u8*)&save, sizeof(save));
 
// After loading:
u8 storedChecksum = save.checksum;
save.checksum = 0;
if (sramChecksum((u8*)&save, sizeof(save)) != storedChecksum) {
    // Save data corrupted!
}

sramClear()

void sramClear

(

u16

size

)

Clear SRAM to zero.

Erases all SRAM contents by filling with zeros. Useful for "delete save" functionality.

Parameters

size	Number of bytes to clear (usually your save data size)

// Clear first 256 bytes (one save slot)
sramClear(256);

sramLoad()

void sramLoad	(	u8 *	data,
		u16	size
	)

Load data from SRAM.

Copies data from battery-backed SRAM to Work RAM.

Parameters

data	Pointer to destination buffer in Work RAM
size	Number of bytes to load (max 32KB)

u8 saveData[64];
sramLoad(saveData, 64);

Note

If no valid save exists, SRAM contents are undefined

sramLoadOffset()

void sramLoadOffset	(	u8 *	data,
		u16	size,
		u16	offset
	)

Load data from SRAM at offset.

Like sramLoad() but reads from a specific offset within SRAM. Useful for multiple save slots.

Parameters

data	Pointer to destination buffer in Work RAM
size	Number of bytes to load
offset	Starting offset in SRAM (0-32767)

// Load slot 2 (each slot is 256 bytes)
sramLoadOffset(saveData, 256, 512);

sramSave()

void sramSave	(	u8 *	data,
		u16	size
	)

Save data to SRAM.

Copies data from Work RAM to battery-backed SRAM. The data will persist when the console is powered off.

Parameters

data	Pointer to data in Work RAM to save
size	Number of bytes to save (max 32KB)

u8 saveData[64] = { ... };
sramSave(saveData, 64);

Note

Uses bank $70 (LoROM SRAM), starting at address $0000

Warning

Ensure ROM header has SRAM enabled!

sramSaveOffset()

void sramSaveOffset	(	u8 *	data,
		u16	size,
		u16	offset
	)

Save data to SRAM at offset.

Like sramSave() but writes to a specific offset within SRAM. Useful for multiple save slots.

Parameters

data	Pointer to data in Work RAM to save
size	Number of bytes to save
offset	Starting offset in SRAM (0-32767)

// Save slot 2 (each slot is 256 bytes)
sramSaveOffset(saveData, 256, 512);