Add Save States to Your Emulator

Emulator Save States

Save states serialize entire machine RAM, registers, and peripheral state so players jump backward instantly. Unlike battery saves (just game SRAM), snapshots freeze the full hardware at one instant.

What to Serialize

Minimum payload:

CPU registers, flags, PC, SP
All RAM (main memory, VRAM, OAM)
PPU/APU internal latches and timers
Cartridge mapper state (bank indices, RAM enable)
Emulator metadata: version, CRC of ROM

Load must restore bit-identical state or games desync. Version your snapshot format for when you add new chips.

For example, saving mid-jump in Super Mario Bros. requires capturing PPU scroll latch and OAM positions, not just CPU registers.

\`\`\`c fwrite(cpu, sizeof(cpu), 1, fp); fwrite(ram, ram_size, 1, fp); fwrite(&ppu, sizeof(ppu), 1, fp); \`\`\`

Design Notes

Include a hash of the loaded ROM in snapshots so loading state against the wrong game fails fast. Silent mismatches produce ghosts that are miserable to debug.

For This Exercise

This exercise asks you to design save/load for your emulator's CPU and memory subsystems. You will implement serialization that restores identical execution state across save and load cycles.

You will use the same mental model here when reading production interpreter source later in the track. Sketch one concrete input on paper, predict the outcome, then confirm with code. That discipline catches logic errors early and makes debugging far faster when you extend the implementation in follow-on tasks.

Implement save state serialization for a simple emulator in C.

Requirements:

Define a SaveState struct containing CPU state, memory, timers, and display state
Implement save_state_to_file() that writes the SaveState struct to a binary file
Implement load_state_from_file() that reads the file and validates a magic header
Implement dump_state_text() that prints a human-readable summary of all state fields
Implement restore_state() that copies a SaveState back into the running emulator
Include a version field and ROM checksum in the save state header
Write a test program that creates an emulator, runs a few cycles, saves state, modifies state, then restores and verifies
Print before/after comparisons showing that restoration was successful

Emulator Save States

Save states serialize entire machine RAM, registers, and peripheral state so players jump backward instantly. Unlike battery saves (just game SRAM), snapshots freeze the full hardware at one instant.

What to Serialize

Minimum payload:

CPU registers, flags, PC, SP
All RAM (main memory, VRAM, OAM)
PPU/APU internal latches and timers
Cartridge mapper state (bank indices, RAM enable)
Emulator metadata: version, CRC of ROM

Load must restore bit-identical state or games desync. Version your snapshot format for when you add new chips.

For example, saving mid-jump in Super Mario Bros. requires capturing PPU scroll latch and OAM positions, not just CPU registers.

\`\`\`c fwrite(cpu, sizeof(cpu), 1, fp); fwrite(ram, ram_size, 1, fp); fwrite(&ppu, sizeof(ppu), 1, fp); \`\`\`

Design Notes

Include a hash of the loaded ROM in snapshots so loading state against the wrong game fails fast. Silent mismatches produce ghosts that are miserable to debug.

For This Exercise

This exercise asks you to design save/load for your emulator's CPU and memory subsystems. You will implement serialization that restores identical execution state across save and load cycles.

Implement save state serialization for a simple emulator in C.

Requirements:

Define a SaveState struct containing CPU state, memory, timers, and display state
Implement save_state_to_file() that writes the SaveState struct to a binary file
Implement load_state_from_file() that reads the file and validates a magic header
Implement dump_state_text() that prints a human-readable summary of all state fields
Implement restore_state() that copies a SaveState back into the running emulator
Include a version field and ROM checksum in the save state header
Write a test program that creates an emulator, runs a few cycles, saves state, modifies state, then restores and verifies
Print before/after comparisons showing that restoration was successful

Topic Overview

Emulator Save States

What to Serialize

Design Notes

For This Exercise

Implementation Task

Progressive Hints

Resources

Open Source

Add Save States to Your Emulator

Topic Overview

Emulator Save States

What to Serialize

Design Notes

For This Exercise

Implementation Task

Progressive Hints

Resources

Open Source