Older but still works.

To understand where ZSO stands, let’s do a quick comparison:

| Feature | ISO | CSO (zlib) | ZSO (Zstd) | CHD (LZMA) | | :--- | :--- | :--- | :--- | :--- | | Compression Ratio | None | Medium (Good) | High (Better) | Very High (Best) | | Decomp Speed | Instant | Slow (Stutters) | Fast | Medium | | PSP Real Hardware | Yes | Yes (Slow) | Via plugin only | No | | PPSSPP Support | Yes | Yes | Native | Yes (via r/w) | | Best Use Case | SSD/NVMe | Old HDDs | PSP/Retro Handhelds | Archival/PS1 |

The Verdict: Use ZSO for PSP games on PPSSPP or actual modded PSPs. Use CHD for PS1/Dreamcast games to save space.

Using MaxCompress as an example:

That’s it. Your game now loads faster and eats less space.

Created by the developer of the PPSSPP emulator (Henrik Rydgård), ziso is the official tool. It is lightweight, fast, and open source.

Open your emulator (PPSSPP, DuckStation, PCSX2) and browse directly to the .zso file. The emulator will treat it exactly like an ISO.

The conversion from ISO to ZSO represents a mature compromise between space efficiency and access speed. By leveraging modern compression algorithms (LZ4, Zstd) and a sector-indexed layout, ZSO enables users to reduce their disc image libraries by 30–70% while maintaining – or even improving – loading performance on contemporary hardware.

For retro-gaming enthusiasts managing terabytes of PS2, PSP, or Wii ISOs, batch-converting to ZSO can reclaim hundreds of gigabytes. For archival purposes, ZSO offers a verifiable, reversible transformation that respects the original media’s logical structure. As emulator developers continue to optimize for compressed formats, ZSO is poised to remain a cornerstone of efficient optical disc emulation.

However, no single format fits all. Mixed-mode audio CDs and heavily random-access workloads may still favor raw ISO or CHD. Nonetheless, for the vast majority of DVD- and UMD-based systems, ISO-to-ZSO conversion is a simple, effective, and highly recommended practice.

Word count: approx. 1,450

Most gamers are familiar with the ISO file—an exact sector-by-sector copy of a disc. While accurate, ISOs are uncompressed and bulky. A typical PS2 game can range from 1GB to over 4GB.

ZSO (which stands for Zlib Standard Optimized) is a compressed file format designed specifically for the PS2 homebrew ecosystem. It is similar to the older CSO (Compressed ISO) format commonly used on the PSP, but optimized for the PlayStation 2’s Open PS2 Loader (OPL).