rom-weaver

Patch a compressed CHD disc in the browser

Apply two supplied patches to a compressed disc, download another CHD, and verify the extracted bytes. You need only a browser and the three practice files below.

This tiny synthetic disc contains test data, not a commercial game. It is not a playable disc; this exercise checks the patching workflow.

Step 1: download the practice files

Save these three files to your device:

Keep the original CHD. Each patch changes the same 16-byte region in its extracted BIN, so the order matters.

Step 2: add the compressed input and patches

  1. Open Apply Patches.
  2. Add the CHD and both BPS files to Inputs.
  3. Wait for reading and checksumming to finish. If asked to choose a file from the CHD, select its .bin entry.
  4. Open Checks on the ROM card. The extracted BIN's SHA-1 should be ba1af47af57aed5590c30358f4b7dc247cf26e94.

The patch runs on the extracted BIN, not on the CHD's compressed bytes. The CHD can also supply a CUE sheet describing the disc.

Step 3: set the patch order

  1. Put 01-first.bps above 02-second.bps in Patches & Cheats. Drag a numbered handle if necessary.
  2. Keep both patches switched on.
  3. Set the first patch's input to Original ROM.
  4. Set the second patch's input to Previous patch output.
  5. Leave checksum validation enabled.

The first patch produces a BIN with SHA-1 b94e0943f9d487bb1a8961dfdce4494bf83d2160. The second patch expects that intermediate result. Applying it directly to the original fails its source checksum.

Step 4: download a compressed result

  1. In Apply, enter practice-patched as the output filename, without an extension.
  2. Choose CHD in Output format.
  3. Choose APPLY & DOWNLOAD, wait for completion, and save practice-patched.chd.

Keep the default compression settings. The compressed file's checksum is not the checksum of the BIN inside it.

Step 5: verify the result

  1. Open Extract and add practice-patched.chd.
  2. If offered a track layout, choose One BIN file.
  3. In Files, choose Clear all, select only the .bin file, then choose Download 1 file.
  4. Open Checksum file and add that BIN.
  5. Select SHA-1 and compare it with ebce631d802abe7c450e3f6cb658f9679701fdd6.

The extracted BIN is 32,768 bytes. A matching SHA-1 confirms the supplied patches produced the expected disc data after decompression.

This verifies the patched BIN, not emulator compatibility or a byte-identical CHD encoding. Your original CHD remains the starting point for another run.

For your own discs, follow Apply ROM patches. For GameCube and Wii compression, see Convert ISO to RVZ.