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patch_sdi.py builds a modified SDI file by appending a custom WinRE.wim to a stock boot.sdi and patching the WIM blob entry in the blob table so that the Windows boot manager loads the custom WIM instead of the original. The stock boot.sdi is never touched — the script always writes a new output file.
The stock SDI is never modified. The script reads it, then creates a separate patched copy at the output path you specify.

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Usage

python patch_sdi.py --sdi boot.sdi --wim custom_winre.wim -o boot_patched.sdi
Full syntax: 
patch_sdi.py --sdi <path> --wim <path> [-o <output>]

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Arguments

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--sdi
string
required
Path to the stock boot.sdi file. This file is read but never modified — a patched copy is written to the output path.
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--wim
string
required
Path to the custom WIM file to append — for example, a modified WinRE.wim that has cmd.exe set as the launch application.
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-o / --output
string
default:"boot_patched.sdi"
Output path for the patched SDI file. Must differ from the input --sdi path; the script exits with an error if both resolve to the same file.
The output path must differ from the input SDI path. If --sdi and --output resolve to the same absolute path, the script will exit with Error: output path must differ from input SDI path.

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How it works

1

Read and validate the stock SDI

The script reads the entire stock SDI into memory and checks that the first eight bytes match the $SDI0001 signature. A warning is printed if the signature does not match, but execution continues.
2

Locate the WIM and PART blob entries

The blob table lives at offset 0x200 and contains 64 entries of 64 bytes each. The script scans those entries for the WIM\x00 and PART type tags. It exits with an error if either tag is not found.
3

Calculate an aligned append offset

The append position is computed by rounding the stock SDI size up to the next 4 KiB page boundary (align_up(stock_size, 0x1000)). Any gap between the end of the stock SDI and that boundary is filled with null bytes.
4

Copy the stock SDI and append the custom WIM

The stock SDI is copied verbatim to the output path using shutil.copy2. The padding bytes are then appended, followed by the custom WIM in 1 MiB chunks.
5

Patch the WIM blob entry

The script reopens the output file in read-write mode and writes two 64-bit little-endian values into the WIM blob entry:
  • data_offset (at entry offset 0x10) — set to the calculated append position so the boot manager finds the custom WIM.
  • data_size (at entry offset 0x18) — set to the byte size of the custom WIM file.
6

Print the ramdisk buffer layout summary

After patching, the script prints a map showing where each region will reside in the ramdisk buffer when the boot manager loads the SDI: the header and blob table, the PART (NTFS) data, the appended custom WIM, and the position of the trusted WIM that the boot manager hash-verifies but does not actually boot from.

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Example output

[*] Stock SDI     : boot.sdi (5243000 bytes, 0x500078)
[*] Custom WIM    : custom_winre.wim (314572800 bytes, 300.0 MiB)
[*] Stock blob table:
    PART entry #0 : offset=0x2000, size=0x4fe000 (5234688 bytes)
    WIM  entry #1 : offset=0x500000, size=0x12c00000 (314572800 bytes)

[*] Patch plan:
    Append custom WIM at file offset 0x501000 (after 3976 bytes padding)
    Set WIM blob offset to 0x501000 (= position in ramdisk buffer)
    Set WIM blob size to 0x12c00000 (314572800 bytes)
    New file size: 319819776 bytes (305.0 MiB)

[*] Copying stock SDI to boot_patched.sdi...
[*] Appending custom WIM...
[*] Patching WIM blob entry (slot #1)...

[+] Patched SDI written: boot_patched.sdi (319819776 bytes)

[*] Ramdisk buffer layout at boot time:
    0x00000000  SDI header + blob table
    0x00002000  PART data (NTFS volume, 5234688 bytes)
    0x00501000  Custom WIM (appended, 314572800 bytes) <-- WIM blob points here
    0x13101000  Trusted WIM (loaded separately by boot loader)
               ^-- hash verified on this one, but NOT booted

[+] Done! Replace boot.sdi with this file and reboot into WinRE.
[+] Verify with: python parse_sdi.py boot_patched.sdi

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Verify the result

After patching, run parse_sdi.py on the output file to confirm the blob table is correct before booting: 
python parse_sdi.py boot_patched.sdi
Check that the WIM entry offset matches 0x501000 (or whatever append offset was printed in the patch plan), that the WIM entry size matches your custom WIM file size, and that no WARNING: extends past EOF lines appear. See parse_sdi.py for full details on the output.

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Getting a custom WinRE.wim

The custom WIM must have cmd.exe configured as the WinRE launch application so that a command prompt opens automatically after the SDI is loaded, giving access to the decrypted OS volume. Building a custom WIM requires mounting and modifying a stock WinRE.wim. If you do not want to build your own, the pre-built boot_patched.sdi available in the project Releases already contains a WIM with cmd.exe set as the launch application — download it and place it directly in USB/sdi/ or TFTP-root/sdi/ without running this script.

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