A focused repair tool for BIOS parity errors
Fix-BiosParity 1.00 is a small PowerShell utility for correcting the parity condition checked by the HOLLY GPU in Dreamcast and Naomi BIOS ROMs. The release contains the Fix-BiosParity.ps1 script, which accepts an input BIOS file and produces a separate output file.
The project is intended for cases where a BIOS image fails the hardware parity check. A corrected image can satisfy the condition needed for the G1 bus to unlock and for the BIOS to run, while preserving the executable instructions in the image, according to the project documentation.
What changed in version 1.00
Version 1.00 was published on October 9, 2026. Its release notes contain no separate fix or feature details, so the available build represents the initial published release of the parity-repair script.
The included script reads the complete BIOS image, calculates the XOR of its bytes, and reduces that result to a single parity value. If the result is already even, the output can retain the original data. If the result is odd, the script changes one bit in the trailing zero-padding area, where the project says the SH-4 does not execute code, then writes the resulting image to the requested output path.
Why the correction works
The HOLLY validation process treats the BIOS as a sequence of four-byte blocks. It XORs the bytes within each block, reduces each result to a parity bit, and combines those bits into one final value. The BIOS passes when that final value is zero, meaning the overall parity is even.
The same result can be calculated by XORing every byte in the ROM first and then counting the one bits in the combined value. Flipping a single bit changes the parity of one block and toggles the final result. The script therefore needs only one bit change when the image fails the check.
The repair is designed to avoid altering executable BIOS instructions. The README says the script locates the first byte of the trailing zero-padding and flips its least-significant bit. That leaves the code fetched by the SH-4 unchanged while changing the value seen by the parity check.
Using the script
The documented command takes an input BIOS file and an output filename: .\Fix-BiosParity.ps1 -InputFile dc_boot.bin -OutputFile dc_boot_fixed.bin. This keeps the source image separate from the generated result.
The utility processes BIOS files; it is not a Dreamcast or Naomi emulator and does not provide console firmware. The README explains the BIOS parity calculation but does not include broader setup instructions for using the corrected image.
A narrow solution with a clear practical benefit
Fix-BiosParity addresses one specific failure mode: a BIOS image whose global byte-XOR has odd parity. Its approach is deliberately minimal, changing only a padding bit when correction is required and leaving the BIOS's executable content intact according to the project documentation.
The README gives an example in which the complete BIOS XOR is 0x3C. That value contains four one bits, so it has even parity and passes the stated check without modification. This illustrates why the script may leave an image effectively unchanged when the required condition is already satisfied, while producing a corrected copy for an image with odd parity.
