Firmware for an Apple iMac logic board 820-2542, revision A in the name, provided as a 4MB BIOS image inside a 1.29 MB RAR. Apple marks its logic boards with 820- part numbers, and this boot ROM is written to the SPI flash when a Mac shows no chime or will not start. Use a programmer that supports the fitted chip, save the original contents first since they hold serial data, and confirm the 820 number on the board matches.
The button opens the matching file on RM Schematics, or a source page if it is not catalogued there yet. The original attachment is from @biosarchive. Enginerys does not host this file.
How to open a .rar file
Compressed archive — the usual wrapper for a schematic + boardview pair or a BIOS + EC set.
Back up your own chip before writing any dump, and match the full board number and revision. Flashing guide →