.mac File Extension

What kind of file is a .mac?

A .mac file is ambiguous. One longstanding meaning is a MacPaint bitmap, but early Monkey's Audio releases also used the suffix. Some specialist programs use .mac for plain-text macro scripts. There is no single registered MIME type or universal structure for every MAC file.

MacPaint files contain one fixed 576 × 720 pixel, 1-bit black-and-white image. Apple's format note specifies a 512-byte header followed by 720 scan lines compressed separately with PackBits. The uncompressed bitmap is 51,840 bytes, while a typical file is about 10 KB and the documented maximum is 53,072 bytes.¹

Identification should use content and provenance, not the filename alone:

• A Monkey's Audio file starts with ASCII MAC , or hexadecimal 4D 41 43 20, and FFmpeg recognizes .mac as a legacy APE suffix.²
• A MacPaint file normally begins with a big-endian version value of 0 or 2, but that weak check is not a unique magic number. A decoder should also validate 720 PackBits rows.¹
• A readable text file may be a program-specific macro. For example, the spec instrument-control environment uses plain-text .mac scripts containing executable commands.³

How do I open a .mac on my computer?

First inspect a copy with a format-identification tool or hex viewer.

• For a MacPaint image, use XnView MP, whose format list includes .mac MacPaint files, or convert it with Netpbm's macptopbm tool.⁴⁵
• For Monkey's Audio, FFmpeg can inspect and decode the stream. Save converted lossless audio as .flac, and use .ape for the original format's modern filename convention.
• For a macro, open it as text. Execute it only in the documented host application after reviewing every command.

Renaming a file does not convert its contents. In particular, changing a MacPaint image to .png or audio to .ape only changes its name.

Poly support for .mac

Poly stores, syncs, shares, versions, and finds .mac files by filename and basic properties. Because the suffix can mean an image, audio stream, or executable macro, Poly does not infer a format-specific preview, content index, or agent-readable representation from it.

Identify and convert a copy first when you need richer behavior. PNG preserves MacPaint's exact monochrome pixels. FLAC preserves decoded Monkey's Audio samples without introducing lossy compression.

History of .mac

Bill Atkinson wrote MacPaint for the original Macintosh, which shipped in January 1984.⁶ Classic Mac systems identified its documents with the PNTG file type rather than relying on a filename extension.¹ The .mac spelling became one interchange convention when those files moved to systems organized around suffixes.⁴

Monkey's Audio later reused .mac in its early releases before version 3.00 changed the default extension to .ape. Current FFmpeg retains .mac as a recognized legacy input name.²

Working with .mac and alternatives

Keep the original until the converted result has been checked. Use PNG or TIFF for a MacPaint image, FLAC or WAV for Monkey's Audio, and the host application's documented format for a macro. MacPaint is already lossless and monochrome, so JPEG adds artifacts without a useful benefit.