FMAC2COM

FMAC2COM is a DOS utility that converts a MacPaint image file to a COM file that, when executed, displays the image in a bare-bones viewer. It was developed by Keith P. Graham.

The COM file remains sort-of compatible with MacPaint format, though most software that supports MacPaint format will reject the files.

Format details
FMAC2COM works by replacing the first 640 bytes of the MacPaint file with an image viewer. It assumes the MacPaint file starts with a MacBinary header.

There was probably only one version of FMAC2COM, but there seem to be two widely-distributed variants of it, which differ by only a single bit. In the FMAC2COM.COM file, the byte at offset 411 is sometimes 0x10, and sometimes 0x18. It appears at offset 408 in the generated COM files. Best guess is the 0x18 variant is the original, just because the 0x10 variant is usually found without the documentation file.

Identification
The first 640 bytes of the file are probably always the same, except for the bytes at offset 3 and 4, and the byte at offset 408 as noted above.

For example, ASCII "" appears at offset 26.

Software

 * - 0x18 variant
 * - 0x18 variant
 * - 0x10 variant

Sample files

 * http://cd.textfiles.com/bthevhell/200/150/girl25.pic (NSFW!)