The MS-DOS EXE format, also known as MZ after its signature (the initials of Microsoft engineer Mark Zbykowski), was introduced with MS-DOS 2.0 (version 1.0 only sported the simple COM format). It is designed as a relocatable executable running under real mode. As such, only DOS and Windows 9x can use this format natively, but there are several free DOS emulators (e.g., DOSBox) that support it and that run under various operating systems (e.g., Linux, Amiga, Windows NT, etc.). Although they can exist on their own, MZ executables are embedded in all NE, LE, and PE executables, usually as stubs so that when they are ran under DOS, they display:
This program cannot be run in MS-DOS mode.
However, they can also be used so that a single executable can provide 2 ports of the same application (e.g.,one for DOS and one for Windows). Windows 9x will run the MZ executable if the program is started from the command line prompt, or the PE executable if started normally. In the case of boot loaders, they can help provide a DOS version, esp. since (U)EFI requires the PE format which contains a MZ executable.
MZ File Structure
MZ executables only consists of 2 structures: the header and the relocation table. The header, which is followed by the program image, looks like this:
|0||Signature||word||0x5A4D (ASCII for 'M' and 'Z')|
|2||Extra bytes||word||Number of bytes in the last page.|
|4||Pages||word||Number of whole/partial pages.|
|6||Relocation items||word||Number of entries in the relocation table.|
|8||Header size||word||The number of paragraphs taken up by the header.|
|10||Minimum allocation||word||The number of paragraphs required by the program, excluding the PSP and program image. If no free block is big enough, the loading stops.|
|12||Maximum allocation||word||The number of paragraphs requested by the program. If no free block is big enough, the biggest one possible is allocated.|
|14||Initial SS||word||Relocatable segment address for SS.|
|16||Initial SP||word||Initial value for SP.|
|18||Checksum||word||When added to the sum of all other words in the file, the result should be zero.|
|20||Initial IP||word||Initial value for IP.|
|22||Initial CS||word||Relocatable segment address for CS.|
|24||Relocation table||word||The (absolute) offset to the relocation table.|
|26||Overlay||word||Value used for overlay management. If zero, this is the main executable.|
|28||Overlay information||N/A||Files sometimes contain extra information for the main's program overlay management.|
Note: A paragraph is 16 bytes in size.
Each pointer in the relocation table looks as such:
|Field||Size (in bytes)|
If both the minimum and maximum allocation fields are cleared, MS-DOS will attempt to load the executable as high as possible in memory. Otherwise, the image will be loaded just above the 256-byte PSP structure, in low memory.