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, especially since UEFI 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||0x00||Signature||word||0x5A4D (ASCII for 'M' and 'Z')|
|2||0x02||Extra bytes||word||Number of bytes in the last page.|
|4||0x04||Pages||word||Number of whole/partial pages.|
|6||0x06||Relocation items||word||Number of entries in the relocation table.|
|8||0x08||Header size||word||The number of paragraphs taken up by the header. It can be any value, as the loader just uses it to find where the actual executable data starts. It may be larger than what the "standard" fields take up, and you may use it if you want to include your own header metadata, or put the relocation table there, or use it for any other purpose.|
|10||0x0A||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||0x0C||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||0x0E||Initial SS||word||Relocatable segment address for SS.|
|16||0x10||Initial SP||word||Initial value for SP.|
|18||0x12||Checksum||word||When added to the sum of all other words in the file, the result should be zero.|
|20||0x14||Initial IP||word||Initial value for IP.|
|22||0x16||Initial CS||word||Relocatable segment address for CS.|
|24||0x18||Relocation table||word||The (absolute) offset to the relocation table.|
|26||0x1A||Overlay||word||Value used for overlay management. If zero, this is the main executable.|
|28||0x1C||Overlay information||N/A||Files sometimes contain extra information for the main's program overlay management.|
A paragraph is 16 bytes in size. A page (or block) is 512 bytes long.
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.
After loading the executable into memory, the program loader goes through every entry in relocation table. For each relocation entry, the loader adds the start segment address into word value pointed to by the segment:offset pair. So, for example, a relocation entry 0001:001A will make the loader add start segment address to the value at offset 1*0x10+0x1A=0x2A within the program data.
Each pointer in the relocation table looks as such:
|0||0x00||Offset||word||Offset of the relocation within provided segment.|
|2||0x02||Segment||word||Segment of the relocation, relative to the load segment address.|
CS and SS registers are relocated in a similar fashion.
Initial Program State
- ES and DS registers both point to the segment containing the PSP structure.
- CS equals value specified in the header, relocated by adding the start segment address to it.
- IP equals value specified in the header. Note, that unlike in COM executables, MZ programs don't start at offset 0x100.
- SS equals value specified in the header, relocated, just like CS.
- SP equals value specified in the header.
- AL is 0x00 if the first FCB in the PSP has a valid drive identifier, 0xFF otherwise.
- AH is the same as AL, but for the second FCB in the PSP.
- All other registers may, or may not be set to 0. You should consider them undefined.
With the advent of the PE executable, Microsoft added items to the MZ header, as defined in WinNT.h
|36||0x24||OEM identifier||word||Defined by name but no other information is given; typically zeroes|
|38||0x26||OEM info||word||Defined by name but no other information is given; typically zeroes|
|60||0x3C||PE header start||dword||Starting address of the PE header|