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{{File formats}}
The MS-DOS EXE format, also known as MZ after its signature (the initials of Microsoft engineer Mark
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.
==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:
Line 13:
{| {{wikitable}}
|-
! colspan=2|Offset
! Field
! Size
Line 19:
|-
| 0
| 0x00
| Signature
| word
Line 24 ⟶ 25:
|-
| 2
| 0x02
| Extra bytes
| word
Line 29 ⟶ 31:
|-
| 4
| 0x04
| Pages
| word
Line 34 ⟶ 37:
|-
| 6
| 0x06
| Relocation items
| word
Line 39 ⟶ 43:
|-
| 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
Line 59 ⟶ 67:
|-
| 16
| 0x10
| Initial SP
| word
Line 64 ⟶ 73:
|-
| 18
| 0x12
| Checksum
| word
| When added to the sum of all other words in the file, the
|-
| 20
| 0x14
| Initial IP
| word
Line 74 ⟶ 85:
|-
| 22
| 0x16
| Initial CS
| word
Line 79 ⟶ 91:
|-
| 24
| 0x18
| Relocation table
| word
Line 84 ⟶ 97:
|-
| 26
| 0x1A
| Overlay
| word
Line 89 ⟶ 103:
|-
| 28
| 0x1C
| Overlay information
| N/A
Line 94 ⟶ 109:
|}
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.▼
===Relocations===
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:
Line 100 ⟶ 121:
{| {{wikitable}}
|-
! colspan=2|Offset
! Field
!Size (in bytes)▼
! Size
! Description
|-
| 0
| 0x00
| Offset
|
| Offset of the relocation within provided segment.
|-
| 2
| 0x02
| Segment
|
| Segment of the relocation, relative to the load segment address.
|}
CS and SS registers are relocated in a similar fashion.
▲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.
==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.
==PE Extension==
With the advent of the [[PE]] executable, Microsoft added items to the MZ header, as defined in WinNT.h
(Actually, that's incorrect – this extension was originally added in the New Executable format, which was first released as part of Windows 1.0 in 1985, and the multitasking MS-DOS 4.0 in 1986. The "PE header start" is called "e_lfanew" in Microsoft's headers, since it was originally the pointer to the NE header, although later was also used for LE, LX and PE; in principle it can be used for *any* executable format which is defined as an extension of MZ.)
{| {{wikitable}}
|-
! colspan=2|Offset
! Field
! Size
! Description
|-
| 28
| 0x1C
| Reserved
| qword
|
|-
| 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
|-
| 40
| 0x28
| Reserved
|-
| 60
| 0x3C
| PE header start
| dword
| Starting address of the PE header
|}
==See Also==
* [http://web.archive.org/web/20120204110033/http://www.nondot.org/sabre/os/files/Executables/EXE.txt OSRC]
* [http://www.delorie.com/djgpp/doc/exe/ D.J. Delorie: MZ Header format]
* [http://www.pinvoke.net/default.aspx/Structures.IMAGE_DOS_HEADER IMAGE_DOS_HEADER]
* [https://marcin-chwedczuk.github.io/a-closer-look-at-portable-executable-msdos-stub A closer look at Portable Executable MS-DOS Stub]
* [https://github.com/FDOS/kernel/blob/master/kernel/task.c#L601 FreeDOS kernel's MZ loader source code]
[[Category:Executable Formats]]
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