View source for UNIX a.out file

'''UNIX a.out files''' are used to hold [[binary]] [[file]]s of various sorts, both [[relocatable binary]] and also [[object code|executable binary]] files (ready to be loaded into [[main memory]] and run).

They generally hold, in order:

* a header giving overall information about the contents
* [[object code]]
* initialized data
* relocation information
* a [[symbol table]]

The relocation information is used during [[linking]] of relocatable binary files into an executable binary file; the last two may not exist in finalized executable binary files.

==PDP-11 a.out format==

The format of the a.out header on a [[PDP-11]] (always 8 words long) is:

{| class="wikitable"
! Offset !! Contents
|-
| 0 || A magic number (below)
|-
| 2 || Program text size
|-
| 4 || Initialized data size
|-
| 6 || Uninitialized ([[BSS]]) data size
|-
| 010 || Symbol table size
|-
| 012 || Entry location
|-
| 014 || Unused
|-
| 016 || Flag indicating relocation information has been suppressed
|}

Magic number values are: 0407 (text is not write-protected and not shared), 0410 (text is write-protected, and one copy in [[main memory]] will be shared by all [[process]]es executing that file), or 0411 (as for 0410, but [[instruction]] and data space are separate, with both beginning at 0; i.e. 'split I&D'). The origin of the '0407' will be obvious to anyone familiar with PDP-11 object code; in the early days of UNIX, an executable binary file was loaded into memory without stripping off the header, and started at location '0'; the '0407' is a [[jump|BR]] instruction which skips over the header to the first location after it.

The 'entry location' was always 0 on the PDP-11.

The symbol table consists of an array of 6-word entries. The first four words contain the symbol in [[ASCII]], padded on the end with '0' bytes. The next word is a field indicating the type of symbol (below). The final word is the value (possibly not final in relocatable binary files). Symbol types are:

{| class="wikitable"
! Value !! Symbol type
|- 
| 00 || undefined
|-
| 01 || absolute
|-
| 02 || text
|-
| 03 || data
|-
| 04 || BSS
|-
| 24 || register assignment
|-
| 37 || file name (produced by the linker, 'ld')
|-
| 40 || undefined [[external]]
|-
| 41 || absolute external
|-
| 42 || text external
|-
| 43 || data external
|-
| 44 || BSS external
|}

If the symbol's type is un-defined external, and the value field is non-zero, the symbol names a [[common region]], of a size indicated by the value.

If a word in the text or data section involves a reference to an un-defined external symbol (as indicated by the relocation bits for that word - below), the value of the word as stored in the file is an offset from the associated external symbol. When the file is bound into an object file by the [[linker]], the eventual value of the symbol will be added into the word.

If relocation information is present, there is one word per word of text or initialized data. Bits 3-1 of a relocation word indicate the entity referred to by the word associated with the relocation word:

{| class="wikitable"
! Value !! Reference type
|-
| 00 || the reference is absolute
|-
| 02 || the reference is to the text segment
|-
| 04 || the reference is to initialized data
|-
| 06 || the reference is to BSS
|-
| 10 || the reference is to an un-defined external symbol
|}

For references to un-defined external symbols, he remaining bits of the relocation word (15-4) contain the index of the symbol's entry in the table (numbered sequentially from 0).

==External links==

* [https://minnie.tuhs.org/cgi-bin/utree.pl?file=V6/usr/man/man5/a.out.5 A.OUT - V] - [[UNIX Sixth Edition|UNIX V6]] man page

[[Category: UNIX]]
[[Category: PDP-11 File Formats]]