+.TH A.OUT 5 "UNIX/32V"
+.SH NAME
+a.out \- assembler and link editor output
+.SH SYNOPSIS
+.B #include <a.out.h>
+.SH DESCRIPTION
+.I A.out
+is the output file of the assembler
+.IR as (1)
+and the link editor
+.IR ld (1).
+Both programs make
+.I a.out
+executable if there were no
+errors and no unresolved external references.
+Layout information as given in the include file for the VAX-11 is:
+.PP
+.nf
+.ta 8n +9n +11n
+.PP
+.so /usr/include/a.out.h
+.fi
+.PP
+The file has four sections:
+a header, the program text and data,
+relocation information, and a symbol table
+(in that order).
+The last two may be empty
+if the program was loaded
+with the `\-s' option
+of
+.I ld
+or if the symbols and relocation have been
+removed by
+.IR strip (1).
+.PP
+In the header the sizes of each section are given in bytes.
+The size of the header is not included in any of the other sizes.
+.PP
+When an
+.I a.out
+file is loaded into core for execution, three logical segments are
+set up: the text segment, the data segment
+(with uninitialized data, which starts off as all 0, following
+initialized),
+and a stack.
+The text segment begins at 0
+in the core image; the header is not loaded.
+If the magic number in the header is 0407(8), it indicates that the text
+segment is not to be write-protected and shared,
+so the data segment is immediately contiguous
+with the text segment.
+If the magic number is 0410,
+the data segment begins at the first 0 mod 512 byte
+boundary following the text segment,
+and the text segment is not writable by the program;
+if other processes are executing the same file,
+they will share the text segment.
+.PP
+The stack will occupy the highest possible locations
+in the core image: growing downwards from 80000000(16).
+The stack is automatically extended as required.
+The data segment is only extended as requested by
+.IR break (2).
+.PP
+The start of the text segment in the file is 32(10);
+the start of the data segment is 32+a_text;
+the start of the text relocation is 32+a_text+a_data;
+the start of the data relocation is 32+a_text+a_data+a_trsize;
+the start of the symbol table is 32+a_text+a_data+a_trsize+a_drsize.
+.PP
+The layout of a symbol table entry and the principal flag values
+that distinguish symbol types are given in the include file.
+.PP
+If a symbol's type is undefined external,
+and the value field is non-zero,
+the symbol is interpreted by the loader
+.I ld
+as
+the name of a common region
+whose size is indicated by the value of the
+symbol.
+.PP
+The value of a byte in the text or data which is not
+a portion of a reference to an undefined external symbol
+is exactly that value which will appear in memory
+when the file is executed.
+If a byte in the text or data
+involves a reference to an undefined external symbol,
+as indicated by the relocation information,
+then the value stored in the file
+is an offset from the associated external symbol.
+When the file is processed by the
+link editor and the external symbol becomes
+defined, the value of the symbol will
+be added to the bytes in the file.
+.PP
+If relocation
+information is present, it amounts to six bytes per
+relocatable dataum.
+There is no relocation information if a_trsize+a_drsize==0.
+The relocation information is structured as
+.nf
+.ta 4n,+17n
+.PP
+struct relocation_info {
+ long address; /* relative to current segment */
+ short pcrel:1, /* if so, segment offset has already */
+ /* been subtracted */
+ length:2, /* 0=byte, 1=word, 2=long */
+ extern:1, /* does not include value */
+ /* of symbol referenced */
+ offset:1, /* already includes origin */
+ /* of this segment (?) */
+ symbolnum:11;
+ /* if extern then symbol table */
+ /* ordinal (0, 1, 2, ...) else */
+ /* segment number (same as symbol types) */
+};
+.fi
+.SH "SEE ALSO"
+as(1), ld(1), nm(1)
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