A.OUT(5) A.OUT(5)
NAME
a.out - assembler and link editor output
SYNOPSIS
#include <<a.out.h>>
DESCRIPTION
A.out is the output file of the assembler as(1) and the link editor
ld(1). Both programs make a.out executable if there were no errors and
no unresolved external references. Layout information as given in the
include file for the PDP11 is:
The file has four sections: a header, the program and data text, relo-
cation information, and a symbol table (in that order). The last two
may be empty if the program was loaded with the `-s' option of ld or if
the symbols and relocation have been removed by strip(1).
In the header the sizes of each section are given in bytes, but are
even. The size of the header is not included in any of the other
sizes.
When an a.out file is loaded into core for execution, three logical
segments are set up: the text segment, the data segment (with unini-
tialized 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 8K
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. If the magic number is 411,
the text segment is again pure, write-protected, and shared, and more-
over instruction and data space are separated; the text and data seg-
ment both begin at location 0. If the magic number is 0405, the text
segment is overlaid on an existing (0411 or 0405) text segment and the
existing data segment is preserved.
The stack will occupy the highest possible locations in the core image:
from 0177776(8) and growing downwards. The stack is automatically
extended as required. The data segment is only extended as requested
by brk(2).
The start of the text segment in the file is 020(8); the start of the
data segment is 020+St (the size of the text) the start of the reloca-
tion information is 020+St+Sd; the start of the symbol table is
020+2(St+Sd) if the relocation information is present, 020+St+Sd if
not.
The layout of a symbol table entry and the principal flag values that
distinguish symbol types are given in the include file. Other flag
values may occur if an assembly language program defines machine
instructions.
If a symbol's type is undefined external, and the value field is non-
zero, the symbol is interpreted by the loader ld as the name of a com-
mon region whose size is indicated by the value of the symbol.
The value of a word in the text or data portions which is not a refer-
ence to an undefined external symbol is exactly that value which will
appear in core when the file is executed. If a word in the text or
data portion involves a reference to an undefined external symbol, as
indicated by the relocation information for that word, then the value
of the word as 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
into the word in the file.
If relocation information is present, it amounts to one word per word
of program text or initialized data. There is no relocation informa-
tion if the `relocation info stripped' flag in the header is on.
Bits 3-1 of a relocation word indicate the segment referred to by the
text or data word associated with the relocation word:
000 absolute number
002 reference to text segment
004 reference to initialized data
006 reference to uninitialized data (bss)
010 reference to undefined external symbol
Bit 0 of the relocation word indicates, if 1, that the reference is
relative to the pc (e.g. `clr x'); if 0, that the reference is to the
actual symbol (e.g., `clr *$x').
The remainder of the relocation word (bits 15-4) contains a symbol num-
ber in the case of external references, and is unused otherwise. The
first symbol is numbered 0, the second 1, etc.
SEE ALSO
as(1), ld(1), nm(1)
A.OUT(5)
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