A.OUT(V) 9/9/73 A.OUT(V)
NAME
a.out - assembler and link editor output
DESCRIPTION
A.out is the output file of the assembler as and the link
editor ld. Both programs make a.out executable if there
were no errors and no unresolved external references.
This file has four sections: a header, the program and data
text, a symbol table, and relocation bits (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.
The header always contains 8 words:
1 A magic number (407, 410, or 411(8))
2 The size of the program text segment
3 The size of the initialized portion of the data segment
4 The size of the uninitialized (bss) portion of the data
segment
5 The size of the symbol table
6 The entry location (always 0 at present)
7 Unused
8 A flag indicating relocation bits have been suppressed
The sizes of each segment are in bytes but are even. The
size of the header is not included in any of the other
sizes.
When a file produced by the assembler or loader 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 (word 0) is 407,
it indicates that the text segment is not to be write-pro-
tected and shared, so the data segment is immediately con-
tiguous with the text segment. If the magic number is 410,
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 moreover instruction and data
space are separated; the text and data segment both begin at
location 0. See the 11/45 handbook for restrictions which
apply to this situation.
The stack will occupy the highest possible locations in the
core image: from 177776(8) and growing downwards. The stack
is automatically extended as required. The data segment is
only extended as requested by the break system call.
The start of the text segment in the file is 20(8); the
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start of the data segment is 20+St (the size of the text)
the start of the relocation information is 20+St+Sd; the
start of the symbol table is 20+2(St+Sd) if the relocation
information is present, 20+St+Sd if not.
The symbol table consists of 6-word entries. The first four
words contain the ASCII name of the symbol, null-padded.
The next word is a flag indicating the type of symbol. The
following values are possible:
00 undefined symbol
01 absolute symbol
02 text segment symbol
03 data segment symbol
37 file name symbol (produced by ld)
04 bss segment symbol
40 undefined external (.globl) symbol
41 absolute external symbol
42 text segment external symbol
43 data segment external symbol
44 bss segment external symbol
Values other than those given above may occur if the user
has defined some of his own instructions.
The last word of a symbol table entry contains the value of
the symbol.
If the 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 common 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 reference to an undefined external symbol is exactly
that value which will appear in core when the file is exe-
cuted. If a word in the text or data portion involves a
reference to an undefined external symbol, as indicated by
the relocation bits 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 ed-
itor 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 information if the ``suppress relocation'' 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:
00 indicates the reference is absolute
02 indicates the reference is to the text segment
04 indicates the reference is to initialized data
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06 indicates the reference is to bss (uninitialized data)
10 indicates the reference is to an undefined external
symbol.
Bit 0 of the relocation word indicates if on that the refer-
ence is relative to the pc (e.g. ``clr x''); if off, that
the reference is to the actual symbol (e.g., ``clr *$x'').
The remainder of the relocation word (bits 15-4) contains a
symbol number in the case of external references, and is un-
used otherwise. The first symbol is numbered 0, the second
1, etc.
SEE ALSO
as(I), ld(I), strip(I), nm(I)
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