A.OUT (V) 3/15/72 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. In both cases, a.out may be executed provided
there were no errors and no unresolved external refer-
ences.
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(8))
2 The size of the program text segment
3 The size of the initialized data segment
4 The size of the uninitialized (bss) segment
5 The size of the symbol table
6 The entry location (always 0 at present)
7 The stack size required (0 at present)
8 A flag indicating relocation bits have been sup-
pressed
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 segments are set up: the
text segment, the data segment, and the bss (uninitial-
ized data) segment, in that order. The text segment be-
gins at the lowest location in the core image; the header
is not loaded. The data segment begins immediately after
the text segment, and the bss segment immediately after
the data segment. The bss segment is initialized by 0's.
In the future the text segment will be write-protected
and shared.
The start of the text segment in the file is 20(8); the
start of the data segment is 20+S (the size of the text)
t
the start of the relocation information is 20+S +S ; the
t d
start of the symbol table is 20+2(S +S ) if the reloca-
t d
tion information is present, 20+S +S if not.
t d
The symbol table consists of 6-word entries. The first
four 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
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 ex-
actly that value which will appear in core when the file
is executed. If a word in the text or data portion in-
volves 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 pro-
cessed 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 information if the "suppress relocation"
flag in the header is on.
Bits 3-1 of a relocation word indicate the segment re-
ferred 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 the data segment
06 indicates the reference is to the bss segment
10 indicates the reference is to an undefined external
symbol.
Bit 0 of the relocation word indicates if on that the
reference is relative to the pc (e.g. "clr x"); if off,
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 unused otherwise. The first symbol is numbered 0, the
second 1, etc.
SEE ALSO as, ld, strip, nm, un(I)