A.OUT(V) − PWB/UNIX 9/9/73
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:
1A magic number (407, 410, or 411(8))
2The size of the program text segment
3The size of the initialized portion of the data segment
4The size of the uninitialized (bss) portion of the data segment
5The size of the symbol table
6The entry location (always 0 at present)
7Unused
8A 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-protected and shared, so the data segment is immediately contiguous 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 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:
00undefined symbol
01absolute symbol
02text segment symbol
03data segment symbol
37file name symbol (produced by ld)
04bss segment symbol
40undefined external (.globl) symbol
41absolute external symbol
42text segment external symbol
43data segment external symbol
44bss 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 executed. 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 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 referred to by the text or data word associated with the relocation word:
00indicates the reference is absolute
02indicates the reference is to the text segment
04indicates the reference is to initialized data
06indicates the reference is to bss (uninitialized data)
10indicates 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, 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 unused otherwise. The first symbol is numbered 0, the second 1, etc.