1A.OUT(5) File Formats Manual A.OUT(5)
2
6 a.out - assembler and link editor output
7
9 #include <a.out.h>
10
12 A.out is the output file of the assembler as(1) and the link editor
13 ld(1). Both programs make a.out executable if there were no errors and
14 no unresolved external references. Layout information as given in the
15 include file for the PDP11 is:
16 #ifndef __A_OUT_GNU_H__
18 #define __A_OUT_GNU_H__
19 #include <bits/a.out.h>
21 #define __GNU_EXEC_MACROS__
23 struct exec
25 {
26 unsigned long a_info; /* Use macros N_MAGIC, etc for access. */
27 unsigned int a_text; /* Length of text, in bytes. */
28 unsigned int a_data; /* Length of data, in bytes. */
29 unsigned int a_bss; /* Length of uninitialized data area for file, in bytes. */
30 unsigned int a_syms; /* Length of symbol table data in file, in bytes. */
31 unsigned int a_entry; /* Start address. */
32 unsigned int a_trsize;/* Length of relocation info for text, in bytes. */
33 unsigned int a_drsize;/* Length of relocation info for data, in bytes. */
34 };
35 enum machine_type
37 {
38 M_OLDSUN2 = 0,
39 M_68010 = 1,
40 M_68020 = 2,
41 M_SPARC = 3,
42 M_386 = 100,
43 M_MIPS1 = 151,
44 M_MIPS2 = 152
45 };
46 #define N_MAGIC(exec) ((exec).a_info & 0xffff)
48 #define N_MACHTYPE(exec) ((enum machine_type)(((exec).a_info >> 16) & 0xff))
49 #define N_FLAGS(exec) (((exec).a_info >> 24) & 0xff)
50 #define N_SET_INFO(exec, magic, type, flags) ((exec).a_info = ((magic) & 0xffff) | (((int)(type) & 0xff) << 16) | (((flags) & 0xff) << 24))
51 #define N_SET_MAGIC(exec, magic) ((exec).a_info = ((exec).a_info & 0xffff0000) | ((magic) & 0xffff))
52 #define N_SET_MACHTYPE(exec, machtype) ((exec).a_info = ((exec).a_info&0xff00ffff) | ((((int)(machtype))&0xff) << 16))
53 #define N_SET_FLAGS(exec, flags) ((exec).a_info = ((exec).a_info&0x00ffffff) | (((flags) & 0xff) << 24))
54 /* Code indicating object file or impure executable. */
56 #define OMAGIC 0407
57 /* Code indicating pure executable. */
58 #define NMAGIC 0410
59 /* Code indicating demand-paged executable. */
60 #define ZMAGIC 0413
61 /* This indicates a demand-paged executable with the header in the text.
62 The first page is unmapped to help trap NULL pointer references. */
63 #define QMAGIC 0314
64 /* Code indicating core file. */
65 #define CMAGIC 0421
66 #define N_TRSIZE(a) ((a).a_trsize)
68 #define N_DRSIZE(a) ((a).a_drsize)
69 #define N_SYMSIZE(a) ((a).a_syms)
70 #define N_BADMAG(x) (N_MAGIC(x) != OMAGIC&& N_MAGIC(x) != NMAGIC && N_MAGIC(x) != ZMAGIC && N_MAGIC(x) != QMAGIC)
71 #define _N_HDROFF(x) (1024 - sizeof (struct exec))
72 #define N_TXTOFF(x) (N_MAGIC(x) == ZMAGIC ? _N_HDROFF((x)) + sizeof (struct exec) : (N_MAGIC(x) == QMAGIC ? 0 : sizeof (struct exec)))
73 #define N_DATOFF(x) (N_TXTOFF(x) + (x).a_text)
74 #define N_TRELOFF(x) (N_DATOFF(x) + (x).a_data)
75 #define N_DRELOFF(x) (N_TRELOFF(x) + N_TRSIZE(x))
76 #define N_SYMOFF(x) (N_DRELOFF(x) + N_DRSIZE(x))
77 #define N_STROFF(x) (N_SYMOFF(x) + N_SYMSIZE(x))
78 /* Address of text segment in memory after it is loaded. */
80 #define N_TXTADDR(x) (N_MAGIC(x) == QMAGIC ? 4096 : 0)
81 /* Address of data segment in memory after it is loaded. */
83 #define SEGMENT_SIZE 1024
84 #define _N_SEGMENT_ROUND(x) (((x) + SEGMENT_SIZE - 1) & ~(SEGMENT_SIZE - 1))
86 #define _N_TXTENDADDR(x) (N_TXTADDR(x)+(x).a_text)
87 #define N_DATADDR(x) (N_MAGIC(x)==OMAGIC? (_N_TXTENDADDR(x)) : (_N_SEGMENT_ROUND (_N_TXTENDADDR(x))))
89 #define N_BSSADDR(x) (N_DATADDR(x) + (x).a_data)
90 #if !defined (N_NLIST_DECLARED)
92 struct nlist
93 {
94 union
95 {
96 char *n_name;
97 struct nlist *n_next;
98 long n_strx;
99 } n_un;
100 unsigned char n_type;
101 char n_other;
102 short n_desc;
103 unsigned long n_value;
104 };
105 #endif /* no N_NLIST_DECLARED. */
106 #define N_UNDF 0
108 #define N_ABS 2
109 #define N_TEXT 4
110 #define N_DATA 6
111 #define N_BSS 8
112 #define N_FN 15
113 #define N_EXT 1
114 #define N_TYPE 036
115 #define N_STAB 0340
116 #define N_INDR 0xa
117 #define N_SETA 0x14 /* Absolute set element symbol. */
118 #define N_SETT 0x16 /* Text set element symbol. */
119 #define N_SETD 0x18 /* Data set element symbol. */
120 #define N_SETB 0x1A /* Bss set element symbol. */
121 #define N_SETV 0x1C /* Pointer to set vector in data area. */
122 #if !defined (N_RELOCATION_INFO_DECLARED)
124 /* This structure describes a single relocation to be performed.
125 The text-relocation section of the file is a vector of these structures,
126 all of which apply to the text section.
127 Likewise, the data-relocation section applies to the data section. */
128 struct relocation_info
130 {
131 int r_address;
132 unsigned int r_symbolnum:24;
133 unsigned int r_pcrel:1;
134 unsigned int r_length:2;
135 unsigned int r_extern:1;
136 unsigned int r_pad:4;
137 };
138 #endif /* no N_RELOCATION_INFO_DECLARED. */
139 #endif /* __A_OUT_GNU_H__ */
141 The file has four sections: a header, the program and data text, relo‐
143 cation information, and a symbol table (in that order). The last two
144 may be empty if the program was loaded with the `-s' option of ld or if
145 the symbols and relocation have been removed by strip(1).
146 In the header the sizes of each section are given in bytes, but are
148 even. The size of the header is not included in any of the other
149 sizes.
150 When an a.out file is loaded into core for execution, three logical
152 segments are set up: the text segment, the data segment (with unini‐
153 tialized data, which starts off as all 0, following initialized), and a
154 stack. The text segment begins at 0 in the core image; the header is
155 not loaded. If the magic number in the header is 0407(8), it indicates
156 that the text segment is not to be write-protected and shared, so the
157 data segment is immediately contiguous with the text segment. If the
158 magic number is 0410, the data segment begins at the first 0 mod 8K
159 byte boundary following the text segment, and the text segment is not
160 writable by the program; if other processes are executing the same
161 file, they will share the text segment. If the magic number is 411,
162 the text segment is again pure, write-protected, and shared, and more‐
163 over instruction and data space are separated; the text and data seg‐
164 ment both begin at location 0. If the magic number is 0405, the text
165 segment is overlaid on an existing (0411 or 0405) text segment and the
166 existing data segment is preserved.
167 The stack will occupy the highest possible locations in the core image:
169 from 0177776(8) and growing downwards. The stack is automatically
170 extended as required. The data segment is only extended as requested
171 by brk(2).
172 The start of the text segment in the file is 020(8); the start of the
174 data segment is 020+St (the size of the text) the start of the reloca‐
175 tion information is 020+St+Sd; the start of the symbol table is
176 020+2(St+Sd) if the relocation information is present, 020+St+Sd if
177 not.
178 The layout of a symbol table entry and the principal flag values that
180 distinguish symbol types are given in the include file. Other flag
181 values may occur if an assembly language program defines machine
182 instructions.
183 If a symbol's type is undefined external, and the value field is non-
185 zero, the symbol is interpreted by the loader ld as the name of a com‐
186 mon region whose size is indicated by the value of the symbol.
187 The value of a word in the text or data portions which is not a refer‐
189 ence to an undefined external symbol is exactly that value which will
190 appear in core when the file is executed. If a word in the text or
191 data portion involves a reference to an undefined external symbol, as
192 indicated by the relocation information for that word, then the value
193 of the word as stored in the file is an offset from the associated
194 external symbol. When the file is processed by the link editor and the
195 external symbol becomes defined, the value of the symbol will be added
196 into the word in the file.
197 If relocation information is present, it amounts to one word per word
199 of program text or initialized data. There is no relocation informa‐
200 tion if the `relocation info stripped' flag in the header is on.
201 Bits 3-1 of a relocation word indicate the segment referred to by the
203 text or data word associated with the relocation word:
204 000 absolute number
206 002 reference to text segment
207 004 reference to initialized data
208 006 reference to uninitialized data (bss)
209 010 reference to undefined external symbol
210 Bit 0 of the relocation word indicates, if 1, that the reference is
212 relative to the pc (e.g. `clr x'); if 0, that the reference is to the
213 actual symbol (e.g., `clr *$x').
214 The remainder of the relocation word (bits 15-4) contains a symbol num‐
216 ber in the case of external references, and is unused otherwise. The
217 first symbol is numbered 0, the second 1, etc.
218
220 as(1), ld(1), nm(1)
221 A.OUT(5)