1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
|
/* Startup code for ARM & ELF
Copyright (C) 1995, 1996, 1997, 1998, 2001, 2002 Free Software Foundation, Inc.
This file is part of the GNU C Library.
The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
The GNU C Library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with the GNU C Library; if not, write to the Free
Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
02111-1307 USA. */
/* This is the canonical entry point, usually the first thing in the text
segment.
Note that the code in the .init section has already been run.
This includes _init and _libc_init
At this entry point, most registers' values are unspecified, except:
a1 Contains a function pointer to be registered with `atexit'.
This is how the dynamic linker arranges to have DT_FINI
functions called for shared libraries that have been loaded
before this code runs.
sp The stack contains the arguments and environment:
0(sp) argc
4(sp) argv[0]
...
(4*argc)(sp) NULL
(4*(argc+1))(sp) envp[0]
...
NULL
*/
/*
For uClinux it looks like this:
argc argument counter (integer)
argv char *argv[]
envp char *envp[]
argv[0] program name (pointer)
argv[1...N] program args (pointers)
argv[argc-1] end of args (integer)
NULL
env[0...N] environment variables (pointers)
NULL
ARM register quick reference:
Name Number ARM Procedure Calling Standard Role
a1 r0 argument 1 / integer result / scratch register / argc
a2 r1 argument 2 / scratch register / argv
a3 r2 argument 3 / scratch register / envp
a4 r3 argument 4 / scratch register
v1 r4 register variable
v2 r5 register variable
v3 r6 register variable
v4 r7 register variable
v5 r8 register variable
sb/v6 r9 static base / register variable
sl/v7 r10 stack limit / stack chunk handle / reg. variable
fp r11 frame pointer
ip r12 scratch register / new-sb in inter-link-unit calls
sp r13 lower end of current stack frame
lr r14 link address / scratch register
pc r15 program counter
*/
.text
.globl _start
.type _start,#function
_start:
/* Clear the frame pointer and link register since this is the outermost frame. */
mov fp, #0
mov lr, #0
/* Pop argc off the stack and save a pointer to argv */
ldr a2, [sp], #4
mov a3, sp
/* Push stack limit */
str a3, [sp, #-4]!
/* Push rtld_fini */
str a1, [sp, #-4]!
#ifdef L_Scrt1
ldr sl, .L_GOT
.L_GOT_OFF:
add sl, pc, sl
ldr ip, .L_GOT+4 /* _fini */
ldr a1, [sl, ip]
str a1, [sp, #-4]! /* Push _fini */
ldr ip, .L_GOT+8 /* _init */
ldr a4, [sl, ip]
ldr ip, .L_GOT+12 /* main */
ldr a1, [sl, ip]
/* __uClibc_main (main, argc, argv, init, fini, rtld_fini, stack_end) */
/* Let the libc call main and exit with its return code. */
b __uClibc_main(PLT)
#else
/* Fetch address of fini */
ldr ip, =_fini
/* Push fini */
str ip, [sp, #-4]!
/* Set up the other arguments in registers */
ldr a1, =main
ldr a4, =_init
/* __uClibc_main (main, argc, argv, init, fini, rtld_fini, stack_end) */
/* Let the libc call main and exit with its return code. */
b __uClibc_main
#endif
#ifdef L_Scrt1
.L_GOT:
.word _GLOBAL_OFFSET_TABLE_-(.L_GOT_OFF+8)
.word _fini(GOT)
.word _init(GOT)
.word main(GOT)
#endif
/* Define a symbol for the first piece of initialized data. */
.data
.globl __data_start
__data_start:
.long 0
.weak data_start
data_start = __data_start
|
