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/* When we enter this piece of code, the program stack looks like this:
argc argument counter (integer)
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
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
When we are done here, we want
a1=argc
a2=argv[0]
a3=argv[argc+1]
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
*/
#include <features.h>
.text
.global _start
.type _start,%function
.type _init,%function
.type _fini,%function
.type main,%function
.type __uClibc_main,%function
.text
_start:
/* Save a copy of rtld_fini before r0 gets nuked */
mov r5, r0
/* clear the frame pointer */
mov fp, #0
/* Load register r0 with main */
#ifdef __PIC__
adr r8, .L_main
ldr r0, .L_main
add r0, r0, r8
ldr r4, .L_init + 4
add r4, r4, r8
#else
ldr r0, =main
#endif
#ifdef __ARCH_HAS_MMU__
/* Load register r1 from the stack to its final resting place */
ldr r1, [sp], #4
/* Copy argv pointer into r2 -- which its final resting place */
mov r2, sp
#else
/*
* uClinux stacks look a little different from normal
* MMU-full Linux stacks (for no good reason)
*/
/* pull argc and argv off the stack */
ldr r1,[sp, #0]
ldr r2,[sp, #4]
#endif
/* Store _init and _fini to r3 and r4 */
#ifdef __PIC__
adr r8, .L_init
ldr r3, .L_init
add r3, r3, r8
ldr r4, .L_init + 4
add r4, r4, r8
#else
ldr r3, =_init
ldr r4, =_fini
#endif
/* Store _fini(r4), rtld_fini(r5), and stack_end(r2) on the stack */
str r2, [sp, #-4]!
str r5, [sp, #-4]!
str r4, [sp, #-4]!
/* We need to call __uClibc_main which should not return.
__uClibc_main (int (*main) (int, char **, char **), int argc,
char **argv, void (*init) (void), void (*fini) (void),
void (*rtld_fini) (void), void *stack_end)
*/
bl __uClibc_main
/* Crash if somehow `exit' returns anyways. */
bl abort
#ifdef __PIC__
.L_init:
.word _init
.word _fini
.L_main:
.word main
#endif
/* We need this stuff to make gdb behave itself, otherwise
gdb will choke with SIGILL when trying to debug apps.
*/
.section ".note.ABI-tag", "a"
.align 4
.long 1f - 0f
.long 3f - 2f
.long 1
0: .asciz "GNU"
1: .align 4
2: .long 0
.long 2,0,0
3: .align 4
/* 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
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