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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
.text
_start:
/* clear the frame pointer */
mov fp, #0
#ifdef __UCLIBC_HAS_MMU__
/* Load register r0 (argc) from the stack to its final resting place */
ldr r0, [sp], #4
/* Copy argv pointer into r1 -- which its final resting place */
mov r1, sp
/* Skip to the end of argv and put a pointer to whatever
we find there (hopefully the environment) in r2 */
add r2, r1, r0, lsl #2
add r2, r2, #4
#else
/*
* uClinux stacks look a little different from normal
* MMU-full Linux stacks (for no good reason)
*/
/* pull argc, argv and envp off the stack */
ldr r0,[sp, #0]
ldr r1,[sp, #4]
ldr r2,[sp, #8]
#endif
#ifdef __UCLIBC_CTOR_DTOR__
/* Store the address of _init in r3 as an argument to main() */
ldr r3, =_init
/* Push _fini onto the stack as the final argument to main() */
ldr r4, =_fini
stmfd sp!, {r4}
/* Ok, now run uClibc's main() -- shouldn't return */
bl __uClibc_start_main
#else
bl __uClibc_main
#endif
/* Crash if somehow `exit' returns anyways. */
bl abort
/* Stick in a dummy reference to main(), so that if an application
* is linking when the main() function is in a static library (.a)
* we can be sure that main() actually gets linked in */
L_dummy_main_reference:
.long main
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