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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:
- /* clear the frame pointer */
- mov fp, #0
- #ifdef __PIC__
- /* Store the address of main in r0 */
- adr r5, .L_main
- ldr r0, .L_main
- add r0, r0, r5
- #else
- /* Store the address of main in r0 */
- ldr r0, =main
- #endif
- #ifdef __ARCH_HAS_MMU__
- /* Load register r1 (argc) 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, argv and envp off the stack */
- ldr r1,[sp, #0]
- ldr r2,[sp, #4]
- #endif
- #ifdef __PIC__
- /* Store the address of _init in r3 */
- adr r5, .L_init
- ldr r3, .L_init
- add r3, r3, r5
- /* Push _fini onto the stack as an argument to main() */
- ldr r4, .L_init + 4
- add r4, r4, r5
- stmfd sp!, {r4}
- /* Push rtld_fini onto the stack as an argument to main() */
- ldr r4, .L_init + 8
- add r4, r4, r5
- stmfd sp!, {r4}
- #else
- /* Store the address of _init in r3 as an argument to main() */
- ldr r3, =_init
- /* Push _fini onto the stack as an argument to main() */
- ldr r4, =_fini
- stmfd sp!, {r4}
- /* Push rtld_fini onto the stack as an argument to main() */
- ldr r4, =rtld_fini
- stmfd sp!, {r4}
- #endif
- /* 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
- .word rtld_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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