/* 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 #if defined L_crt0 || ! defined __UCLIBC_CTOR_DTOR__ .type __uClibc_main,%function #else .weak _init .weak _fini .type __uClibc_start_main,%function #endif /* 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 */ .type main,%function .text _start: /* clear the frame pointer */ mov fp, #0 #ifdef __ARCH_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 #if (defined L_crt1 || defined L_gcrt1 ) && defined __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 /* We need this stuff to make gdb behave itself, otherwise gdb will chokes 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 #if defined L_gcrt1 && defined __UCLIBC_PROFILING__ # include "./gmon-start.S" #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