/* 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 .global __uClibc_main .type _start,%function .type __uClibc_main,%function .text _start: #if 0 /* some old code the I feel should not be here - davidm */ @ adjust the data segment base pointer ldr r3,=__data_start sub sl,sl,r3 mov BASEREG,sl #endif /* 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 to MMU 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 /* Ok, now run uClibc's main() -- shouldn't return */ bl __uClibc_main #if 0 /* this is already provided by crtbegin/crtend in the arm-elf compiler */ /* a little bit of stuff to support C++ */ .section .ctors,"aw" .align 4 .global __CTOR_LIST__ __CTOR_LIST__: .long -1 .section .dtors,"aw" .align 4 .global __DTOR_LIST__ __DTOR_LIST__: .long -1 #endif