entry_point_of_C_program
#if defined(__ARMCC_VERSION) && !defined(NOSTD)
bl __rt_entry
#else
bl _start
#endif
.text 0x000000000c068ec8 0x7aa50 load address 0x000000003c068ec8
*(.text*)
.text 0x000000000c068ec8 0x5c /usr/local/Cellar/gcc-arm-none-eabi-49/20150925/bin/../lib/gcc/arm-none-eabi/4.9.3/armv7e-m/fpu/crtbegin.o
.text 0x000000000c068f24 0x74 /usr/local/Cellar/gcc-arm-none-eabi-49/20150925/bin/../lib/gcc/arm-none-eabi/4.9.3/../../../../arm-none-eabi/lib/armv7e-m/fpu/crt0.o
0x000000000c068f24 _mainCRTStartup
0x000000000c068f24 _start
0c068f24 <_mainCRTStartup>:
$t():
c068f24: 4b15 ldr r3, [pc, #84] ; (c068f7c <_mainCRTStartup+0x58>)
c068f26: 2b00 cmp r3, #0
c068f28: bf08 it eq
c068f2a: 4b13 ldreq r3, [pc, #76] ; (c068f78 <_mainCRTStartup+0x54>)
c068f2c: 469d mov sp, r3
c068f2e: f5a3 3a80 sub.w sl, r3, #65536 ; 0x10000
c068f32: 2100 movs r1, #0
c068f34: 468b mov fp, r1
c068f36: 460f mov r7, r1
c068f38: 4813 ldr r0, [pc, #76] ; (c068f88 <_mainCRTStartup+0x64>)
c068f3a: 4a14 ldr r2, [pc, #80] ; (c068f8c <_mainCRTStartup+0x68>)
c068f3c: 1a12 subs r2, r2, r0
c068f3e: f07a fc1b bl c0e3778 <__memset_veneer>
c068f42: 4b0f ldr r3, [pc, #60] ; (c068f80 <_mainCRTStartup+0x5c>)
c068f44: 2b00 cmp r3, #0
c068f46: d000 beq.n c068f4a <_mainCRTStartup+0x26>
c068f48: 4798 blx r3
c068f4a: 4b0e ldr r3, [pc, #56] ; (c068f84 <_mainCRTStartup+0x60>)
c068f4c: 2b00 cmp r3, #0
c068f4e: d000 beq.n c068f52 <_mainCRTStartup+0x2e>
c068f50: 4798 blx r3
c068f52: 2000 movs r0, #0
c068f54: 2100 movs r1, #0
c068f56: 0004 movs r4, r0
c068f58: 000d movs r5, r1
c068f5a: 480d ldr r0, [pc, #52] ; (c068f90 <_mainCRTStartup+0x6c>)
c068f5c: 2800 cmp r0, #0
c068f5e: d002 beq.n c068f66 <_mainCRTStartup+0x42>
c068f60: 480c ldr r0, [pc, #48] ; (c068f94 <_mainCRTStartup+0x70>)
c068f62: f07a fc79 bl c0e3858 <__atexit_veneer>
c068f66: f079 fe97 bl c0e2c98 <____libc_init_array_veneer>
c068f6a: 0020 movs r0, r4
c068f6c: 0029 movs r1, r5
c068f6e: f000 f869 bl c069044 <main>
c068f72: f07a fb39 bl c0e35e8 <__exit_veneer>
c068f76: bf00 nop
The symbol _start is the entry point of your program. That is, the address of that symbol is the address jumped to on program start. Normally, the function with the name _start is supplied by a file called crt0.o which contains the startup code for the C runtime environment. It sets up some stuff, populates the argument array argv, counts how many arguments are there, and then calls main. After main returns, exit is called.
If a program does not want to use the C runtime environment, it needs to supply its own code for _start. For instance, the reference implementation of the Go programming language does so because they need a non-standard threading model which requires some magic with the stack. It's also useful to supply your own _start when you want to write really tiny programs or programs that do unconventional things.
