os_kernel_lab/labcodes/lab5/user/libs/initcode.S

.text
.globl _start
_start:
    # set ebp for backtrace
    movl $0x0, %ebp

    # move down the esp register
    # since it may cause page fault in backtrace
    subl $0x20, %esp

    # call user-program function
    call umain
1:  jmp 1b