/*
 * vlibc — x86_64 _start (todo 3).
 *
 * The kernel enters the program here with the initial stack holding, in
 * order from %rsp: argc, argv[0..argc-1], a NULL argv terminator, the
 * environment strings, a NULL envp terminator, and the auxiliary vector.
 * The kernel leaves %rsp 16-byte aligned at entry.
 *
 * _start forwards (main, argc, argv, envp) to __libc_start_main, matching
 * vlibc's startup signature (src/start/start.h). The frame pointer is
 * zeroed first so the outermost frame terminates backtraces, and %rsp is
 * 16-byte aligned at the call site per the SysV AMD64 ABI. __libc_start_main
 * never returns (it ends in exit), so the hlt loop below is unreachable; it
 * exists only to satisfy the assembler and to fault loudly (hlt in user
 * mode raises #GP) if a broken build ever falls through.
 */

    .text
    .global _start
    .type _start, @function
_start:
    xor %ebp, %ebp              # outermost frame: zero frame pointer
    mov %rsp, %r8               # keep the raw initial stack for envp
    pop %rsi                    # rsi = argc
    mov %rsp, %rdx              # rdx = argv
    lea 16(%r8, %rsi, 8), %rcx  # rcx = envp = initial rsp + 8 + 8*(argc+1)
    lea main(%rip), %rdi        # rdi = &main
    and $-16, %rsp              # ABI: rsp % 16 == 0 at the call site
    call __libc_start_main      # _Noreturn: routes main's result into exit
1:  hlt
    jmp 1b
    .size _start, . - _start

    .section .note.GNU-stack, "", @progbits