/* SPDX-License-Identifier: Apache-2.0
 * SchOS — process entry point.
 *
 * This has to be assembly, and the reason is worth recording.
 *
 * exec() hands a new process its arguments in registers, not on the stack:
 * kernel/fs/exec.c sets proc->tf->rdi = argc and proc->tf->rsi = argv. Those
 * are already the first two SysV argument registers, so _start's entire job
 * is to call main and then exit — it must not touch %rdi or %rsi at all.
 *
 * It used to be written in C, capturing the incoming registers with GCC's
 *
 *     register int argc_reg asm("rdi");
 *
 * That is not what local register variables mean. Both GCC and clang only
 * guarantee the binding when the variable is used as an operand of an inline
 * asm statement; reading one to pick up an incoming argument is undefined.
 * GCC happened to do the useful thing. clang compiled it to
 *
 *     movl  -0x4(%rbp), %eax        # two uninitialised stack slots
 *     movq  -0x10(%rbp), %rax
 *
 * and every program was handed a garbage argc and argv. Nothing crashed --
 * init, shmtest and socktest take no arguments and passed their whole test
 * suites -- so the only symptom was `inputtest -n 6` quietly ignoring its
 * limit. A compiler-dependent way to read registers has no business being in
 * C when four instructions of assembly are exactly right.
 */

.text
.globl _start
_start:
    xorq    %rbp, %rbp          /* end the frame-pointer chain here */

    /* SysV wants %rsp 16-byte aligned immediately before a call, so that the
     * callee sees %rsp+8 aligned once the return address is pushed. Rounding
     * down is safe: argc and argv arrive in registers, so nothing below is
     * needed. */
    andq    $-16, %rsp

    callq   main                /* main(%rdi = argc, %rsi = argv) */

    /* main's return value is the exit status, and it used to be thrown away.
     *
     * sys_exit() reads the status from %rdi, and nothing put it there -- so
     * every first-party program exited with whatever happened to be in that
     * register, and `if socktest; then` could not work for any of them. The
     * test suites all end in `return failures ? 1 : 0` and none of it reached
     * the shell.
     *
     * %eax, not %rax: the status is the low eight bits, and a `return 0` from
     * a C function leaves only %eax defined. */
    movl    %eax, %edi
    callq   exit_group          /* noreturn, but be certain */
1:  hlt
    jmp     1b