mmap from asm code return bad descriptor, but in c is success

Question

i want to call mmap syscall. In C code this was success, but from asm i have returned -9(bad file descritor) returned value after syscall instruction.

1. Where is the mistake?
2. Do I understand correctly that if after the system call instruction in the register eax the number is less than zero, then it can be converted to errno, and if more, then this is the returned address?

asm code:

   .text
   .globl _start

   .set PROT_READ,          0x1
   .set PROT_WRITE,     0x2
   .set MAP_PRIVATE,        0x2
   .set MAP_ANONYMOUS,     0x20

   _start:
    pushq   %rbp
    movq    %rsp, %rbp

    movl     $PROT_READ, %ecx        # PROT_READ
    orl      $PROT_WRITE, %ecx       # PROT_WRITE
    movl     $MAP_PRIVATE, %r9d      # MAP_PRIVATE
    orl      $MAP_ANONYMOUS, %r9d    # MAP_ANONYMOUS

    movl    $9, %eax
    movq    $0, %rdi
    movq    $24, %rsi
    movl    %ecx, %edx
    movl    %r9d, %ecx
    movl    $-1, %r8d
    movl    $0, %r9d
    syscall

    mov %rax, %rbx 
    movl $1, (%rbx) # store value

    popq    %rbp
    retq

C code:

 #define MAP_ANONYMOUS 0x20
 int a = PROT_READ | PROT_WRITE;         // result 3
 int b = MAP_PRIVATE | MAP_ANONYMOUS;    // result 34
 void *ptr = mmap (NULL, 7, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);