push instruction.. push %rbx

| push instruction |

(gdb) disass
Dump of assembler code for function _IO_default_uflow:
   0x00007f7c1e9964d0 :	mov    0xd8(%rdi),%rax
=> 0x00007f7c1e9964d7 :	push   %rbx
   0x00007f7c1e9964d8 :	mov    %rdi,%rbx
   0x00007f7c1e9964db :	callq  *0x20(%rax)
   0x00007f7c1e9964de :	cmp    $0xffffffffffffffff,%eax
   0x00007f7c1e9964e1 :	je     0x7f7c1e9964f2 
   0x00007f7c1e9964e3 :	mov    0x8(%rbx),%rdx
   0x00007f7c1e9964e7 :	movzbl (%rdx),%eax
   0x00007f7c1e9964ea :	add    $0x1,%rdx
   0x00007f7c1e9964ee :	mov    %rdx,0x8(%rbx)
   0x00007f7c1e9964f2 :	pop    %rbx
   0x00007f7c1e9964f3 :	retq
End of assembler dump.

rbx is the 64 bit equivelant for ebx register

PUSH instruction pushes  a value onto  the stack.
Here i think push instruction pushes the value of rbx register
on to the stack.