yes.S

  .text

  .globl _start
_start:
  pop %rcx                      # argc
  loop arg                      # argc == 1 will cause no jump
noarg:
  push $0x79                    # place y\0 on stack
  push %rsp                     # push then pop is 2 bytes, while movq is 3
  push %rsp                     # garbage to be discarded
arg:
  pop %rsi                      # Discard Program Name
  pop %rsi                      # Get First Argument
lenloop:
  incl %edx                     # Starts at 0 at prog start
  cmpb %al, -1(%rsi,%rdx,1)     # Check for end of string (al is 0)
  jne lenloop
  movb $10, -1(%rsi,%rdx,1)     # Set the 0 to a newline (ASCII 10)
printloop:
  incl %eax                     # Set eax to 1 = SYS_WRITE (started at 0)
  # Perform the system call. At this point %rsi contains the string pointer
  # %rdx contains the string length, %rdi contains 0 (STDOUT), as it was
  # never set, and %rax contains 1 (SYS_WRITE)
  syscall                       # Registers except %rax persist across call
  subl %edx, %eax               # Reset eax to 0, and check for failure
  je printloop                  # on failure continue, else loop

  # Exit program with status 130
  push $60                      # Push/pop is 3 bytes instead of 5
  pop %rax
  push $0xffffffffffffff82      # Don't care about upper bytes,
                                # so set them to 1 to get 2 byte push imm
  pop %rdi
  syscall