-- Prologue.
  | push rbp
  | mov rbp, rsp
  -- Accumulative sum (16-bit).
  | xor r9, r9                  -- Clear out r9. Stores accumulated sum.
  | mov rcx, 0                  -- Set index to 0.
  | 1:
  | cmp rcx, rsi                -- Rsi stores size arg. Compare index to size.
  | jg >2                       -- Go to branch '2' if equals.
  | mov ah, [rdi + rcx]         -- Read 16-bit of data and swapping byte order.
  | mov al, [rdi + rcx+1]
  | add r9, rax                 -- Sum and accumulate into r9.
  | add rcx, 2                  -- Increase index by 2.
  | jmp <1                      -- Go to beginning of loop.
  | 2:
  | mov rax, r9                 -- Save accumulated sum in rax.
  -- Sum carry until done.
  | 4:
  | shr r9, 16                  -- Right-shift r9 16-bit to obtain value of carry.
  | cmp r9, 0                   -- Check whether there's carry.
  | jz >5                       -- In case not, go to '4'.
  | and rax, 0xffff             -- Clear out higher part of rax.
  | add rax, r9                 -- Sum carry to rax.
  | mov r9, rax                 -- Assign rax to r9 to check if there's carry again.
  | jmp <4                      -- Go to beggining of loop.
  -- One-complement.
  | 5:
  | not rax                     -- One-complement of rax.
  | and rax, 0xffff             -- Clear out higher part of rax.
  -- Epilogue.
  | mov rsp, rbp
  | pop rbp
  -- Return.
  | ret