read character from stdin and write stdout by system call

echo.s

.intel_syntax noprefix
.comm mem, 30000
.globl main

main:
    lea     rbx, mem

    # read systemcall
    mov     rax, 0              # specify read system call
    mov     edx, 0x1            # 3nd argument (count)
    mov     rsi, rbx            # 2nd argument (string pointer)
    mov     edi, 0x0            # 1st argument (stdout)
    syscall

   # write systemcall
   mov     rax, 1              # specify write system call
   mov     edx, 0x1            # 3nd argument (count)
   mov     rsi, rbx            # 2nd argument (string pointer)
   mov     edi, 0x1            # 1st argument (stdout)
   syscall