write character for stdout by system call

write.s

.intel_syntax noprefix
.comm mem, 30000
.globl main

main:
    lea     rbx, mem
    mov     byte ptr [rbx], 0x41
    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