Simple program emulating UNIX cat

cat.asm

; Simple program emulating UNIX cat
;
; Assembler: YASM
;
; Fernando Giannasi
; July 09, 2023

section .data

SYS_EXIT        equ 60
SYS_READ        equ 0
SYS_WRITE       equ 1
SYS_OPEN        equ 2
SYS_CLOSE       equ 3
STD_OUT         equ 1
O_RDONLY        equ 0

EXIT_SUCCESS    equ 0
BUFFER_SIZE     equ 256

errorMsg        db  "Usage: ./cat <filename_1> ... <filename_n>",10,0
errorMsgLen     equ $ - errorMsg
errorMsg2       db  "Error opening the file",10,0
errorMsg2Len    equ $ - errorMsg2


section .bss
    buffer      resb BUFFER_SIZE

section .text
    global _start

%macro printMacro 2
    mov rsi, %1
    mov rdx, %2
    mov rax, SYS_WRITE
    mov rdi, STD_OUT
    syscall
%endmacro

_start:
    ; check args (argc >= 2)
    pop rcx
    cmp rcx, 2
    jl _error

    mov r15, rcx
_outerloopRW:
    dec r15
    cmp r15, 0
    je _sysExit ; exit outerloop

    ; get filename, open read-mode and check
    mov rax, SYS_OPEN
    pop rdi
    pop rdi
    mov rsi, O_RDONLY
    mov rdx, 0
    syscall
    push rax
    cmp rax, 0
    jle _filerror

    ; read from file
_innerloopRW:
    mov rdi, [rsp]
    mov rax, SYS_READ
    mov rsi, buffer
    mov rdx, BUFFER_SIZE
    syscall

    ; check number of bytes read, exit innerloop if 0
    cmp rax, 0
    je _outerloopRW

    ; print
    printMacro buffer, rax
    jmp _innerloopRW

_sysExit:
    mov rax, SYS_EXIT
    mov rdi, EXIT_SUCCESS
    syscall
    ret

_error:
    printMacro errorMsg, errorMsgLen
    call _sysExit

_filerror:
    printMacro errorMsg2, errorMsg2Len
    call _sysExit