linux - 在汇编中查找数字是偶数/奇数

Question

我正在尝试查找给定数字(用户输入)是偶数还是奇数。

I'm simply applying AND operation on binary digits of a no. with 1, If the number is odd then operation will result 0 and we will Output Number is odd, otherwise we will output Number is even.

虽然逻辑看似简单，但在下面的代码中却行不通。我不知道代码中的问题在哪里。谁能告诉我问题出在哪里

section .data
    userMsg db 'Please enter a number'
    lenuserMsg equ $ - userMsg  
    even_msg db 'Even Number!'
    len1 equ $ - even_msg
    odd_msg db 'Odd Number!'
    len2 equ $ - odd_msg

section .bss
    num resb 5    ;Reserved 5 Bytes for Input

section .text

global _start     ;must be declared for linker (gcc)

_start:
    ;User Prompt
    mov ebx, 1           ;file descriptor (stdout)
    mov ecx, userMsg     ;message to write 'Please enter a number'
    mov edx, lenuserMsg  ;message length
    mov eax, 4           ;system call number (sys_write)
    int 0x80             ;call kernel

    ;Taking user input
    mov ebx, 0           ;(stdin)
    mov ecx, num 
    mov edx, 5           ;i/p length
    mov eax, 3           ;system call number (sys_read)
    int 0x80             ;call kernel

    mov ax, [num]
    and ax, 1
    jz evnn              ;Jump on Even

    ;Printing No. is Odd
    mov ebx, 1           ;file descriptor (stdout) 
    mov ecx, odd_msg     ;message to write 'Odd Number!'
    mov edx, len2        ;message length
    mov eax, 4           ;system call number (sys_write)
    int 0x80             ;call kernel
    jmp outprog          ;Jump to exit

    ;Printing No. is Even
    evnn:
    mov ebx, 1           ;file descriptor (stdout) 
    mov ecx, even_msg    ;message to write 'Even Number!'
    mov edx, len1        ;message length  
    mov eax, 4           ;system call number (sys_write)
    int 0x80             ;call kernel

    ;Exit
    outprog: 
    mov eax, 1           ;system call number (sys_exit)
    int 0x80             ;call kernel

Answer 1

只关注手头的实际问题，好吗？如果说一个 ASCII 字符被放入 AL 寄存器，只需将其转换为一个数字，其余的应该是自然的。在计算(二进制数和系统)中，整数的奇数或偶数由位0决定。如果为1，则为奇数。如果为0，则为偶数。 (令我惊讶的是，到目前为止还没有人特别强调这一点)。

    ...                  ;OS puts a char in AL.
    sub al,30h           ;turn an ASCII char to one integer digit
    shr al,1             ;Lets see how the flags responds below
    jc .odd              ;CF is set if the first bit (right-most, bit 0) is 1.
      ;do Even things
      ;skip pass .odd
.odd:
      ;do Odd things