I’m currently learning assembly language and trying to write a simple calculator program. The program is supposed to read two numbers and an operator (+, -, *, /) from stdin, perform the specified operation, and print the result. However, I’m encountering some issues with the IMUL and DIV instructions in my code. Here’s what I have so far

section .bss
    num1 resb 8         ; Buffer to store the first number (max 8 bytes)
    num2 resb 8         ; Buffer to store the second number (max 8 bytes)
    operator resb 1     ; Buffer to store the operator (1 byte)
    result resb 16      ; Buffer to store the result (max 16 bytes)

section .text
    global _start

_start:
    ; Read the first number
    mov rax, 0          ; syscall number (sys_read)
    mov rdi, 0          ; file descriptor (stdin)
    mov rsi, num1       ; buffer to store input
    mov rdx, 8          ; number of bytes to read (max 8 bytes)
    syscall             ; call kernel

    ; Read the operator
    mov rax, 0          ; syscall number (sys_read)
    mov rdi, 0          ; file descriptor (stdin)
    mov rsi, operator   ; buffer to store input
    mov rdx, 1          ; number of bytes to read (1 byte)
    syscall             ; call kernel

    ; Read the second number
    mov rax, 0          ; syscall number (sys_read)
    mov rdi, 0          ; file descriptor (stdin)
    mov rsi, num2       ; buffer to store input
    mov rdx, 8          ; number of bytes to read (max 8 bytes)
    syscall             ; call kernel

    ; Convert first number from ASCII to integer
    movzx eax, byte [num1]
    sub eax, '0'

    ; Convert second number from ASCII to integer
    movzx ebx, byte [num2]
    sub ebx, '0'

    ; Perform the specified operation
    mov cl, byte [operator]
    cmp cl, '+'
    je add_operation
    cmp cl, '-'
    je sub_operation
    cmp cl, '*'
    je mul_operation
    cmp cl, '/'
    je div_operation

add_operation:
    add eax, ebx
    jmp output

sub_operation:
    sub eax, ebx
    jmp output

mul_operation:
    imul eax, ebx
    jmp output

div_operation:
    xor edx, edx
    div ebx
    jmp output

output:
    add eax, '0'
    mov [result], al
    mov [result+1], 0 ; null terminator for safety
    mov rax, 1          ; syscall number (sys_write)
    mov rdi, 1          ; file descriptor (stdout)
    mov rsi, result     ; buffer to store output
    mov rdx, 2          ; number of bytes to write (result + null terminator)
    syscall             ; call kernel

    ; Exit
    mov rax, 60         ; syscall number (sys_exit)
    xor rdi, rdi        ; exit code 0
    syscall             ; call kernel

When I run this code, I get the following errors:

main.asm:60: error: invalid combination of opcode and operands
main.asm:71: error: operation size not specified

I am not sure how to fix these issues. Any help would be greatly appreciated!