Writing a simple calculator program in assembly language can be a great learning exercise. Since assembly language is related to specific processor architectures, I will provide an example of an assembly language calculator based on the x86 architecture. Please note that this example will be very basic, supporting only integer addition and subtraction.
Below is a simple calculator example written in x86 assembly language. This program will run in the command line interface and allow the user to input two integers and an operator (+ or -).
section .data prompt1 db "Enter first number: ", 0 prompt2 db "Enter second number: ", 0 prompt3 db "Enter operation (+ or -): ", 0 result db "Result: ", 0 newline db 10, 0 ; newline character section .bss num1 resb 4 ; reserve 4 bytes for the first number num2 resb 4 ; reserve 4 bytes for the second number op resb 1 ; reserve 1 byte for the operation section .text global _start _start: ; Display prompt for the first number mov eax, 4 mov ebx, 1 mov ecx, prompt1 mov edx, 20 int 0x80 ; Read the first number from the user mov eax, 3 mov ebx, 0 mov ecx, num1 mov edx, 4 int 0x80 ; Display prompt for the second number mov eax, 4 mov ebx, 1 mov ecx, prompt2 mov edx, 20 int 0x80 ; Read the second number from the user mov eax, 3 mov ebx, 0 mov ecx, num2 mov edx, 4 int 0x80 ; Display prompt for the operation mov eax, 4 mov ebx, 1 mov ecx, prompt3 mov edx, 23 int 0x80 ; Read the operation from the user mov eax, 3 mov ebx, 0 mov ecx, op mov edx, 1 int 0x80 ; Perform the operation mov al, [op] cmp al, '+' je add_numbers cmp al, '-' je subtract_numbers jmp invalid_operation add_numbers: ; Add the numbers and store the result in num1 mov eax, [num1] add eax, [num2] mov [num1], eax jmp display_result subtract_numbers: ; Subtract the second number from the first and store the result in num1 mov eax, [num1] sub eax, [num2] mov [num1], eax jmp display_result invalid_operation: ; Display an error message for invalid operation mov eax, 4 mov ebx, 1 mov ecx, error_msg mov edx, 26 int 0x80 jmp exit display_result: ; Display the result mov eax, 4 mov ebx, 1 mov ecx, result mov edx, 8 int 0x80 ; Display the calculated result mov eax, [num1] add eax, 0x30303030 ; convert to ASCII (assuming EAX < 1000) mov dl, al mov ah, 2 int 0x21 ; Display a newline mov eax, 4 mov ebx, 1 mov ecx, newline mov edx, 1 int 0x80 exit: ; Exit the program mov eax, 1 xor ebx, ebx int 0x80 error_msg db "Invalid operation!", 0Please note that this code has several important issues and limitations:
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The input numbers are assumed to be single-byte integers (0-255), which is impractical in the real world. For larger integers, you would need to handle multi-byte values and possible overflows.
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The display of the result assumes the result is a number less than 1000 and simply converts it to ASCII for display. This is not a generic solution.
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The program does not perform error checking, such as ensuring that the user inputs valid numbers or operators.
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Linux system calls (
int 0x80) are used to interact with the operating system, meaning it can only run on Linux and needs to be compatible with a 32-bit environment. -
int 0x21is used for character display, which is actually a DOS interrupt and should not appear in a Linux program. This is an error and should be replaced with anothermovandint 0x80call.
This code example requires quite a bit of correction to work in practice. To keep it simple, I have not included complete input validation and error handling, which are necessary in production code.
If you want to write and run assembly programs on Windows, you need to use Windows APIs instead of Linux system calls, and you might need to use a different assembler (like MASM or FASM).
Additionally, I recommend checking the documentation for your assembler and target operating system for the correct usage of system calls and APIs. For learning purposes, you may also want to use a simulator or debugger to step through your code and observe its behavior.
Step Debugging This Program
Step debugging assembly language programs typically requires a debugger, such as GDB (GNU Debugger) for x86 programs on Linux, or on Windows, you might use WinDbg, OllyDbg, or the Visual Studio debugger. Since the example code I provided earlier was written for the Linux environment and contains some errors, I will first correct those errors and then provide a simplified version so you can use GDB for step debugging.
Below is a simplified and corrected x86 assembly program that implements a simple addition and subtraction calculator:
section .data prompt1 db "Enter first number: ", 0 prompt2 db "Enter second number: ", 0 prompt3 db "Enter operation (1 for +, 2 for -): ", 0 result db "Result: ", 0 newline db 10, 0 error_msg db "Invalid operation!", 0 section .bss num1 resb 4 num2 resb 4 op resb 1 section .text global _start _start: ; Display prompt for the first number mov eax, 4 mov ebx, 1 mov ecx, prompt1 mov edx, 20 int 0x80 ; Read the first number from the user mov eax, 3 mov ebx, 0 mov ecx, num1 mov edx, 4 int 0x80 ; Display prompt for the second number mov eax, 4 mov ebx, 1 mov ecx, prompt2 mov edx, 20 int 0x80 ; Read the second number from the user mov eax, 3 mov ebx, 0 mov ecx, num2 mov edx, 4 int 0x80 ; Display prompt for the operation mov eax, 4 mov ebx, 1 mov ecx, prompt3 mov edx, 35 int 0x80 ; Read the operation from the user mov eax, 3 mov ebx, 0 mov ecx, op mov edx, 1 int 0x80 ; Subtract '0' from the operation to get the actual operation code movzx eax, byte [op] sub al, '0' cmp al, 2 ja invalid_operation ; Perform the operation cmp al, 1 je add_numbers jmp subtract_numbers add_numbers: ; Add the numbers mov eax, [num1] add eax, [num2] jmp display_result subtract_numbers: ; Subtract the numbers mov eax, [num1] sub eax, [num2] display_result: ; Display the result mov [num1], eax ; Store the result in num1 ; Display "Result: " mov eax, 4 mov ebx, 1 mov ecx, result mov edx, 8 int 0x80 ; Convert the result to ASCII and display it ; Note: This only works for results 0-9999. For larger numbers, you need a more complex conversion. add eax, 0x30303030 mov dl, al mov ah, 2 int 0x21 ; DOS interrupt, replace with Linux system call if targeting Linux ; Display a newline mov eax, 4 mov ebx, 1 mov ecx, newline mov edx, 1 int 0x80 jmp exit invalid_operation: ; Display an error message for invalid operation mov eax, 4 mov ebx, 1 mov ecx, error_msg mov edx, 19 int 0x80 exit: ; Exit the program mov eax, 1 xor ebx, ebx int 0x80Note: The code above still contains an error, namely using int 0x21 to display result characters. On Linux, you should use int 0x80 system calls to accomplish this task. However, to simplify, we assume you are only concerned with the logic of step debugging, rather than the specific implementation of system calls.
To debug this program in GDB, you first need to use the nasm assembler to assemble it into an object file, then link it into an executable file. Below are the commands executed in the Linux terminal:
nasm -f elf32 -o calculator.o calculator.asm
ld -m elf_i386 -o calculator calculator.oThen, you can use GDB to debug this program:
gdb ./calculatorIn GDB, you can use the following commands to step through the program:
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break _start: Set a breakpoint at_start. -
run: Run the program until it hits the first breakpoint. -
stepiors: Step through the current instruction. -
nextiorni: Execute the next instruction but do not enter the function if it is a function call. -
continueorc: Continue executing the program until the next breakpoint. -
info registers: View the contents of the registers. -
x/x $ecx: View the contents of the memory address pointed to by the ECX register (in hexadecimal format). -
print $eax: Print the value of the EAX register.
Using these commands, you can step through the program, view the register states, and monitor memory changes to understand how the program works. Remember, when inputting numbers, since the program does not implement input buffer handling, you need to press the enter key after each input and input only 1 or 2 for the operation code.