x86-64LINUXcalculator

This is a simple x86-64 Assembly calculator, written for a Linux kernel

Installation/Setting up

To install and use the program, you must follow those steps:

1. Download the repository with either the GitHub Download button or with typing

git clone https://github.com/fif3x/x86-64LINUXcalculator.git

in the terminal

2. Install dependencies with running the installDependencies.sh script note: it's written for debian-based systems
3. Build project with running the build.sh script
4. Go to bin/ and open the .out file

How it works

Lets break down the program into multiple parts:

1. .data section and its contests: In the .data section, we store the string for output and its length like this:

output db 0, 10 ; space for 1 char and a newline char
length equ $ - output

2. Operation selection To select an operation, we change the value of the r14 register in line 25:

mov r14, 1  ; mode (1:add, 2:sub, 3:mul, 4:div)

3. Printing After preforming an operation, we store the result in r8 and call the _print function using

mov r8,  r12      ; store in r8 to print
call     _print   ; call print function

then in the _print function we firstly make r8 an ascii text, add the last byte of r8b to output, and then also add a new line to the output

add r8,      '0'
mov byte [output],   r8b
mov byte [output+1], 10

lastly, we call the Linux write function, using the following values: rax = 1 for syscall:write rdi = 1 for stdout rsi = address of output to point to it rdx = length of output so the code looks like this:

mov rax,             1              ; syscall: write
mov rdi,             1              ; stdout
lea rsi,             [rel output]   ; output address
mov rdx,             length         ; output length
syscall
ret ; return control

4. Ending program After completing everything needed, we clear the registers using xor reg, reg, which is a bitwise operator making reg equal to 0, and we call the syscall for exiting:

; clear registers
xor r12, r12
xor r13, r13
xor rdx, rdx
xor r8,  r8
xor rsi, rsi
xor r14, r14

mov rax, 60   ; exit program
xor rdi, rdi  ; code 0
syscall