Rework the second chapter

Signed-off-by: Alexander Kuleshov <kuleshovmail@gmail.com>

.gitignore

@@ -50,3 +50,6 @@ modules.order
 Module.symvers
 Mkfile.old
 dkms.conf
+
+# Our programs
+sum/sum

README.md

@@ -22,7 +22,7 @@ Have fun!
 Here are links to each post:
 
  * [Say hello to x86_64 Assembly part 1](https://github.com/0xAX/asm/blob/master/content/asm_1.md)
- * [Say hello to x86_64 Assembly part 2](https://github.com/0xAX/asm/blob/master/content/asm_2.md)
+ * [Part 2. The `x86_64` concepts](https://github.com/0xAX/asm/blob/master/content/asm_2.md)
  * [Say hello to x86_64 Assembly part 3](https://github.com/0xAX/asm/blob/master/content/asm_3.md)
  * [Say hello to x86_64 Assembly part 4](https://github.com/0xAX/asm/blob/master/content/asm_4.md)
  * [Say hello to x86_64 Assembly part 5](https://github.com/0xAX/asm/blob/master/content/asm_5.md)

sum/README.md

@@ -1,8 +1,9 @@
-This is "sum of 2 numbers" with assembly for Linux x86-64
+This is a basic program that gets the sum of two integer numbers and check that it is correct. It prints the message if the sum is correct, otherwise just exits.
 
-Build it with: `make`
+Build the program executing:
 
-More details at - [Say hello to x64 Assembly [part 2]](https://github.com/0xAX/asm/blob/master/content/asm_2.md)
-
-[@0xAX](https://x.com/0xAX)
+```bash
+make
+```
 
+Read the details in the - [Part 2. The `x86_64` concepts](https://github.com/0xAX/asm/blob/master/content/asm_2.md)

sum/sum.asm

@@ -1,47 +1,53 @@
-;initialised data section
+;; Definition of the .data section
 section .data
- ; Define constants
- num1: equ 100
- num2: equ 50
- ; http://geekswithblogs.net/MarkPearl/archive/2011/04/13/more-nasm.aspx
- msg: db "Sum is correct", 10
+ ;; The first number
+ num1 dq 0x64
+ ;; The second number
+ num2 dq 0x32
+ ;; The message to print if the sum is correct
+ msg db "The sum is correct!", 10
 
-;;
-;; program code
-;;
+;; Definition of the .text section
 section .text
- global _start
+ ;; Reference to the entry point of our program
+ global _start
 
-;; entry point
+;; Entry point
 _start:
- ; get sum of num1 and num2
- mov rax, num1
- mov rbx, num2
- add rax, rbx
- ; compare rax with correct sum - 150
- cmp rax, 150
- ; if rax is not 150 go to exit
- jne .exit
- ; if rax is 150 print msg
- jmp .rightSum
+ ;; Set the value of the num1 to the rax
+ mov rax, [num1]
+ ;; Set the value of the num2 to the rbx
+ mov rbx, [num2]
+ ;; Get sum of the rax and rbx. The result is stored in the rax.
+ add rax, rbx
+.compare:
+ ;; Compare the value of the rax with `150`
+ cmp rax, 150
+ ;; Go to the .exit label if the values of the rax and 150 are not equal
+ jne .exit
+ ;; Go to the .correctSum label if the values of the rax and 150 are equal
+ jmp .correctSum
 
-; Print message that sum is correct
-.rightSum:
- ;; write syscall
- mov rax, 1
- ;; file descritor, standard output
- mov rdi, 1
- ;; message address
- mov rsi, msg
- ;; length of message
- mov rdx, 15
- ;; call write syscall
- syscall
- ; exit from program
- jmp .exit
+; Print message that the sum is correct
+.correctSum:
+ ;; Number of the sytem call. 1 - `sys_write`.
+ mov rax, 1
+ ;; The first argument of the `sys_write` system call. 1 is `stdout`.
+ mov rdi, 1
+ ;; The second argument of the `sys_write` system call. Reference to the message.
+ mov rsi, msg
+ ;; The third argument of the `sys_write` system call. Length of the message.
+ mov rdx, 20
+ ;; Call the `sys_write` system call.
+ syscall
+ ; Go to the exit of the program.
+ jmp .exit
 
 ; exit procedure
 .exit:
- mov rax, 60
- mov rdi, 0
- syscall
+ ;; Number of the system call. 60 - `sys_exit`.
+ mov rax, 60
+ ;; The first argument of the `sys_exit` system call.
+ mov rdi, 0
+ ;; Call the `sys_exit` system call.
+ syscall