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asm/4.md

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+# Assembly x86_64 - User Input
+
+## Assembly Code 
+
+We're going to use vim to write our code
+    
+    
+    [ 192.168.0.18/24 ] [ /dev/pts/88 ] [~/binexp/asm]
+    → vim 4.asm
+    
+    
+    
+    
+    section .data
+            mytext1 db "What is your name? "
+            mytext2 db "Hello, "
+    
+    section .bss
+            myname resb 16  ; we reserve 16 bytes and we can access it with the label 'myname'
+    
+    section .text
+            global _start
+    
+    _start:
+            call _printText1
+            call _getName
+            call _printText2
+            call _printName
+    
+    
+            mov rax, 60
+            mov rdi, 0
+            syscall
+    
+    
+    _printText1:            ;print 'what your the name ?'
+            mov rax, 1              ; syscall ID 1 (write text)
+            mov rdi, 1              ; first arg  = 1
+            mov rsi, mytext1        ; second arg = our text1 (what is your name?)
+            mov rdx, 19             ; third arg  = the text limit
+            syscall                 ;run the syscall
+            ret             ; finish the function
+    
+    _getName:               ;get user input
+            mov rax, 0              ; syscall ID 0 (read user input)
+            mov rdi, 0              ; first arg 0 is stdin, 1 is stdout, 2 is stderr
+            mov rsi, myname         ; second arg (where will we store user input?)
+            mov rdx, 16             ; third arg the limit of user input characters we want
+            syscall         ;make the syscall
+            ret             ;finish the function
+    
+    _printText2:            ;print 'Hello'
+            ;same as printtext1
+            mov rax, 1
+            mov rdi, 1
+            mov rsi, mytext2
+            mov rdx, 7
+            syscall
+            ret
+    
+    _printName:             ;print the name
+            ;same as pritntext 1 and 2
+            mov rax, 1
+            mov rdi, 1
+            mov rsi, myname ; this is the myname label with it's 16 reserved bytes from earlier
+            mov rdx, 16
+            syscall
+            ret
+    
+    
+
+Now this code contains what we learned previously, with the exception of the following chunks of code: 
+    
+    
+    section .bss
+            myname resb 16 
+    
+
+this is used to declare a variable called 'myname' and we reserve 16 bytes for it. Ideally to store our user input string of text.
+    
+    
+    _getName:               
+            mov rax, 0              ; syscall ID 0 (read user input)
+            mov rdi, 0              
+            mov rsi, myname         
+            mov rdx, 16     
+            syscall         
+            ret             
+    
+    
+
+The _getName function first sets the rax register as 0 (syscall ID 0) to read the user input, more info [here](https://chromium.googlesource.com/chromiumos/docs/+/HEAD/constants/syscalls.md)
+
+the first arguement (rdi) is set to 0 because we want stdin (1 is stdout and 2 is stderr)
+
+the second arguement (rsi) is the variable we want to store our user input to,
+
+the third arguement (rdx) is set to 16 because that is the maximum size of reserved bytes for our 'myname' variable. 
+
+and lastly we just run the syscall and end the function.
+
+## Compiling 
+
+Here we're going to use nasm to compile our assembly code and then use ld to create the binary out of the .o file: 
+    
+    
+    [ 192.168.0.18/24 ] [ /dev/pts/89 ] [~/binexp/asm]
+    → nasm -f elf64 4.asm -o 4.o
+    
+    [ 192.168.0.18/24 ] [ /dev/pts/89 ] [~/binexp/asm]
+    → ld 4.o -o 4
+    
+    [ 192.168.0.18/24 ] [ /dev/pts/89 ] [~/binexp/asm]
+    → ./4
+    What is your name? Pierre
+    Hello, Pierre
+    
+    
+    
+
+And that's it ! in the next tutorial we will cover math operations and the stack, you can click [here](5.html).
+