Question

I’m trying to learn assembly for various reasons and I’m trying to write a code that says hello bla bla bla
then reads your name, prints your name and says more bla bla bla
but it does not show the text after it gets your name
Im sure its something stupid… sorry…

here is what I have so far:

.section .bss
    .lcomm bufname, 256 
.section .data
    msg1:
    .ascii  "Hello, please enter your name!\n"  # Message to write
    len1 = . - msg1                 # Length of string

    msg2:
    .ascii "Good to meet you "
    len2 = . - msg2

    msg3:
    .ascii ". Iam your first assembly program!\n"
    len3 = . - msg3

.section .text

.globl _start

_start:
    call get_name
get_name:
    #should print msg1
    mov $4, %eax        # system call number (sys_write)
    mov $1, %ebx        # file descriptor (stdout)
    mov $msg1, %ecx     # Message to write
    mov $len1, %edx     # Lenght of message
    int $0x80       # Call kernel

    #should get user input
    mov $3, %eax        # System call number (sys_read)
    mov $0, %ebx        # File descriptor (stdin)
    mov $bufname, %ecx  # Buffer to store the name
    mov $256, %edx      # Lenght of buffer
    int $0x80 

    #should print msg2    
    mov $4, %eax
    mov $1, %ebx
    mov $msg2, %ecx
    mov $len2, %edx
    int $0x80

    #should print bufname (doesn't)
    mov $bufname, %ecx
    mov $256, %edx  
    int $0x80

    #should print msg3 (doesn't) 
    mov $msg3, %ecx
    mov $len3, %edx
    int $0x80 

    call exit   
exit:
    mov $1, %eax
    mov $0, %ebx
    int $0x80 

To compile i use

as Jes.s -o Jes.o
ld Jes.o -o Jes

This is the output i get

$ ./Jes
Hello, please enter your name!
renato
Good to meet you

It should show

Good to meet you renato. Iam your first assembly program!

Why is this wrong?
and much thanks for your time!

Answer 1

The problem is that when you call the interrupt (int) the registers can get overwritten. It’s not like making a function call where the registers are saved and restored for you. You need to place duplicates of these lines before all of your int calls:

mov $4, %eax        # system call number (sys_write)
mov $1, %ebx        # file descriptor (stdout)