Chars, Lists, and Strings

CHARS, LISTS, AND STRINGS

Character Data

section .data
    char db 'A'  ; Define a single character 'A'

section .text
    global _start

_start:
    mov bl, [char]  ; Move the value of char to bl
    mov eax, 1      ; Move 1 to eax
    int 80h         ; Interrupt

If I run the code and print the exit code with echo $? in bash, I will see the ASCII value of 'A' (65). I can search for the ASCII table to verify this or use the man ascii command in the terminal.

List Data

section .data
    list db 1, 2, 3, 4  ; Define a list of values with no null terminator
                        ; Each value is a byte in size

section .text
    global _start

_start:
    mov bl, [list]  ; Move the first value of list to bl
    mov eax, 1      ; Move 1 to eax
    int 80h         ; Interrupt

One thing I omitted in this code is the null terminator, something that I learned in C. In this case, I have a list of values, so I need to add the null terminator manually so I know when the list ends. I can use whatever value I want as long as it’s not part of the list. For example, in this case, using 0 or -1 would be a good choice list db 1, 2, 3, 4, -1.

String Data

section .data
    string1 db 'ABA', 0  ; Define a string 'ABA' with null terminator
    string2 db 'CDE', 0  ; Define another string 'CDE' with null terminator

section .text
    global _start

_start:
    mov bl, [string1]  ; Move the first character of string1 to bl
    mov eax, 1         ; Move 1 to eax
    int 80h            ; Interrupt

In strings, it’s a little bit easier to add the null terminator because I can just add it at the end of the string. The importance of adding a null terminator is to know when the string ends; otherwise, I don’t know where a slot of memory ends and another begins.

GDB Tips

By default, GDB uses AT&T syntax but to ensure consistency with NASM I can set GDB to use Intel syntax. This will make it easier to understand the code during debugging sessions.

Command to set Intel syntax in GDB:

echo "set disassembly-flavor intel" > ~/.gdbinit

Also, in GDB I can use x/x to see the memory address in hex and x/x2 to see the next 2 memory addresses and so on.