Data Types

Here I am introducing different data types in assembly. Read the comments to know more.

;note how I'm using different data types here and the brackets to initialize rax
;with not the address of a variable but the absolute value
global start

section .text

        _start:
        xor rax,rax; nullifying rax to make sure no garbage bytes exist
        mov al,1 ;syscall for write.
        mov rdi,1 ;int fd, which is 1 for stdout since we are not storign the  message anywhere rather printing it out
        mov rsi,hello_world ; The  buffer to be printed
        mov rdx,length ; The length of   hello world buffer we calculated in data section
        syscall ; Executes the syscall 1 with arguments we gave. At this point rax  will store the return value from the executed syscall

        mov rax, var4 ; Loads up the var4 address in rax
        mov rax, [var4] ; Loads up the value in var4 in rax

        mov rax, 60 ; syscall number for exit
        mov rdi, 11 ; Custom error code. Could be anything. 0 in most cases.
        syscall

section .data
        hello_world: db "Hello world! This is Harshit",0x0a
        length: equ $-hello_world

        var1: db 0x11,0x22 ; Assigns 2 bytes 0x11 and 0x22 to var1
        var2: dw 0x3344 ; Assigns a word (2 bytes) to var2
        var3: dd 0xaabbccdd ; Assigns a double word (4 bytes) to var3
        var4: dq  0xaabbccdd11223344 ; Assigns a quad word (8 bytes) to var4

        repeat_buffer: times 128 db 0xAA ; Assigns 128 bytes of 0xaa to repeat_buffer


section .bss
        buffer: resb 64 ; Reserves uninitialized 64 bytes

After assembling and linking the code, we can dissect this in GDB and check what is happening.

1. Here, we can see using "info variables" our variables are initialized

2. In GDB, we can use "x" to examine these. Use help x to display what commands can be used. So, x/x means examine in hex. Then add if you want to see bytes or whatever in x/<size>x<keyword> REGISTER_NAME format

3. We can also examine repeat_buffer (that times initialized with 128 0xaa) and buffer that initializes 64 bytes unreserved (assembler by default assigns it with 0x00)

4. We should run this and set a breakpoint on lines where I am assigning [var4] and var4 to rax to see how registers are behaving when RIP reaches there

Upon hitting breakpoint 2, we can observe that the memory address of variable 4 (var4) is not yet loaded in the rax register. Using "nexti" I can make the RIP jump one point and make it execute the next intstruction.

When I do nexti, I hit breakpoint3. This is the breakpoint where memory address of var4 has been set to rax but "mov rax [var4] is yet to be set. At that point, we can see registers and conclude var4's memory address has been set.

Now, we can examine the memory address in RAX right now and confirm that it is infact the var4 (same string). With "nexti" I jump RIP to next instruction and at that moment, "mov rax [var4]" has been executed and RAX assigned with the 8 byte string.

We can use disas command to see where RIP is at the moment. (notice arrow on the left)