x86 assembly: printing integer to the console afte

Question

I'm trying to learn x86 assembly. The book I'm using is Assembly Language - Step by Step, Programming With Linux (and I'd have to say it's pretty good). I've learned a lot so far, but I feel as though I should also be challenging myself to stay ahead in many respects so I can learn faster through doing (I can do follow along, top-down learning, but I find it tediously slow).

So, I figured it would be a cool idea to try and multiply two registers (32-bit) and then output the data to the console.

The problem is that when I execute the program (I'm using NASM, just as the book does - no Insight debugger though), I receive a segmentation fault. I've done a fair amount of debugging in gdb with this little hammer out, but for whatever reason I can't seem to figure out what the issue is.

I'd like to know why I'm receiving a segmentation fault, and what a good way would be to reprimand the issue. Also, if the comments I've made in the code don't match up with what exactly is happening, I'd be grateful if anyone could correct me on that.

Here's my code so far (it's well commented)

Thanks.

teh codez

section .data
;TODO

section .bss
valueToPrint: resb 4            ;alloc 4 bytes of data in 'valueToPrint'

section .text

global _start

_mul:
    mov eax, 0x2A ;store 42 in eax
    mov edx, 0x2A ;store 42 in edx
    mul eax
    ret

_safe_exit:
    mov eax, 1  ;initiate 'exit' syscall
    mov ebx, 0  ;exit with error code 0
    int 0x80    ;invoke kernel to do its bidding 

_start:
    nop                         ;used to keep gdb from complaining

    call _mul                       ;multiply the values
    mov [valueToPrint], eax         ;store address of eax in the contents of valueToPrint
    mov eax, 4                      ;specify a system write call - aka syswrite
    mov ebx, 1                      ;direction used to make the syswrite call output to console - i.e. stdout
    mov dword [ecx], valueToPrint   ;store valueToPrint in ecx: ecx represents the syswrite register
    int 0x80                        ;invoke kernel based on the given parameters

    call _safe_exit

Edit

Also, I'm running Arch Linux, if that makes a difference.

Answer 1

EDITIT! (forgot a cmp!)

To output value in base 10...

; assuming value is in EAX (only)
.loop1:
    div   10      ; divide by 10, leave result in eax, REMAINDER in edx
    push  eax     ; save value
    mov   eax,edx
    or    eax,0x30 ; convert 0-9 to '0'-'9' (ascii 0x30='0')
    call  display_a_char ; (you write this!)
    pop   eax
    or    eax,eax ; set flags (edit!)
    jnz   .loop1
; all done, maybe put a \n return or something here

Answer 2

This line is causing the segmentation fault:

mov dword [ecx], valueToPrint

You're telling it to store valueToPrint in the memory location at address ecx. You never initialize ecx (the kernel probably initializes it to 0 on program start for you), so when you dereference it, you're going to access an invalid memory location.

The write(2) system call takes 3 parameters: the file descriptor number in register ebx, a pointer to the string to write in ecx, and the number of bytes to write in edx. So, if you want to just print the raw binary data of the result, you can pass the address of valueToPrint, and tell it to print 4 bytes from that address. In this case, valueToPrint is 1764 (0x6e4 in hex), so this code would print out the 4 bytes e4 06 00 00 on x86, which is little-endian:

mov [valueToPrint], eax   ; store the result into memory
mov eax, 4                ; system call #4 = sys_write
mov ebx, 1                ; file descriptor 1 = stdout
mov ecx, valueToPrint     ; store *address* of valueToPrint into ecx
mov edx, 4                ; write out 4 bytes of data
int 0x80                  ; syscall