flat assembler

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Rock24

Joined: 13 Feb 2015
Posts: 5

Rock24 13 Feb 2015, 22:05

Hello.
I am new in Assembler and I have my first big problem Smile

I would like to display on the screen second element of the list ( table).

Code:

format ELF64 executable ;wersja pliku
entry _start
segment readable executable
_start:
        mov rax,1
        mov rdi,1
        mov rsi,table
        mov rdx,[dlugosc]
        syscall
        
        mov rax,60
        syscall

segment readable writeable
table db "1","2","3","4"
dlugosc dq $ - table

I could write only this code. It shows all elements of the list

13 Feb 2015, 22:05

alkap

Joined: 18 Feb 2015
Posts: 44
Location: Dnipro, Ukraine

alkap 16 Mar 2015, 11:05

I could be wrong, but perhaps the code below might do the trick. I'm not familiar with the x86-64 syscall calling convention, nor do I have access to an x86-64 machine, so I haven't been able to test my suggestion:

Code:

...
mov rax,1 
mov rdi,1
mov rsi, byte [table+1]
mov rdx, 1
syscall
...

Can you please give this a try and let us know how you go.

16 Mar 2015, 11:05

HaHaAnonymous

Joined: 02 Dec 2012
Posts: 1178
Location: Unknown

HaHaAnonymous 16 Mar 2015, 16:19

Quote:

I would like to display on the screen second element of the list ( table).

If it is a table, then you may want an aligned table of strings... Because strings can have varying lengths.

Code:

format ELF64 executable ;wersja pliku 
entry _start 
segment readable executable 
_start: 
        mov rcx,table
        mov rax,1 
        mov rdi,1 
        ; writes the 2nd item
        lea rsi,[rcx+32+8]
        mov rdx,[rcx+32]
        syscall 
         
        mov rax,60 
        syscall 

segment readable writeable 

x = 32

align x
; alignment 32 is enough in this case because no item exceeds 32 bytes, in case one of your strings does, align to higher boundaries
table:
       dq $0000000000000002
       db ":D"

       align x
       dq $0000000000000004
       db "woof"

       align x
       dq $000000000000000A
       db "woof woof!"

       align x
       dq $0000000000000003
       db "o_O"

or alternatively this method:

Code:

format ELF64 executable ;wersja pliku 
entry _start 
segment readable executable 
_start: 
        mov rcx,table
        mov rax,1 
        mov rdi,1 
        ; writes the 2nd item
        mov rsi,[rcx+8*1]
        mov rdx,[rsi-8]
        syscall 
         
        mov rax,60 
        syscall 

segment readable writeable 

x = 8

align x
table:
       dq data0+x
       dq data1+x
       dq data2+x
       dq data3+x

align x
data0: dq $02
       db ":D"

align x
data1: dq $04
       db "woof"

align x
data2: dq $0A
       db "woof woof!"

align x
data3: dq $03
       db "o_O"

Not sure.

16 Mar 2015, 16:19

Endre

Joined: 29 Dec 2003
Posts: 215
Location: Budapest, Hungary

Endre 22 Mar 2015, 11:55

I guess the OP wants something like this

Code:

format ELF64 executable ;wersja pliku
entry _start
segment readable executable
_start:
        mov rax,1
        mov rdi,1
        mov rsi,table+1
        mov rdx,1
        syscall
        
        mov rax,60
        syscall

segment readable writeable
table db "1","2","3","4"
dlugosc dq $ - table

22 Mar 2015, 11:55

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