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    <p>The first problem here is that you need to understand the sys_time sys call. There is a handy chart at <a href="http://syscalls.kernelgrok.com/" rel="noreferrer">http://syscalls.kernelgrok.com/</a> that tells you what the various sys calls require as input in registers.</p> <p>sys_time is system call 13, so</p> <pre><code>mov eax,13 </code></pre> <p>is good</p> <p>However sys_time also requires a memory address to be passed in ebx, where it writes the actual time.</p> <p>A quick way would be to allocate some space on the stack (we can push anything on the stack, the sys_time value will overwrite it, why not stick the value of eax onto it).</p> <pre><code>push eax </code></pre> <p>Then feed the stack pointer into ebx</p> <pre><code>mov ebx, esp </code></pre> <p>Now make the system call</p> <pre><code>int 80h </code></pre> <p>Now we can pop the time off the stack (into e.g. eax)</p> <pre><code>pop eax </code></pre> <p>Now eax contains the current unix time (i.e. number of seconds since Jan 01 1970.)</p> <p>To avoid the trickyness of printing numbers directly to the unix console I will cheat and provide a complete example that compiles in nasm and links with the c library via gcc and uses printf</p> <pre><code>[SECTION .data] PrintNum db "%d",10,0 ;this is a c string so is null terminated [SECTION .text] extern printf global main main: push ebp mov ebp,esp push ebx push esi push edi ; stuff before this for glibc compatibility mov eax, 13 push eax mov ebx, esp int 0x80 pop eax push eax ; push eax onto stack then the format string, then call printf to write eax to console, unwind stack pointer push PrintNum call printf add esp,8 pop edi ; stuff after this for glibc compatibility pop esi pop ebx mov esp,ebp pop ebp ret </code></pre> <p>Compile with</p> <pre><code>nasm -f elf sys_time.asm gcc sys-time.o -o sys-time </code></pre> <p>Though if you are on 64-bit linux you might have to do (and have the relevant multilib gcc and glibc). You can't compile this program as a native 64-bit executable because it uses push and pop and you can't push a 32 bit register onto a 64 bit stack.</p> <pre><code>nasm -f elf32 sys_time.asm gcc -m32 sys-time.o -o sys-time </code></pre> <p>Then you should get</p> <pre><code>$ ./systime 1310190574 </code></pre> <p>I've tested this on 32 bit and 64 bit linux and managed to get the above code compiled. Let me know if you have any problems.</p> <p>To answer your question regarded nasm tutorials, I have been learning recently from "Assembly Language Step By Step, Third Edition" by Jeff Duntemann. See <a href="http://www.duntemann.com/assembly.html" rel="noreferrer">http://www.duntemann.com/assembly.html</a> for details and sample chapters.</p>
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